Earthquake Report: Turkey!

Posted on October 30, 2020 by Jay Patton

I awakened to be late to attending the GSA meeting today. I had not checked the time. 7am is too early, but i understand the time differences…
As i was logging into Zoom, my coworker emailed our Tsunami Unit group about a M7 in the eastern Mediterranean. So, I shifted gears a bit. But i had my poster to present, so i had to stay somewhat focused on that.
https://earthquake.usgs.gov/earthquakes/eventpage/us7000c7y0/executive
Today, in the wee hours (my time in California), there was a M 7.0 earthquake offshore of western Turkey in the Icarian Sea. The earthquake mechanism (i.e. focal mechanism or moment tensor) was for an extensional type of an earthquake, slip along a normal fault.
I immediately thought about some quakes/deprems that happened there several years ago. This area is an interesting and complicated part of the world, tectonically.

2017.07.20 M 6.7 Turkey
2017.06.12 M 6.3 Turkey/Greece

To the north is a strike-slip plate boundary localized along the North Anatolia fault system. This is a right lateral fault system, where the plates move side by side, relative to each other. See the introductory information links below to learn more about different types of faults.
To the south is a convergent plate boundary (plates are moving towards each other) related to (1) the Alpide Belt, a convergent plate boundary formed in the Cenozoic that extends from Australia to Morocco. On the southern side of Greece and western Turkey, there are subduction zones where the Africa plate dives northward beneath the Eurasia and Anatolia plates.
The region of today’s earthquake is in a zone of north-south oriented extension. This extension appears to be in part due to gravitational collapse of uplifted metamorphic core complexes.
There are several “massifs” that were emplaced in the past, lifted up, creating gravitational potential. The normal faults may have formed as the upper crust extended. It is complicated here, so i am probably missing some details. But, with the references i provide below, y’all can read more on your own. Feel free to contact me if i wrote something incorrect. I love my peer reviewers (you).
So, this N-S extension creates east-west oriented valleys/basins with E-W striking (trending) faults. There are south dipping faults on the north sides and north dipping faults on the south side of these valleys.
These structures are called rifts. A famous rift is the East Africa Rift.
There are two main rifts in western Turkey, the Büyük Menderes Graben and the Küçük Menderes Graben Systems. If we project these rifts westward, we can see another rift, the rift that forms the Gulf of Corinth in Greece, the Gulf of Corinth Rift. This is one of the most actively spreading rifts in the world.
In addition to the large earthquake, which caused lots of building damage and also caused over a dozen deaths so far (sadly), there was recorded a tsunami on the tide gages in the region. I use the IOC website to obtain tide gage data. This is an excellent service. There are only a few national tide gage online websites that rival this one.
It is also highly likely that there were landslides or that there was liquefaction somewhere in the region. The USGS models i present below show a high likelihood for these earthquake triggered processes.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 7.0 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

On the left is a map from Armijo et al. (1999) that shows the plate boundary faults and tectonic plates in the region. This M 7.0 earthquake, denoted by the blue circle.
In the upper left corner is a map that shows the tectonic strain in the region. Areas of red are deforming more from tectonic motion than are areas that are blue. Learn more about the Global Strain Rate Map project here.
To the right of the strain map is a comparison of the shaking intensity modeled by the USGS and the shaking intensity based on peoples’ “boots on the ground” observations. A modeled estimate of intensity is shown by the color overlay and labels MMI 4, 5, 6, 7. The USGS Did You Feel It observations are the colored circles (color = intensity) and labeled dyfi 6.2 for example.
On the upper right and right center are two maps that show (bottom) liquefaction susceptibility and (top) landslide probability. These are based on empirical models from the USGS that show the chance an area may have experienced these processes that may have happened as a result of the ground shaking from the earthquake. I spend more time explaining these types of models and what they represent in this Earthquake Report for the recent event in Albania.
Faults shown on these maps come from the DISS fault database from INGV and their collaborators. These data have been incorporated into the Global Earthquake Model. The red lines represent the top of the fault plane and the green shapes represent the fault planes as they dip into the Earth. Note how the North Anatolia fault, which is a vertically dipping strike-slip fault, appears to not have fault planes. Why do you think that is?
In the lower right corner is a map showing epicenters for earthquakes since 30 July 2020 (from EMSC).
Along the bottom of the poster are several tsunami plots from the region. The Bodrum tide gage is on a south facing shoreline, so the waves are not directed directly at this gage. The Kos Marina and Hrakleio gages are more directly facing the earthquake. Note which gages have larger waves. Why do you think this is so?

Here is the map with a month’s seismicity plotted.
For the century record of M>7 earthquakes I use the USGS National Earthquake Information Center as a source of data.
However, there is a local network of earthquake records from CSEM EMSC here.

Here are the main tide gages that have decent tsunami records in the Aegean region. I offset these records vertically a modest amount for the plot, so disregard the absolute elevation values.
I made a crude measurements for the wave height of these tsunami records (neglecting to take into account changes in tide). The locations are shown in the map.

Other Report Pages

Some Relevant Discussion and Figures

Here is a lovely plate tectonic overview map, highlighting the plate boundary faults, as well as the crustal faults (Taymaz et a., 2007).

Seismicity of the Eastern Mediterranean region and surroundings reported by USGS–NEIC during 1973–2007 with magnitudes for M . 3 superimposed on a shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b).

Here is the tectonic map from Dilek and Sandvol (2009).

Tectonic map of the Aegean and eastern Mediterranean region showing the main plate boundaries, major suture zones, fault systems and tectonic units. Thick, white arrows depict the direction and magnitude (mm a21) of plate convergence; grey arrows mark the direction of extension (Miocene–Recent). Orange and purple delineate Eurasian and African plate affinities, respectively. Key to lettering: BF, Burdur fault; CACC, Central Anatolian Crystalline Complex; DKF, Datc¸a–Kale fault (part of the SW Anatolian Shear Zone); EAFZ, East Anatolian fault zone; EF, Ecemis fault; EKP, Erzurum–Kars Plateau; IASZ, Izmir–Ankara suture zone; IPS, Intra–Pontide suture zone; ITS, Inner–Tauride suture; KF, Kefalonia fault; KOTJ, Karliova triple junction; MM, Menderes massif; MS, Marmara Sea; MTR, Maras triple junction; NAFZ, North Anatolian fault zone; OF, Ovacik fault; PSF, Pampak–Sevan fault; TF, Tutak fault; TGF, Tuzgo¨lu¨ fault; TIP, Turkish–Iranian plateau (modified from Dilek 2006).

This is the Wouldloper (2009) tectonic map of the Mediterranean Sea.

This is a fantastic figure, yet quite complicated. This map shows teh plate boundaries, the GPS motions, and the tectonic strain for the region (Perouse et al., 2012).
We use GPS sites at specific locations to measure how fast the Earth’s crust moves due to plate tectonics and other reasons. These GPS sites are almost constantly recording their geographic position. If a GPS site is moving, we can take two observations (lets say a year apart), measure their relative distance, and divide the time between the measurements to get the velocity (the speed) that this GPS site is moving. The white vectors (the arrows) show the direction those GPS sites are moving and the length of the vector represents its velocity. The black arrows show what the plate motion rates are at the plate boundaries and these are modeled using lots of different data sources (not just GPS).
Tectonic strain is a measure of how much the Earth’s crust is deforming over time. The higher the tectonic strain rate (i.e. red), the more tectonic stress is being accumulated in the crust and along faults. Areas of higher strain are places where there are more likely to be larger or more (or both) earthquakes.

Present-day kinematic and tectonic map encompassing the Central and Eastern Mediterranean, summarizing our main results and interpretations. Our kinematic model includes rigid-block motions as well as localized and distributed strain. Central-SW Aegean block (CSW AEG block) and East Anatolian block (East Anat. block) are purely kinematic and directly results from strain modeling (Figure 5). AP-IO Block is our Apulian-Ionian block with tentative tectonic boundaries. Rotation pole of this Apulian-Ionian block relative to Nubia (Nu WAp-Io) and to Eurasia (Eu WAp-Io) are shown with their 95% confidence ellipse.

This is the Ersoy et al. (2014) map showing their interpretation of the modern deformation in the northern Aegean Sea and western Turkey.

Geological map showing the distribution of the Menderes Extensional Metamorphic Complex (MEMC), Oligocene–Miocene volcanic and sedimentary units and volcanic centers in the Aegean Extensional Province (compiled from geological maps of Greece (IGME) and Turkey (MTA), and adapted from Ersoy and Palmer, 2013). Extensional deformation field with rotation (rotational extension) is shown with gray field, and simplified from Brun and Sokoutis (2012), Kissel et al. (2003) and van Hinsbergen and Schmid (2012). İzmir–Balıkesir Transfer zone (İBTZ) give the outer limit for the rotational extension, and also limit of ellipsoidal structure of the MEMC. MEMC developed in two stages: the first one was accommodated during early Miocene by the Simav Detachment Fault (SDF) in the north; and the second one developed during Middle Miocene along the Gediz (Alaşehir) Detachment Fault (GDF) and Küçük Menderes Detachment Fault (KMDF). Extensional detachments were also accommodated by strike-slip movement along the İBTZ (Ersoy et al., 2011) and Uşak–Muğla Transfer Zone (Çemen et al., 2006; Karaoğlu and Helvacı, 2012). Other main core complexes in the Aegean, the Central Rhodope (CRCC), Southern Rhodope (SRCC), Kesebir–Kardamos Dome (KKD) and Cycladic (CCC) Core Complexes are also shown. The area bordered with dashed green line represents the surface trace of the asthenospheric window between the Aegean and Cyprean subducted slabs (Biryol et al., 2011; de Boorder et al., 1998). See text for detail.

This is a great figure showing another interpretation to explain the extension in this region (slab rollback and mantle flow) from Brun and Sokoutis (2012).

Mantle flow pattern at Aegean scale powered by slab rollback in rotation around vertical axis located at Scutary-Pec (Albania). A: Map view of fl ow lines above (red) and below (blue) slab. B: Three-dimensional sketch showing how slab tear may accommodate slab rotation. Mantle fl ow above and below slab in red and blue, respectively. Yellow arrows show crustal stretching.

Below is a series of figures from Jolivet et al. (2013). These show various data sets and analyses for Greece and Turkey.

Upper Panel (A): This is a tectonic map showing the major faults and geologic terranes in the region. The fault possibly associated with today’s earthquake is labeled OU on the map, for the Ula-Oren fault.

Lower Panel (B): This shows historic seismicity for the region. Note the general correlation with the faults in the upper panel.

A: Tectonic map of the Aegean and Anatolian region showing the main active structures
(black lines), the main sutures zones (thick violet or blue lines), the main thrusts in the Hellenides where they have not been reworked by later extension (thin blue lines), the North Cycladic Detachment (NCDS, in red) and its extension in the Simav Detachment (SD), the main metamorphic units and their contacts; AlW: Almyropotamos window; BD: Bey Daglari; CB: Cycladic Basement; CBBT: Cycladic Basement basal thrust; CBS: Cycladic Blueschists; CHSZ: Central Hellenic Shear Zone; CR: Corinth Rift; CRMC: Central Rhodope Metamorphic Complex; GT: Gavrovo–Tripolitza Nappe; KD: Kazdag dome; KeD: Kerdylion Detachment; KKD: Kesebir–Kardamos dome; KT: Kephalonia Transform Fault; LN: Lycian Nappes; LNBT: Lycian Nappes Basal Thrust; MCC: Metamorphic Core Complex; MG: Menderes Grabens; NAT: North Aegean Trough; NCDS: North Cycladic Detachment System; NSZ: Nestos Shear Zone; OlW: Olympos Window; OsW: Ossa Window; OSZ: Ören Shear Zone; Pel.: Peloponnese; ÖU: Ören Unit; PQN: Phyllite–Quartzite Nappe; SiD: Simav Detachment; SRCC: South Rhodope Core Complex; StD: Strymon Detachment; WCDS: West Cycladic Detachment System; ZD: Zaroukla Detachment. B: Seismicity. Earthquakes are taken from the USGS-NEIC database. Colour of symbols gives the depth (blue for shallow depths) and size gives the magnitude (from 4.5 to 7.6).

Upper Panel (C): These red arrows are Global Positioning System (GPS) velocity vectors. The velocity scale vector is in the lower left corner. The main geodetic (study of plate motions and deformation of the earth) signal here is the westward motion of the North Anatolian fault system as it rotates southward as it traverses Greece. The motion trends almost south near the island of Crete, which is perpendicular to the subduction zone.

Lower Panel (D): This map shows the region of mid-Cenozoic (Oligo-Miocene) extension (shaded orange). It just happens that there is still extension going on in parts of this prehistoric extension.

C: GPS velocity field with a fixed Eurasia after Reilinger et al. (2010) D: the domain affected by distributed post-orogenic extension in the Oligocene and the Miocene and the stretching lineations in the exhumed metamorphic complexes.

Upper Panel (E): This map shows where the downgoing slab may be located (in blue), along with the volcanic centers associated with the subduction zone in the past.

Lower Panel (F): This map shows the orientation of how seismic waves orient themselves differently in different places (anisotropy). We think seismic waves travel in ways that reflects how tectonic strain is stored in the earth. The blue lines show the direction of extension in the asthenosphere, green lines in the lithospheric mantle, and red lines for the crust.

E: The thick blue lines illustrate the schematized position of the slab at ~150 km according to the tomographic model of Piromallo and Morelli (2003), and show the disruption of the slab at three positions and possible ages of these tears discussed in the text. Velocity anomalies are displayed in percentages with respect to the reference model sp6 (Morelli and Dziewonski, 1993). Coloured symbols represent the volcanic centres between 0 and 3 Ma after Pe-Piper and Piper (2006). F: Seismic anisotropy obtained from SKS waves (blue bars, Paul et al., 2010) and Rayleigh waves (green and orange bars, Endrun et al., 2011). See also Sandvol et al. (2003). Blue lines show the direction of stretching in the asthenosphere, green bars represent the stretching in the lithospheric mantle and orange bars in the lower crust.

Upper Panel (G): This is the map showing focal mechanisms in the poster above. Note the strike slip earthquakes associated with the North Anatolian fault and the thrust/reverse mechanisms associated with the thrust faults.

G: Focal mechanisms of earthquakes over the Aegean Anatolian region.

Here is another map showing the GPS plate motion rates from Perouse et al. (2012). Note the scale on the two map panels are different. The rates on the map on the right are much faster than the rates in Africa.

Input GPS velocities of the model. Velocities are in Eurasia fixed reference frame with their respective 95% confidence ellipse. Velocity vectors are color coded relative to the study they have been taken from (see paper for more details). (a) GPS velocities of the entire Nubian plate used to constrain the Nubia–Eurasia relative motion. Nubia–Eurasia rotation pole defined in this and previous studies are shown with their 1s confidence ellipse: circle, Calais et al. [2003]; diamond, Le Pichon and Kreemer [2010]; open square, D’Agostino et al. [2008]; triangle, Argus et al. [2010]; filled square, Reilinger et al. [2006]; red star, present study. Parameters of these rotation poles are summarized in Table 2. (b) Focus on the GPS velocities in the Central and Eastern Mediterranean region.

Here is a map that shows historic earthquake mechanisms (Perouse et al., 2012).

Input seismic moment tensors of the model. Fault plane solutions are from the Harvard CMT catalog (from 1976 to 2007) and the Regional Centroid Moment Tensor (RCMT) catalog (from 1995 to 2007). Location and hypocenter depth of the events are relocalized according to the Engdahl et al. [1998] catalog.

Those Rifts

First we can see this map that highlights all the grabens mapped in the region. A graben is basically a block of Earth that has moved relatively down, forming a valley.
These grabens are bound on at least one side by a normal fault (shown here with stippled lines pointing in the direction that the faults dip into the Earth.

Outline geological map of western Anatolia showing Neogene and Quaternary basins [simplified from Bingo1 (1989).

Here is a map of the western part of the Buyuk Menderes Graben valley (Bozcurt 2000). The main reason to show this is because it shows the location of the cross-section shown next (in the box labeled “Figure 6b”).
The island labeled Chios here is also called Samos on other maps.

Simplified geological map of the northern margin of the Btiytik Menderes Graben in the area between Germencik and Umurlu.

Here is the cross section that shows their interpretation of the tectonic faults in the subsurface.

Geological cross-section of the northern margin of the Bt~yt~k Menderes Graben (see Fig. 6b for location) based on fig. llb of Cohen et al. (1995). This cross-section indicates a total of c. 5 km of extension. Assuming a uniform extension rate, the age of the fault zone is (c. 5 km/1 mm a -1) 5 Ma. More details in the paper.

Here is a low-angle oblique illustrative view of the Graben forming basin common in the region (Emre and Sozbilir, 2007..

Let’s now venture offshore into the ocean. This map shows some geologic units, some mapped crustal faults, and some seismic lines (Ocakoglu et al., 2005). These seismic lines are shown as rows of dots.
Each straight dotted line represents a path that a research vessel took to make observations about the subsurface using seismic waves. The 30 Oct 2020 M 7.0 earthquake was to the north of Samos.
None of the seismic lines are optimally located to look for the fault that ruptured earlier today, but they may help us learn about what might be possible here.

Geology map of the study area (simplified from MTA 1: 500,000 scale geology map) and location of the seismic lines. Active faults are marked onland with bold lines.

Here are some seismic lines (seismic reflection profiles), whose locations are shown on the above map. The upper two panels are relevant (see line 10 on the map). These are consistent with normal faults on the north side of the basin.

Time migrated seismic sections, offshore Teke and Karaburun, showing active normal faults marked with white lines and strike-slip faults with black lines (see Fig. 3A for locations). Vertical exaggeration is ~2. Observed vertical displacement on the seafloor and basement surface by normal fault (marked with bold circle on Line-10) looks the same, thus this normal fault is Quaternary age. On line-18, vertical displacement seen on basement units are greater than displacement on Pliocene–Quaternary deposits due to fault marked with a bold circle thus this normal fault can be interpreted as Later Miocene–Pliocene age.

I include this map to show that there are lots of faults in this area. This is their final fault map based on the interpretations of many seismic lines.

(A) The correlations between offshore and onshore active fault systems in the study region. N–S, NE–SW and NW–SE oriented lines and dashed-lines show interpreted active strike-slip faults and their possible extensions. These faults are annotated with dNT for those at north and dST for those at south. E–W oriented lines and dashed lines show interpreted active normal faults and their possible continuations, with footwalls indicated by the plus symbol. (B) Simplified active fault map of the study area. The bold lines show the master active faults. (C) Pureshear model can explain the development of active structures in the study area.

Below are a map and a cross section further to the east, in the eastern part of the Büyük Menderes Graben (Kaya, 2015). They were studying geotherm water in the region as it relates to the fault geometry and other factors. and, well, who doesn’t like a little pre-planning at a hot spring?

Geological map of western Turkey showing the Menderes massif and its subdivision into the AG Alasehir graben, the BMG Büyük Menderes graben, the CMM Central Menderes massif, the KMG Küçük Menderes graben, the NMM Northern Menderes massif and the SMM Southern Menderes massif, modified from Sengör and Bozkurt (2013).

Here is the cross-section, showing normal faults bounding the graben.

(a) A conceptual model of geothermal circulation in the study area, (b) a deep seismic profile with the N–S direction taken from a 30 km west of study area (Nazilli region) (Çifçi et al., 2011). Roman numerals indicate the different sedimentary sequences.

Let’s look at this yet another way. Below is a map and series of cross sections along the Küçük Menderes Graben (KMG). Rojay et al. (2005) take a look at the Plio-Quaternary history of the KMG. The KMG is the rift to the north of the Buyuk Menderes Graben.

Simplified geological map of the KMG showing the positions of geological cross-sections.

Here is a series of cross sections along this basin, locaions are shown on the previous map.

Series of geological cross-sections showing various sectors of the KMG depicting horst and graben structures overprinted onto the huge synclinal structure (see Fig. 3 for positions of geological cross-sections).

Here is their model of how the regional deformation is driven by the metamorphic core complex process.

Schematic tentative cross-sections showing the Miocene to Quaternary evolution of the KMG (modified from Erinç [66]). Note the continuing extension since Miocene.

Regional Cross Sections

The following three figures are from Dilek and Sandvol, 2006. The locations of the cross sections are shown on the map as orange lines. Cross section G-G’ is located in the region of today’s earthquake.
Here is the map (Dilek and Sandvol, 2006). I include the figure caption below in blockquote.

Simplified tectonic map of the Mediterranean region showing the plate boundaries, collisional zones, and directions of extension and tectonic transport. Red lines A through G show the approximate profile lines for the geological traverses depicted in Figure 2. MHSZ—mid-Hungarian shear zone; MP—Moesian platform; RM—Rhodope massif; IAESZ— Izmir-Ankara-Erzincan suture zone; IPS—Intra-Pontide suture zone; ITS—inner Tauride suture zone; NAFZ—north Anatolian fault zone; KB—Kirsehir block; EKP—Erzurum-Kars plateau; TIP—Turkish-Iranian plateau.

Here are cross sections A-D (Dilek and Sandvol, 2006). I include the figure caption below in blockquote.

Simplified tectonic cross-sections across various segments of the broader Alpine orogenic belt.

(A) Eastern Alps. The collision of Adria with Europe produced a bidivergent crustal architecture with both NNW- and SSE-directed nappe structures that involved Tertiary molasse deposits, with deep-seated thrust faults that exhumed lower crustal rocks. The Austro-Alpine units north of the Peri-Adriatic lineament represent the allochthonous outliers of the Adriatic upper crust tectonically resting on the underplating European crust. The Penninic ophiolites mark the remnants of the Mesozoic ocean basin (Meliata). The Oligocene granitoids between the Tauern window and the Peri-Adriatic lineament represent the postcollisional intrusions in the eastern Alps. Modified from Castellarin et al. (2006), with additional data from Coward and Dietrich (1989); Lüschen et al. (2006); Ortner et al. (2006).

(B) Northern Apennines. Following the collision of Adria with the Apenninic platform and Europe in the late Miocene, the westward subduction of the Adriatic lithosphere and the slab roll-back (eastward) produced a broad extensional regime in the west (Apenninic back-arc extension) affecting the Alpine orogenic crust, and also a frontal thrust belt to the east. Lithospheric-scale extension in this broad back-arc environment above the west-dipping Adria lithosphere resulted in the development of a large boudinage structure in the European (Alpine) lithosphere. Modified from Doglioni et al. (1999), with data from Spakman and Wortel (2004); Zeck (1999).

(C) Western Mediterranean–Southern Apennines–Calabria. The westward subduction of the Ionian seafloor as part of Adria since ca. 23 Ma and the associated slab roll-back have induced eastward-progressing extension and lithospheric necking through time, producing a series of basins. Rifting of Sardinia from continental Europe developed the Gulf of Lion passive margin and the Algero-Provencal basin (ca. 15–10 Ma), then the Vavilov and Marsili sub-basins in the broader Tyrrhenian basin to the east (ca. 5 Ma to present). Eastward-migrating lithospheric-scale extension and
necking and asthenospheric upwelling have produced locally well-developed alkaline volcanism (e.g., Sardinia). Slab tear or detachment in the Calabria segment of Adria, as imaged through seismic tomography (Spakman and Wortel, 2004), is probably responsible for asthenospheric upwelling and alkaline volcanism in southern Calabria and eastern Sicily (e.g., Mount Etna). Modified from Séranne (1999), with additional data from Spakman et al. (1993); Doglioni et al. (1999); Spakman and Wortel (2004); Lentini et al. (this volume).

(D) Southern Apennines–Albanides–Hellenides. Note the break where the Adriatic Sea is located between the western and eastern sections along this traverse. The Adria plate and the remnant Ionian oceanic lithosphere underlie the Apenninic-Maghrebian orogenic belt. The Alpine-Tethyan and Apulian platform units are telescoped along ENE-vergent thrust faults. The Tyrrhenian Sea opened up in the latest Miocene as a back-arc basin behind the Apenninic-Maghrebian mountain belt. The Aeolian volcanoes in the Tyrrhenian Sea represent the volcanic arc system in this subduction-collision zone environment. Modified from Lentini et al. (this volume). The eastern section of this traverse across the Albanides-Hellenides in the northern Balkan Peninsula shows a bidivergent crustal architecture, with the Jurassic Tethyan ophiolites (Mirdita ophiolites in Albania and Western Hellenic ophiolites in Greece) forming the highest tectonic nappe, resting on the Cretaceous and younger flysch deposits of the Adria affinity to the west and the Pelagonia affinity to the east. Following the emplacement of the Mirdita- Hellenic ophiolites onto the Pelagonian ribbon continent in the Early Cretaceous, the Adria plate collided with Pelagonia-Europe obliquely starting around ca. 55 Ma. WSW-directed thrusting, developed as a result of this oblique collision, has been migrating westward into the peri-Adriatic depression. Modified from Dilek et al. (2005).

(E) Dinarides–Pannonian basin–Carpathians. The Carpathians developed as a result of the diachronous collision of the Alcapa and Tsia lithospheric blocks, respectively, with the southern edge of the East European platform during the early to middle Miocene (Nemcok et al., 1998; Seghedi et al., 2004). The Pannonian basin evolved as a back-arc basin above the eastward retreating European platform slab (Royden, 1988). Lithospheric-scale necking and boudinage development occurred synchronously with this extension and resulted in the isolation of continental fragments (e.g., the Apuseni mountains) within a broadly extensional Pannonian basin separating the Great Hungarian Plain and the Transylvanian subbasin. Steepening and tearing of the west-dipping slab may have caused asthenospheric flow and upwelling, decompressional melting, and alkaline volcanism (with an ocean island basalt–like mantle source) in the Eastern Carpathians. Modified from Royden (1988), with additional data from Linzer (1996); Nemcok et al. (1998); Doglioni et al. (1999); Seghedi et al. (2004).

(F) Arabia-Eurasia collision zone and the Turkish-Iranian plateau. The collision of Arabia with Eurasia around 13 Ma resulted in (1) development of a thick orogenic crust via intracontinental convergence and shortening and a high plateau and (2) westward escape of a lithospheric block (the Anatolian microplate) away from the collision front. The Arabia plate and the Bitlis-Pütürge ribbon continent were probably amalgamated earlier (ca. the Eocene) via a separate collision event within the Neo-Tethyan realm. BSZ—Bitlis suture zone; EKP—Erzurum-Kars plateau. A slab break-off and the subsequent removal of the lithospheric mantle (lithospheric delamination) beneath the eastern Anatolian accretionary complex caused asthenospheric upwelling and extensive melting, leading to continental volcanism and regional uplift, which has contributed to the high mean elevation of the Turkish-Iranian plateau. The Eastern Turkey Seismic Experiment results have shown that the crustal thickness here is ~ 45–48 km and that the Turkish-Iranian plateau is devoid of mantle lithosphere. The collision-induced convergence has been accommodated by active diffuse north-south shortening and oblique-slip faults dispersing crustal blocks both to the west and the east. The late Miocene through Plio-Quaternary volcanism appears to have become more alkaline toward the south in time. The Pleistocene Karacadag shield volcano in the Arabian foreland represents a local fissure eruption associated with intraplate extension. Data from Pearce et al. (1990); Keskin (2003); Sandvol et al. (2003); S¸engör et al. (2003).

(G) Africa-Eurasia collision zone and the Aegean extensional province. The African lithosphere is subducting beneath Eurasia at the Hellenic trench. The Mediterranean Ridge represents a lithospheric block between the Africa and Eurasian plate (Hsü, 1995). The Aegean extensional province straddles the Anatolide-Tauride and Sakarya continental blocks, which collided in the Eocene. NAF—North Anatolian fault. South-transported Tethyan ophiolite nappes were derived from the suture zone between these two continental blocks. Postcollisional granitic intrusions (Eocone and Oligo-Miocene, shown in red) occur mainly north of the suture zone and at the southern edge of the Sakarya continent. Postcollisional volcanism during the Eocene–Quaternary appears to have migrated southward and to have changed from calc-alkaline to alkaline in composition through time. Lithospheric-scale necking, reminiscent of the Europe-Apennine-Adria collision system, and associated extension are also important processes beneath the Aegean and have resulted in the exhumation of core complexes, widespread upper crustal attenuation, and alkaline and mid-ocean ridge basalt volcanism. Slab steepening and slab roll-back appear to have been at work resulting in subduction zone magmatism along the Hellenic arc.

Here is another cross section that shows the temporal evolution of the tectonics of this region in the area of cross section G-G’ above (Dilek and Sandvol, 2009).

Late Mesozoic–Cenozoic geodynamic evolution of the western Anatolian orogenic belt as a result of collisional
and extensional processes in the upper plate of north-dipping subduction zone(s) within the Tethyan realm. See text
for discussion.

Europe

General Overview

Earthquake Reports

2020.10.30 M 7.0 Turkey
2020.05.02 M 6.6 Crete, Greece
2020.01.24 M 6.7 Turkey
2019.11.26 M 6.4 Albania
2018.10.25 M 6.8 Greece
2017.07.20 M 6.7 Turkey
2017.06.12 M 6.3 Turkey/Greece
2016.10.30 M 6.6 Italy
2016.10.30 M 6.6 Italy Update #1
2016.10.28 M 5.8 Tyrrhenian Sea
2016.10.26 M 6.1 Italy
2016.10.16 M 5.3 Greece/Albania
2016.08.23 M 6.2 Italy
2016.01.24 M 6.1 Mediterranean
2015.11.17 M 6.5 Greece
2015.04.16 M 6.0 Crete

Social Media

#EarthquakeReport for #Earthquake #Deprem and #Tsunami in the eastern #AegeanSea offshore of #Turkey

poster is now updated with aftershocks from @LastQuake
complicated tectonics
also a plot of tide gage data from the region

report here:https://t.co/vNuRdWw0Gs pic.twitter.com/SnYXwg2n3T

— Jason "Jay" R. Patton (@patton_cascadia) October 31, 2020

#EarthquakeReport #TsunamiReport for M7 offshore of #Turkey

small sized tsunami observed across the #AegeanSea https://t.co/i1lZJ0pkb3

analog event in 2017 and more tectonic background herehttps://t.co/jwwXh0SpXl pic.twitter.com/sk1HVbbCKD

— Jason "Jay" R. Patton (@patton_cascadia) October 30, 2020

Unfortunately, with the source so close to the coast, any Tsunami Early Warning System (#TEWS) has little room to warn the population in advanced to save lives. Prepareness/education is then the key ingredient.
Arrival times as prediceted by #Tsunami–#HySEA #IzmirEarthquake pic.twitter.com/8UEwItsLal

— Jorge Macías Sánchez (@JorgeMACSAN) October 30, 2020

İzmir #deprem Alaçatı #tsunami #deliklikoy pic.twitter.com/Fo74diHpBJ

— ulaş tuzak (@ulastuzak) October 30, 2020

M6.9 #earthquake (#deprem) strikes 66 km SW of #İzmir (#Turkey) 21 min ago. Updated map of its effects: pic.twitter.com/Kh3WMz6Hxi

— EMSC (@LastQuake) October 30, 2020

Video forwarded by a friend pic.twitter.com/P5g7H7LInn

— Tiernan Henry (@tiernanhenry) October 30, 2020

I'd be cautious. A similar EQ occurred offshore Bodrum, SW Turkey, in 2017 (Mw 6.6). Many assumed it ruptured the big mapped normal fault, but careful analysis showed it ruptured a smaller conjugate fault that would've been missing from this database. https://t.co/zEKYatNi1O

— Edwin Nissen (@faulty_data) October 30, 2020

#deprem geçmiş olsun İzmir 2020 son hızıyla devam ediyor. pic.twitter.com/cZc3rgWV0e

— jojomiyo (@jojomiyo1) October 30, 2020

Fully automatic processing (beta-version) of the expected permament deformation and #InSAR fringes for the M 7.0 #earthquake in #dodecanese (#Greece), 11:51 (UTC).

Focal mechanism from USGS, both nodal planes used.

With @antandre71 pic.twitter.com/TQiaoDgYf7

— Simone Atzori (@SimoneAtzori73) October 30, 2020

Absolutely terrible scenes coming out of Turkey after the M7.0 earthquake. My thoughts are with all of the people impacted by this event. 💔 https://t.co/gs8Wj8Cj5a

— Dr. Wendy Boo – hon 👻 (@DrWendyRocks) October 30, 2020

The October 30 M7 EQ offshore Samos Island, Greece, occurred as the result of normal faulting at a shallow crustal depth within the Eurasia tectonic plate in the E Aegean Sea. This indicates N-S oriented extension that is common in the Aegean Sea. 🍫 https://t.co/r5i9Ni1S1B pic.twitter.com/C6CyLDOlZ1

— USGS Earthquakes (@USGS_Quakes) October 30, 2020

Η #Σάμος άντεξε στο τρομακτικό μέγεθος των 6,7 ρίχτερ ευτυχώς δεν έχουμε θύματα!! #Σεισμός pic.twitter.com/Sd00bTBOd5

— Θεοδόσης Ζερβουδάκης (@tzervoudakis) October 30, 2020

El terremoto de Turquía de hace un rato llevó a un desastre tremendo. Uno que se da por la falta de preparación ante algo así, sobre todo en la parte ingenieril.

Tendrá magnitud 7, pero a 10 km de profundidad golpea fuerte a las ciudades cercanas, que estaban mal paradas pic.twitter.com/vomUd3Xauu

— Cristian Farías (@cfariasvega) October 30, 2020

30 Ekim 2020 #Seferihisar açıkları (#İzmir)/Sisam (M6.6/6.9) #depremi anaşokundan itibaren 1.0 ile 5.1 arasında değişen toplam 85 deprem oldu. Depremler D-B doğrultulu normal fay boyunca dağılım göstermektedir. pic.twitter.com/65ULhiqEq2

— Dr. Ramazan Demirtaş (@Paleosismolog) October 30, 2020

İzmir'de su seviyesi yükseldi. Tsunami benzeri görüntüler ortaya çıkıyor.#deprem pic.twitter.com/dbxCCgks5C

— Politikaloji🇹🇷 (@politikaloji) October 30, 2020

Location of Samos Mw7 #earthquake on Aegean Sea seismo-tectonic sketch. In yellow, grabens / major extension zones. Today's earthquake happened on a major normal fault bounding one of these grabens. Map from Armijo et al. GJI, 1996 pic.twitter.com/qx46D6peTS

— Robin Lacassin (@RLacassin) October 30, 2020

Seferihisar'da tsunami…
Deniz suyu ilçeyi kapladı…
İzmir Seferihisar'da 6.6 büyüklüğündeki depremin ardından tsunami meydana geldi.#İzmir #deprem #izmirdedeprem #Tsunami pic.twitter.com/kCugei77Zj

— FORUM ATMOSFER (@forumatmosfer) October 30, 2020

Watch the waves from the M7.0 #earthquake near Turkey roll across seismic stations in Europe. https://t.co/SoZMmJHvCU (THREAD) pic.twitter.com/8YKQHaj2yf

— IRIS Earthquake Sci (@IRIS_EPO) October 30, 2020

Map of extension responsible for today's Mw 7.0 earthquake in the Aegean (red star). GPS vectors show motion relative to Anatolia plate. NW Turkey moves N, SW Turkey moves S, so western Turkey stretches N-S. Graph shows how W Turkey opens up like spreading the fingers of a hand pic.twitter.com/JpWklc7YZY

— Edwin Nissen (@faulty_data) October 30, 2020

🌊🇬🇷 Vathí es otra localidad al norte de la isla de #Samos que también registró los efectos del tsunami, inundando las zonas más baja de la ciudad. Se observan estragos menores en el registro.

Vídeo: @atta_fareid pic.twitter.com/CnZzN6c48l

— EarthQuakesTime (@EarthQuakesTime) October 30, 2020

More @NERC_COMET LiCSAR results for yesterday's Aegean earthquake, including filtered/unwrapped interferograms and kmz files for viewing in google earth: https://t.co/TY8ijrUoml
Unwrapped data (below) easier to interpret. Main subsidence (red) is offshore N of Samos. pic.twitter.com/eGGCHL6LXx

— Tim Wright (@timwright_leeds) October 31, 2020

Helpful map showing tectonic setting of today's M7.0 #IzmirEarthquake (yellow dot). The African Plate is subducting under the South Aegean/Anatolian Plate, which is extending as it overrides. The fault that broke today is a "pull-apart" fault (normal fault). #EarthquakeIzmir pic.twitter.com/31Hw4EFWze

— Brian OLSON (@mrbrianolson) October 30, 2020

30 Ekim 2020 / İzmir pic.twitter.com/OYzy0n9hF5

— Son Dakika TV (@sondakikativi) October 30, 2020

GPS velocity & direction of surface monitoring stations in the area of today's M7.0 #IzmirEarthquake showing SSW-directed extension towards the African Plate. The stations near the epicenter are moving ~0.9 – 1.3 inches per year (relative to stable African P.) Data via @UNAVCO pic.twitter.com/iVJJO93Gtl

— Brian OLSON (@mrbrianolson) October 30, 2020

Today's Mw 7.0 #earthquake near the Greek island of Samos ruptured near the Menderes Graben in Western Turkey, a region with a long history of strike-slip and normal faulting. pic.twitter.com/9FiOreCK2R

— Sylvain Barbot (@quakephysics) October 30, 2020

#EarthquakeReport for #Earthquake #Deprem and #Tsunami in the eastern #AegeanSea offshore of #Turkey

poster is now updated with aftershocks from @LastQuake
complicated tectonics
also a plot of tide gage data from the region

report here:https://t.co/vNuRdWw0Gs pic.twitter.com/SnYXwg2n3T

— Jason "Jay" R. Patton (@patton_cascadia) October 31, 2020

AGGIORNAMENTO: Terremoto Mw 7.0 a Nord di Samos (Grecia) del 30 ottobre 2020 https://t.co/pnhYLioLb1

— INGVterremoti (@INGVterremoti) October 30, 2020

It is a bit late in the game, but here is a simulation of yesterdays Turkey/Greece tsunami: pic.twitter.com/HrSnsCl2mA

— Amir Salaree (@amirsalaree) October 31, 2020

My thoughts are with the bereaved, injured and homeless after yesterday's earthquake in Turkey. The size of the quake is shown on these responses from @raspishake seismometers across the globe. The plot is made using @obspy. pic.twitter.com/wrD1Xhfcx8

— Mark Vanstone (@wmvanstone) October 31, 2020

Map of ground displacements calculated from @CopernicusData Sentinel-1 radar (InSAR) by NASA-JPL ARIA project. Western Samos island moved up (blue tones), small area of coast moved down (red) due to M7.0 earthquake yesterday. Other areas affected by atmosphere. pic.twitter.com/26rFem82YN

— Eric Fielding (@EricFielding) October 31, 2020

Jason, also the ~1 days @LastQuake aftershocks distribution seems to be in agreement with the positive stress change related to the @usgs preliminary finite fault model pic.twitter.com/7FNr52Aat2

— Jugurtha Kariche (@JkaricheKariche) October 31, 2020

The red curve below represents the intensity (i.e. shaking and damage level) vs epicentral distance for yesterday M7 #Izmir #Samos #earthquake #deprem. The blue dots are individual felt reports shared by eyewitnesses via LastQuake app.
1/n pic.twitter.com/cBn4sew2xx

— EMSC (@LastQuake) October 31, 2020

Preliminary teleseismic finite fault model of the 30 Oct Mw 7 Greece #earthquake for both planes. Method= Ji et al. (2002). Here rupture started from the KOERI hypocenter (H=10 km). Rupture moved bilaterally; most of the high slip and its peak located up-dip in the shallow depth. pic.twitter.com/7E4jpwJzvu

— Dimas Sianipar (@SianiparDimas) November 1, 2020

Regional tectonics of the area where the M7.0 Samos #earthquake occurred pic.twitter.com/XaWmKMsrlA

— IRIS Earthquake Sci (@IRIS_EPO) November 2, 2020

A bit of lunchtime #dataviz.
Aftershock sequence of the M7.0 Western Turkey as it stands.
Catalogue: @LastQuake pic.twitter.com/5oUa0fyXpL

— Stephen Hicks 🇪🇺 (@seismo_steve) November 2, 2020

Damage Proxy Map from ARIA shows surface changes that may be due to damage measured with radar images. Maps on NASA Disasters Portal: https://t.co/Q4JA2ezU8R
Data also on ARIA-share: https://t.co/CDz2xn2gFo pic.twitter.com/uu5iYqi6WA

— Advanced Rapid Imaging & Analysis (ARIA) (@aria_hazards) November 2, 2020

The "GEER-069: 2020 #Samos Island (Aegean Sea) #earthquake Report" by @HAEE_ETAM, @DepremVakfi, #TDMD, @EERI_tweets and #GEER has been published and is available online (https://t.co/7pb7V8kSMx) ! pic.twitter.com/zpNehqIrTg

— EQUIDAS (@equidas) January 3, 2021

References:

Basic & General References

Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

Specific References

Basili R., G. Valensise, P. Vannoli, P. Burrato, U. Fracassi, S. Mariano, M.M. Tiberti, E. Boschi (2008), The Database of Individual Seismogenic Sources (DISS), version 3: summarizing 20 years of research on Italy’s earthquake geology, Tectonophysics, doi:10.1016/j.tecto.2007.04.014
Brun, J.-P., Sokoutis, D., 2012. 45 m.y. of Aegean crust and mantle flow driven by trench retreat. Geol. Soc. Am., v. 38, p. 815–818.
Caputo, R., Chatzipetros, A., Pavlides, S., and Sboras, S., 2012. The Greek Database of Seismogenic Sources (GreDaSS): state-of-the-art for northern Greece in Annals of Geophysics, v. 55, no. 5, doi: 10.4401/ag-5168
Dilek, Y., 2006. Collision tectonics of the Mediterranean region: Causes and consequences in Dilek, Y., and Pavlides, S., eds., Postcollisional tectonics and magmatism in the Mediterranean region and Asia: Geological Society of America Special Paper 409, p. 1–13
Dilek, Y. and Sandvol, E., 2006. Collision tectonics of the Mediterranean region: Causes and consequences in Dilek, Y., and Pavlides, S., eds., Postcollisional tectonics and magmatism in the Mediterranean region and Asia: Geological Society of America Special Paper 409, p. 1–13
DISS Working Group (2015). Database of Individual Seismogenic Sources (DISS), Version 3.2.0: A compilation of potential sources for earthquakes larger than M 5.5 in Italy and surrounding areas. http://diss.rm.ingv.it/diss/, Istituto Nazionale di Geofisica e Vulcanologia; DOI:10.6092/INGV.IT-DISS3.2.0.
Emre, T. and Sozbilir, H., 2007. Tectonic Evolution of the Kiraz Basin, Küçük Menderes Graben: Evidence for Compression/Uplift-related Basin Formation Overprinted by Extensional Tectonics in West Anatolia in Turkish Journal of Earth Sciences, v. 106, p. 441-470
Ersoy, E.Y., Cemen, I., Helvaci, C., and Billor, Z., 2014. Tectono-stratigraphy of the Neogene basins in Western Turkey: Implications for tectonic evolution of the Aegean Extended Region in Tectonophysics v. 635, p. 33-58.
Jolivet, L., et al., 2013. Aegean tectonics: Strain localisation, slab tearing and trench retreat in Tectonophysics, v. 597-598, p. 1-33
Kaya, A., 2015. The effects of extensional structures on the heat transport mechanism: An example from the Ortakçı geothermal field (Büyük Menderes Graben, SW Turkey) in Journal oF african Easth Sciences, v. 108, p. 74-88, http://dx.doi.org/10.1016/j.jafrearsci.2015.05.002
Kokkalas, S., et al., 2006. Postcollisional contractional and extensional deformation in the Aegean region in GSA Special Papers, v. 409, p. 97-123.
Kurt, H., Demirbag, E., and Kuscu, I., 1999. Investigation of the submarine active tectonism in the Gulf of Gokova, southwest Anatolia–southeast Aegean Sea, by multi-channel seismic reflection data in Tectonophysics, v. 305, p. 477-496
Ocakoglu, N., DEmirbag, E.,. and Kuscu, I., 2005. Neotectonic structures in I˙zmir Gulf and surrounding regions (western Turkey): Evidences of strike-slip faulting with compression in the Aegean extensional regime in Marine Geology, v. 219, p. 155-171, doi:10.1016/j.margeo.2005.06.004
Papazachos, B.C., Papadimitrious, E.E., Kiratzi, A.A., Papazachos, C.B., and Louvari, E.k., 1998. Fault Plane Solutions in the Aegean Sea and the Surrounding Area and their Tectonic Implication, in Bollettino Di Geofisica Terorica Ed Applicata, v. 39, no. 3, p. 199-218.
Pérouse, E., N. Chamot-Rooke, A. Rabaute, P. Briole, F. Jouanne, I. Georgiev, and D. Dimitrov, 2012. Bridging onshore and offshore present-day kinematics of central and eastern Mediterranean: Implications for crustal dynamics and mantle flow, Geochem. Geophys. Geosyst., 13, Q09013, doi:10.1029/2012GC004289.
Rojay, B., Toprak, V., Demirci, C., and Süzen, L., 2005. Plio-Quaternary evolution of the Küçük Menderes Graben Southwestern Anatolia, Turkey in Geodinamica Acta, v. 18, no. 3-4, p. 317-331
Taymaz, T., Yilmaz, Y., and Dilek, Y., 2007. The geodynamics of the Aegean and Anatolia: introduction in Geological Society Special Publications, v. 291, p. 1-16.
Wouldloper, 2009. Tectonic map of southern Europe and the Middle East, showing tectonic structures of the western Alpide mountain belt. Only Alpine (tertiary) structures are shown.

Return to the Earthquake Reports page.

Sorted by Magnitude
Sorted by Year
Sorted by Day of the Year
Sorted By Region

Earthquake Report: Southern California

Posted on September 19, 2020 by Jay Patton

Late last night there was a sequence of earthquakes in southern California. The mainshock is a M 4.5 earthquake.
https://earthquake.usgs.gov/earthquakes/eventpage/ci38695658/executive
This temblor was widely felt across the southland (including by my mom, who was warned by earthquake early warning). This sequence happened in the same area as the 1987 Whittier Narrows Earthquake Sequence (which I felt as a child, growing up in Long Beach, CA).
https://earthquake.usgs.gov/earthquakes/eventpage/ci731691/executive
The tectonics of southern CA are dominated by the San Andreas fault (SAF) system. The SAF system is a right-lateral strike-slip plate boundary fault marking the boundary between the Pacific and North America plates.
Basically, the Pacific plate is moving northwest relative to the North America plate. Both plates are moving northwest relative to an Earth reference frame, but the Pacific plate is moving faster.
The SAF system goes through a bend in southern CA, which causes things to get complicated. There are sibling faults to the SAF, also right-lateral strike-slip (e.g. the San Jacinto and Elsinore faults).
Also, because of the fault geometry, there is considerable north-south compression that forms the mountain ranges to the north of the Los Angeles Basin. Some of the faults formed by this compression are the Sierra Madre, Hollywood, Compton, and Puente Hills faults.
A recent earthquake (2014) happened along one of these thrust fault systems. On 28 March 2014 (one day after the 50th anniversary of the Good Friday Earthquake in Alaska) there was an oblique thrust fault earthquake beneath La Habra, CA. My cousins felt that sequence and I remember them mentioning how their children kept waking up after every aftershock, some epicenters were located within a half km from their house.
https://earthquake.usgs.gov/earthquakes/eventpage/ci15481673/executive
This La Habra sequence appears to be related to the Puente Hills Thrust fault system (same for the Whittier Narrows Earthquake). Last night’s M 4.5 also appears to have slipped along a thrust fault on this system. Based on the depth, it looks like the earthquake slipped along the Lower Elysian Park ramp (see poster).
There were a few aftershocks. However, two of them I would rather interpret them as triggered earthquakes. The M 1.6 and M 1.9 earthquakes have strike-slip earthquake mechanisms (focal mechanisms = orange). ~~These also have shallower [hypocentral] depths.~~ There is mapped the Montebello fault, a right-lateral strike-slip fault, just to the east of the M 4.5 epicenter. The Montebello fault is a strand of the Whittier fault system.
So, while this may be incorrect, my initial interpretation is that these two M1+ events happened on the Montebello fault system and were triggered by the M 4.5 event.
There was also an historic earthquake on the Sierra Madre fault system. On 28 June 1991, there was a M 5.8 earthquake beneath the San Gabriel Mountains to the north of the LA Basin. This was also an oblique thrust earthquake.
https://earthquake.usgs.gov/earthquakes/eventpage/ci2021449/executive
Something that all these earthquakes share is that they occurred on blind thrust faults. Why are they called blind? Because they don’t reach the ground surface, so we cannot see them at the surface (thus, we are blind to them).

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 4.5.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

In the upper left corner I include a map that shows the USGS tectonic faults and the USGS seismicity from the past 3 months. I highlight the North America and Pacific plates and their relative motion along the San Andreas fault system.
In the lower right corner I plot the epicenters related to this sequence. The topographic data here are high resolution LiDAR data from 2016 (publically available).
In the lower center left is a low-angle oblique block diagram from Daout at al. (2016) that shows the geometry of the major faults in this area (along with estimates of the slip rates for these faults).
Between the aftershock map and the oblique block diagram are two panels from Rollins et al. (2018). On the left is a map that shows the major fault systems, some historic earthquake mechanisms, and GPS derived plate motion vectors (the direction of relative motion is the orientation of the arrow and the velocity is the length of the arrow). I placed a blue star in the location of last night’s M 4.5. On the right are some cross sections through the subsurface (the location of these cross sections is shown as a dashed gray line on the map). The M 4.5 hypocentral depth is 16.9 km, which clearly plots on the Lower Elysian Park ramp (part of the Puente Hills fault system). Note how the Whittier fault, a strike-slip fault at the surface, soles into the Peunte Hills thrus.
In the upper right corner is a map where I plot a comparison of the CSIN intensity model results (using the MMI Intensity scale, read more about that here) and the USGS “Did You Feel It?” (dyfi) reports. The intensity map is based on a model of how intensity diminishes with distance from the earthquake. The dyfi results are from real observations from real people. See the plot below the map to check out how these data compare, but in a plot not a map.
To the left of the intensity comparisons is another map from Rollins et al. (2018) that shows how much these thrust fault systems are accumulating energy over time. Basically, the warmer colors (e.g. red) shows an area of the fault that is storing more energy per year relative to part of the fault that have less warm colors (e.g. yellow). The Sierra Madre fault system is storing the most energy, per year, of all thrust faults that Rollins and his colleagues studied.

Here is the map with a month’s seismicity plotted.

Two of the most notable historic earthquakes in southern CA are the 1971 Sylmar and 1994 Northridge earthquakes. Both earthquakes had a significant impact on the growth of knowledge about earthquake hazards in southern CA (and elsewhere), but they also resulted in major changes in how seismic hazards are recognized, codified, and mitigated throughout the state (with impacts nationwide and worldwide). And, both of these earthquakes also happened on blind thrust faults, just like last night’s M 4.5!
The 1906 San Francisco and 1933 Long Beach earthquakes led to major changes in the state too. 1933 Long Beach particularly led to changes in how schools are built and resulted in the strongest building code (relative to earthquakes) in the country as the time. These changes were eventually adopted statewide, nationwide, and globally (via the universal building code). Check out the Field Act to learn more about this.
The 1971 Sylmar Earthquake happened on a previously unrecognized fault (because it is blind) and caused lots of damage and many casualties. Perhaps most notably was the veterans hospital which was built across a fault. This fault slipped during the earthquake (triggered by the mainshock). Because the fault slipped beneath the hospital, the hospital was cut in half.
This was quite educational, to learn that when an earthquake fault slips beneath a building, the building does not (generally) perform well. After this earthquake, state senators Alquist and Priolo wrote and helped to get passed the Alquist-Priolo Act. This act required the state (via the California Geological Survey (CGS), where I work) to identify all active faults in the state. The Board of Mines and Geology (BMG) prepared regulations that help manage development (i.e. construction of buildings) in AP zones. Read more about the AP Act here.
The 1994 Northridge Earthquake, with a similar magnitude as the 1971 Sylmar quake, caused extensive damage throughout the San Fernando Valley (like, totally dude) and beyond. There are famous photos of the damage to bridges of the 5 and 14 interchange (interstate 5 and state route 14). The 1994 Northridge Earthquake led to the development of the Seismic Hazards Mapping Act. The CGS and the BMG both have mandates related to the SHMA (I work as part of the Seismic Hazards Mapping Program at CGS). Read more about the SHMA here.
Read more about the 1971 Sylmar Earthquake here.
Read more about the 1994 Northridge Earthquake here.

Other Report Pages

Some Relevant Discussion and Figures

Here is a great map from Wallace (1990) that shows the major faults associated with the San Andreas fault system.

Generalized topographic map of southern California, showing major faults with Quaternary activity in the San Andreas firnit system. Faults dotted where concealed by water: hachures on contours indicate area of closed low.

This is a more updated map from Tucker and Dolan (2001) prepared for their study of the Sierra Madre fault.

Regional neotectonic map for metropolitan southern California showing major active faults. The Sierra Madre fault is a 75-km-long active reverse fault that extends along the northern edge of the metropolitan region. Fault locations are from Ziony and Jones (1989), Vedder et al. (1986), Dolan and Sieh (1992), Sorlien (1994), and Dolan et al. (1997, 2000b). Closed teeth denote reverse fault surface trace; open teeth on dashed lines show upper edge of blind thrust fault ramps. Strike-slip fault surface traces shown by double arrows. Star denotes location of Oak Hill paleoseismologic trench site of Bonilla (1973). CSI, Clamshell-Sawpit fault; ELATB, East Los Angeles blind thrust system; EPT, Elysian park blind thrust fault; Hol Fl, Hollywood fault; PHT, Puente Hills blind thrust fault; RMF, Red Mountain fault; SCII, Santa Cruz Island fault; SSF, Santa Susana fault; SJcF, San Jacinto fault; SJF, San Jose fault; VF, Verdugo fault; A, Altadena study site of Rubin et al. (1998); LA, Los Angeles; LB, Long Beach; LC, La Crescenta; M, Malibu; NB, Newport Beach; Ox, Oxnard; P, Pasadena; PH, Port Hueneme; S, Horsethief Canyon study site in San Dimas; V, Ventura. Dark shading denotes mountains.

This is a great low angle oblique view of the faults in the southland from Fuis et al. (2001). Note that the SAF geometry creates North-South compression in this area (that causes the thrust faults, some of hem are blind).

Schematic block diagram showing interpreted tectonics in vicinity of LARSE line 1. Active faults are shown in orange, and moderate and large earthquakes are shown with orange stars and attached dates, magnitudes, and names. Gray half-arrows show relative motions on faults. Small white arrows show block motions in vicinities of bright reflective zones A and B (see Fig. 2A). Large white arrows show relative convergence direction of Pacific and North American plates. We interpret a master de´collement ascending from bright reflective zone A at San Andreas fault, above which brittle upper crust is imbricating along thrust and reverse faults and below which lower crust is flowing toward San Andreas fault (brown arrows) and depressing Moho. Fluid injection, indicated by small lenticular blue areas, is envisioned in bright reflective zones A and B.

This is an updated figure from Daout et al. (2016). The slip rates are included for each fault.

Three-dimensional schematic block model across the SGM [after Fuis et al., 2001b] superimposed to the digital elevation model, the seismicity (yellow dots), the Moho model (red line), and interpreted active faults summarizing the average interseismic strike-slip (back arrows) and dip-slip (red arrows) rates extracted from the Bayesian exploration. Shallow faults (dashed lines) that formed a complex three-dimensional system at the surface [Plesch et al., 2007] are locked during the interseismic period, while the ramp-décollement system (solid lines) decouples the upper crust from the lower crust and partitioned the observed uniform velocity field (blue vector) at the downdip end of the structures.

Here is a summary of the historic earthquakes in southern CA from Hauksson et al. (1995). They include earthquake mechanisms (B) and the regions impacted (A).

(a) Significant earthquakes of M > 4.8 that have occurred in the greater Los Angeles basin area since 1920. Aftershock zones are shaded with cross hatching, including the 1994 Northridge earthquake. Dotted areas indicate surface rupture, including the rupture of the 1857 earthquake along the San Andreas fault. (b) Lower hemisphere focal mechanisms (shaded quadrants are compressional) for significant earthquakes that have occurred since 1933 in the greater Los Angeles area.

This is an important figure from Leon et al. (2007) that shows their interpretation of the different faults in the Puente Hills fault system. They highlight the location of the 1987 Whittier Narrows Earthquake, which was to the north of last nights M 4.5.

This is also an important figure as it shows some additional faults (Shaw et al., 2002). The M 4.5 most likely occurred on the Lower Elysian Park fault.

Structure contour map of the PHT in relation to other major thrust and strike-slip systems in the northern LA basin. Contour interval is 1 km; depths are subsea. Map coordinates are UTM Zone 11, NAD27 datum.

What follows are a series of figures from Rollins et al. (2018). They studied the strain accumulation (the accumulation of energy in a fault system over time) for the three main thrust fault systems in the LA Basin.
Here is their first figure that shows the relative plate motions as observed using GPS sites.

(a) Tectonics and shortening in the Los Angeles region. Dark blue arrows are shortening-related GPS velocities relative to the San Gabriel Mountains (Argus et al., 2005). Contours are uniaxial strain rate (rate of change of εxx) in the N ~5° E direction estimated from the GPS using the method of Tape et al. (2009). Background shading is the shear modulus at 100-m depth in the CVM*, a heterogeneous elastic model based on the Community Velocity Model (Süss & Shaw, 2003; Shaw et al., 2015) that we create and use in this study (section 4). Black lines are upper edges of faults, dashed for blind faults. Epicenters of the 1971, 1987, and 1994 earthquakes are from Southern California Earthquake Data Center; focal mechanisms are from Heaton (1982) for 1971 and Global CMT Catalog for 1987 and 1994. Profile A-A0 follows LARSE line 1 (Fuis et al., 2001) onshore and line M-M0 of Sorlien et al. (2013) offshore. SGF = San Gabriel Fault; SSF = Santa Susana Fault. VF = Verdugo Fault. SAF = San Andreas Fault. CuF = Cucamonga Fault. A-DF = Anacapa-Dume Fault. SMoF = Santa Monica Fault. HF = Hollywood Fault. RF = Raymond Fault. UEPF = Upper Elysian Park Fault. ChF = Chino Fault. WF = Whittier Fault. N-IF = Newport-Inglewood Fault. PVF = Palos Verdes Fault. (b) GPS velocities on islands. (c) Tectonic setting. Black lines and pairs of half-arrows, respectively, are major faults and their slip senses. Black arrow is Pacific Plate velocity relative to North American plate from Kreemer et al. (2014). GF = Garlock Fault. SJF = San Jacinto Fault. EF = Elsinore Fault. SB = Santa Barbara. LA = Los Angeles. SD = San Diego.

Here is their cross section through this part of the LA Basin. The location of this cross section is marked on the above map as a gray dashed line.

(a) Cross sections of faults, structure, north-south contraction, and seismicity along profile A-A0 . Red lines are fault surfaces as meshed here (Figure 3), dashed where uncertain (Shaw & Suppe, 1996; Shaw & Shearer, 1999; Fuis et al., 2012). Geometries of basin, basement, and mantle are from Shaw et al. (2015); geometry of base of Fernando Formation (boundary between beige and tan units of the basin) is interpolated from Sorlien et al. (2013; offshore), Wright (1991; coastline to Whittier Fault), and Yeats (2004; Whittier Fault to Sierra Madre Fault); topography is from Fuis et al. (2012). (b) Projections of Argus et al. (2005) GPS velocities (relative to San Gabriel Mountains) onto the direction N 5° E and 1σ uncertainties. Note that stations on Palos Verdes are plotted left of the coastline as the offshore section of profile A-A0 passes alongside Palos Verdes (Figure 1a). (c) Seismotectonic features. Distribution of shear modulus is from the CVM*, the heterogeneous elastic model used in this study (section 4). Translucent white circles are relocated 1981–2016 M ≥ 2 earthquakes whose epicenters lie within the mesh area of the three thrust faults and decollement (Hauksson et al., 2012 and updated). PVF = Palos Verdes Fault; N-IF = Newport-Inglewood Fault; WF = Whittier Fault.

I love this map because it shows how these thrust faults dip into the Earth to the north.

geometries of the three main thrust faults beneath the Los Angeles basin (section 4), colored by depth, and 1981–2016 M ≥ 2.5 earthquakes within the mesh area from Hauksson et al. (2012 and updated), scaled by magnitude (white-filled circles). Gray-filled circles are 1981–2016 M ≥ 4.5 earthquakes outside the mesh area. Inferred paleoearthquakes are from Rubin et al. (1998) and Leon et al. (2007, 2009). SAF = San Andreas Fault.

Finally, we see how they model the amount of plate tectonic motion is accumulated as tectonic strain on these faults. Chris is one of the smartest plate tectonicists I know, so read his paper (several times).

Estimates of moment deficit accumulation rate from combining the four interseismic strain accumulation models. (a) Spatial distribution of moment deficit accumulation rate per area. (Values are on the order of ~108 N m ^-1 yr ^-1 as the moment deficit accumulation rate per patch is on the order of 1015 N m ^-1 yr ^-1 [Figure S11] and the patches are a few kilometers (a few thousand meters) on a side.) (b) Unified PDF of moment deficit accumulation rate (dark blue object) formed by combining the PDFs from the four strain accumulation models. The PDF would follow the red curve if strain accumulation updip of the tips of the Puente Hills and Compton faults (PHF and CF) were counted.

San Andreas plate boundary Earthquake Reports

General Overview

1906.04.18 M 7.9 San Francisco

Earthquake Reports

Social Media

#EarthquakeReport for M 4.5 #Earthquake near El Monte #California

appears related to Puente Hills thrust system, possibly on Lower Elysian Park fault

triggered Montebello fault strike-slip earthquakes (?)https://t.co/3imJv5OUZ7

report here https://t.co/TnsLc2U1AF pic.twitter.com/LCsrcN4mAY

— Jason "Jay" R. Patton (@patton_cascadia) September 19, 2020

Just before midnight a 4.5 magnitude earthquake occurred in the Los Angeles Area. @MyShakeApp sent an alert to 20,000 phones! Be prepared with the #MyShake mobile app! Designed to send users alerts ASAP so they can drop, cover and hold on. download here: https://t.co/9zF3qAPeTh pic.twitter.com/bmfl8YBQXP

— Cal OES (@Cal_OES) September 19, 2020

M4.5 #earthquake S California: Mystery, late surface-wave arrival on seismogram at USC. Particle motion suggests Love wave arriving from northeast or southwest.https://t.co/CAzDqXbva7 pic.twitter.com/PuSS7Jm8oj

— Anthony Lomax 😷🇪🇺🌍 (@ALomaxNet) September 19, 2020

Tonight's M4.6 quake was VERY close to the epicenter of the 1987 Whittier earthquake (M5.9), but almost twice as deep as '87. Focal mechanism shows it was on a reverse fault about 11 miles deep. pic.twitter.com/gO8mE713jq

— Brian OLSON (@mrbrianolson) September 19, 2020

Over 26,000 Did You Feel It? responses already! Stronger shaking than average (orange curve) for a California M4.5, more like an east coast 4.5 (green curve)https://t.co/f1wXvxZExQ pic.twitter.com/AO0ePXkvvC

— Susan Hough 🦖 (@SeismoSue) September 19, 2020

M4.5 #earthquake S California: Upwards bump in felt reports around 100km shows shaking amplification due to waves bouncing off the base of the crust (Moho). Also note usual, mistaken ("internet effect"?) reports at large distance.https://t.co/sc07C0qCXw https://t.co/1ffVEDCxLo pic.twitter.com/PQiiTK9xcl

— Anthony Lomax 😷🇪🇺🌍 (@ALomaxNet) September 19, 2020

The 1987 M5.9 Whittier earthquake is the reason my house, which was not my house at the time, has a modular chimney rather than its original 1920s brick chimney. https://t.co/qHlhNBymfB

— Susan Hough 🦖 (@SeismoSue) September 19, 2020

References:

Basic & General References

Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

Specific References

Daout, S., S. Barbot, G. Peltzer, M.-P. Doin, Z. Liu, and R. Jolivet, 2016. Constraining the kinematics of metropolitan Los Angeles faults with a slip-partitioning model, Geophys. Res. Lett., 43, p. 11,192–11,201 doi:10.1002/2016GL071061.
Fuis, G.S., Ryberg, T., Godfrey, N.J., Okaya, D.A., and Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1, p. 15-18
Hauksson, E., Jones, L.M., and Huttn,K., 1995. The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects in JGR, v. 100, no. B7, p. 12,235-12,255
Leon, L. A., S. A. Christofferson, J. F. Dolan, J. H. Shaw, and T. L. Pratt, 2007. Earthquake-by-earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles, California: Implications for fold kinematics and seismic hazard, J. Geophys. Res., 112, B03S03, doi:10.1029/2006JB004461.
Rollins, C., Avouac, J.-P., Landry, W., Argus, D. F., & Barbot, S., 2018. Interseismic strain accumulation on faults beneath Los Angeles, California. Journal of Geophysical Research: Solid Earth, 123. https://doi.org/10.1029/2017JB015387
Shaw, J.H., Plesch, A., Dolan,. J.F., Pratt, T.L., and Fiore, P/, 2002. Puente Hills Blind-Thrust System, Los Angeles, California in BSSA v. 92, no. 8,pp. 2946-2960
Tucker, A.Z. and Dolan, J.F., 2001. Paleoseismologic Evidence for a 8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault, Northern Los Angeles Metropolitan Region, California in BSSA v. 91, no. 2, p. 232-249
Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

Return to the Earthquake Reports page.

Sorted by Magnitude
Sorted by Year
Sorted by Day of the Year
Sorted By Region

Earthquake Report: Halmahera, Indonesia

Posted on July 14, 2019 by Jay Patton

Well, yesterday I was preparing some updates to the Ridgecrest Earthquake following my field work with my colleagues at the California Geological Survey (where I work) and the U.S. Geological Survey. We spent the week documenting surface ruptures associated with the M 6.4 and M 7.1 Ridgecrest Earthquake Sequence. (it is currently named the Searles Valley Earthquake Sequence, but I am calling it the Ridgecrest Earthquake)
I was just about done with these new maps and getting ready to start writing them up in an updated earthquake report when I noticed that there was an interesting earthquake, with few historic analogues, along the Western Australia Shear Zone offshore of northwestern Australia. I probably won’t get to that earthquake, but I started downloading some material and reviewing my literature for the region. I considered doing both of these tasks on Sunday (today). That was not to be as I awakened to an email about this magnitude M 7.3 earthquake in Halmahera, Indonesia. I have several earthquake reports for the Molucca Strait, west of Halmahera. So, I have some background literature and knowledge about this region already.
There was an earthquake along Molucca Strait that I could not work on due to my field work. So I will briefly mention that quake here. There was also a recent earthquake to the south, in the Banda Sea (here is my earthquake report for that event). The June earthquake had the same magnitude as today’s shaker, M = 7.3. However, the earlier quake was too deep to cause a tsunami (unlike today’s temblor). Earthquakes along the Molucca Strait have generated tsunami with wave heights of over 9 meters (30 feet) according toe Harris and Major, 2016.
https://earthquake.usgs.gov/earthquakes/eventpage/us70004jyv/executive
The Molucca Strait is a north-south oriented seaway formed by opposing subduction zone / thrust faults (convergent plate boundaries). See the “Geologic Fundamentals” section below for an explanation of different fault types. On the west of the Molucca Strait is a thrust fault that dips downwards to the west. On the east, there is a thrust fault that dips down to the east (beneath the island of Halmahera).
There is a major east-west trending (striking) strike-slip fault that comes into the region from the east, called the Sorong fault. There are multiple strands of this system. A splay of this Sorong fault splays northwards through the island of Halmahera. There may be additional details about how this splay relates to the Sorong fault, but I was unable to locate any references (or read the details) today. According to BMKG, the fault that is associated with this earthquake is the Sorong-Bacan fault.
Today’s M 7.3 Halmahera earthquake is a strike-slip earthquake (the plates move side-by-side, like the San Andreas or North Anatolia faults). Often people don’t think of tsunami when a strike-slip earthquake happens because there is often little vertical ground motion. Many people are otherwise familiar with thrust or subduction zone earthquakes, which can produce significant uplift and subsidence (vertical land motion), that can lead to significant tsunami.
However, there is abundant evidence that strike-slip earthquakes do cause tsunami, though often of much smaller size than their thrust/subduction siblings. The main difference is that these strike-slip generated tsunami are much smaller in size.
For example, the 1999 Izmit and 2012 Wharton Basin earthquakes provided empirical evidence of strike-slip earthquake triggered tsunami. More recently, the 28 September 2018 magnitude M 7.5 Dongalla-Palu earthquake caused a tsunami in Palu Bay, Sulawesi, Indonesia that exceeded 10 meters (33 feet) in wave height (wave run up elevation)!!! I just got an email from Dr. Lori Dengler who is an a conference where people claim that the earthquake is possibly singlehandedly responsible for this large wave. Previously people thought that there may have been submarine landslides that contributed to the size.
Here is the tide gage record from a gage near today’s M 7.3 earthquake. The earthquake epicenter appears to be on land, so the tsunami is possibly smaller because of this. Indonesia operates a network of tide gages throughout the region here. The gage data below are from the island of Gebe, about 50 miles to the east of the M 7.3 epicenter.

Here is a quote from the Meteorology, Climatology and Geophysics Agency (BMKG) website:

Impact of Earthquake
Based on community reports, it was shown that shocks were felt in Bitung and Manado with the intensity of IV-V MMI (felt by almost all residents, many people built), and in Ternate III-IV MMI (felt by many people in the house). Until now there have been no reports of damage due to a strong earthquake shock in northern Maluku last night. The impact of the North Maluku earthquake only caused a tremendous panic among the people. In the city of Manado, some of the houses of the walls had cracks in the building walls of the building with very light categories.

Now I can get back to working on a Ridgecrest update… stay tuned. (the maps are already made)

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend).
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange). Due to the high rate of seismicity in this region, I do not have an historic seismicity poster for this event.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours transparently on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab 2.0 contours plotted transparently (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

Magnetic Anomalies

In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

In the upper left corner is a plate tectonic map showing major fault lines for the Molucca Strait and Halmahera region (Waltham et al., 2008). I place a blue star in the general location of today’s M 7.3 earthquake.
In the lower left corner is a low angle oblique view of the tectonic plates in this region (Hall, 2011). The view is from the southeast looking into the Earth towards the northwest.
In the lower right corner are the tide gage data from the tide gage at Pulau Gebe. These data were provided by the Indonesian Government here. These appear to be tsunami waves, they lasted over 5 hours and had a small wave height of 12 centimeters..
In the upper right corner is a part of the Global Earthquake Model (GEM) seismic hazard map that uses cool colors to represent a lower level of shaking intensity than warm colors (Silva et al., 2018). The units are in g (gravitational acceleration). 1 g = Earth’s gravity, so hypothetically, “rocks can get thrown in the air at 1g.” This map is prepared based on the chance an area will have earthquakes of a given size based on a combination of many different seismic hazard models. The region where today’s earthquake happened is colored yellow and has a 10% chance of shaking that 0.2g to 0.35 g (or stronger) over the next 50 years.
Below the hazard map is the GEM seismic risk map presents the geographic distribution of average annual loss (USD) due to ground shaking in the residential, commercial and industrial building stock, considering contents, structural and non-structural components. Warmer colors represent larger loss over time. Risk is the overlap of hazard and population. If there are no people, but there is seismic hazard, there is no seismic risk.
To the left of the GEM maps is a map of Halmahera and some surrounding islands. The color shows the level of seismic hazard for these islands (Zulkifli et al.,l 2017). The color shows the estimated Peak level of ground shaking for a period of 500 years (i.e. 10% probability of exceedance in 50 years). The units are the same (g). The M 7.3 earthquake generated up to ~.25 g, which is higher than the model would suggest (between 0.03 and 0.06 g).

Here is the map with a month’s seismicity plotted.

Here is the map with a century’s seismicity plotted. In the future I hope to get around to plotting earthquake mechanisms on this map. Yellow fault lines are from the Coordinating Committee Geoscience East-Southeast Asia consortium (CCOF). Red fault lines are from the Global Earthquake Model (GEM) Foundation.

Other Report Pages

Shaking Intensity and Potential for Ground Failure

Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.

There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

FOS = Resisting Force / Driving Force

When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.

Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.

Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.

Here is a map with landslide probability on it (Jessee et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a low probability for landslides. However, we have already seen photographic evidence for landslides and the lower limit for earthquake triggered landslides is magnitude M 5.5 (from Keefer 1984)

Nowicki Jessee and others (2018) is the preferred model for earthquake-triggered landslide hazard. Our primary landslide model is the empirical model of Nowicki Jessee and others (2018). The model was developed by relating 23 inventories of landslides triggered by past earthquakes with different combinations of predictor variables using logistic regression. The output resolution is ~250 m. The model inputs are described below. More details about the model can be found in the original publication. We modify the published model by excluding areas with slopes <5° and changing the coefficient for the lithology layer "unconsolidated sediments" from -3.22 to -1.36, the coefficient for "mixed sedimentary rocks" to better reflect that this unit is expected to be weak (more negative coefficient indicates stronger rock).To exclude areas of insignificantly small probabilities in the computation of aggregate statistics for this model, we use a probability threshold of 0.002.

Here is an excellent educational video from IRIS and a variety of organizations. The video helps us learn about how earthquake intensity gets smaller with distance from an earthquake. The concept of liquefaction is reviewed and we learn how different types of bedrock and underlying earth materials can affect the severity of ground shaking in a given location. The intensity map above is based on a model that relates intensity with distance to the earthquake, but does not incorporate changes in material properties as the video below mentions is an important factor that can increase intensity in places.

Here is a map showing liquefaction susceptibility (Zhu et al., 2017).

Zhu and others (2017) is the preferred model for liquefaction hazard. The model was developed by relating 27 inventories of liquefaction triggered by past earthquakes to globally-available geospatial proxies (summarized below) using logistic regression. We have implemented the global version of the model and have added additional modifications proposed by Baise and Rashidian (2017), including a peak ground acceleration (PGA) threshold of 0.1 g and linear interpolation of the input layers. We also exclude areas with slopes >5°. We linearly interpolate the original input layers of ~1 km resolution to 500 m resolution. The model inputs are described below. More details about the model can be found in the original publication.

Here is a map that shows a comparison of modeled shaking intensity for both the M 6.9 Molucca Strait (the left panel) and M 7.3 Halmahera (the right panel) earthquakes. The legend shows the MMI scale, which I discuss above.

Seismic Hazard and Seismic Risk

These are the two maps shown in the map above, the GEM Seismic Hazard and the GEM Seismic Risk maps from Pagani et al. (2018) and Silva et al. (2018).

The GEM Seismic Hazard Map:

The Global Earthquake Model (GEM) Global Seismic Hazard Map (version 2018.1) depicts the geographic distribution of the Peak Ground Acceleration (PGA) with a 10% probability of being exceeded in 50 years, computed for reference rock conditions (shear wave velocity, VS30, of 760-800 m/s). The map was created by collating maps computed using national and regional probabilistic seismic hazard models developed by various institutions and projects, and by GEM Foundation scientists. The OpenQuake engine, an open-source seismic hazard and risk calculation software developed principally by the GEM Foundation, was used to calculate the hazard values. A smoothing methodology was applied to homogenise hazard values along the model borders. The map is based on a database of hazard models described using the OpenQuake engine data format (NRML); those models originally implemented in other software formats were converted into NRML. While translating these models, various checks were performed to test the compatibility between the original results and the new results computed using the OpenQuake engine. Overall the differences between the original and translated model results are small, notwithstanding some diversity in modelling methodologies implemented in different hazard modelling software. The hashed areas in the map (e.g. Greenland) are currently not covered by a hazard model. The map and the underlying database of models are a dynamic framework, capable to incorporate newly released open models. Due to possible model limitations, regions portrayed with low hazard may still experience potentially damaging earthquakes.

The GEM Seismic Risk Map:

The Global Seismic Risk Map (v2018.1) presents the geographic distribution of average annual loss (USD) normalised by the average construction costs of the respective country (USD/m2) due to ground shaking in the residential, commercial and industrial building stock, considering contents, structural and non-structural components. The normalised metric allows a direct comparison of the risk between countries with widely different construction costs. It does not consider the effects of tsunamis, liquefaction, landslides, and fires following earthquakes. The loss estimates are from direct physical damage to buildings due to shaking, and thus damage to infrastructure or indirect losses due to business interruption are not included. The average annual losses are presented on a hexagonal grid, with a spacing of 0.30 x 0.34 decimal degrees (approximately 1,000 km2 at the equator). The average annual losses were computed using the event-based calculator of the OpenQuake engine, an open-source software for seismic hazard and risk analysis developed by the GEM Foundation. The seismic hazard, exposure and vulnerability models employed in these calculations were provided by national institutions, or developed within the scope of regional programs or bilateral collaborations. This global map and the underlying databases are based on best available and publicly accessible datasets and models. Due to possible model limitations, regions portrayed with low risk may still experience potentially damaging earthquakes.

Tsunami Hazard

Here are two maps that show the results of probabilistic tsunami modeling for the nation of Indonesia (Horspool et al., 2014). These results are similar to results from seismic hazards analysis and maps. The color represents the chance that a given area will experience a certain size tsunami (or larger).
The first map shows the annual chance of a tsunami with a height of at least 0.5 m (1.5 feet). The second map shows the chance that there will be a tsunami at least 3 meters (10 feet) high at the coast.

Annual probability of experiencing a tsunami with a height at the coast of (a) 0.5m (a tsunami warning) and (b) 3m (a major tsunami warning).

Some Relevant Discussion and Figures

Here is a tectonic map for this part of the world from Zahirovic et al., 2014. They show a fracture zone where the M 7.3 earthquake happened. I left out all the acronym definitions (you’re welcome), but they are listed in the paper.

Regional tectonic setting with plate boundaries (MORs/transforms = black, subduction zones = teethed red) from Bird (2003) and ophiolite belts representing sutures modified from Hutchison (1975) and Baldwin et al. (2012). West Sulawesi basalts are from Polvé et al. (1997), fracture zones are from Matthews et al. (2011) and basin outlines are from Hearn et al. (2003).

Here are maps showing the regional tectonics (Smoczyk et al., 2013).

Along its western margin, the Philippine Sea plate is associated with a zone of oblique convergence with the Sunda plate. This highly active convergent plate boundary extends along both sides the Philippine Islands, from Luzon in the north to Sulawesi in the south. The tectonic setting of the Philippines is unusual in several respects: it is characterized by opposite-facing subduction systems on its east and west sides; the archipelago is cut by a major transform fault, the Philippine Fault; and the arc complex itself is marked by volcanism, faulting, and high seismic activity. Subduction of the Philippine Sea plate occurs at the eastern margin of the archipelago along the Philippine Trench and its northern extension, the East Luzon Trough. The East Luzon Trough is thought to be an unusual example of a subduction zone in the process of formation, as the Philippine Trench system gradually extends northward (Hamburger and others, 1983).

This shows Global Positioning System (GPS) velocities at various locations. These plate motions are represented as vectors in mm/yr. (see legend) Here note how the vector labeled phil/eura (for the motion of the PSP relative to the Eurasia plate) is oblique to the plate margin along the Philippine trench (i.e. the PSP is not subducting perpendicular to the megathrust fault). The oblique relative motion seems to lead to strain partitioning, leading to a forearc sliver fault (the Philippine fault, shown in maps above). Below I include the text from the original figure caption in blockquote.

Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, ew Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals

This is one of my favorite figures of all time (Hall, 2011). Read below for more details.

3D cartoon of plate boundaries in the Molucca Sea region modified from Hall et al. (1995). Although seismicity identifies a number of plates there are no continuous boundaries, and the Cotobato, North Sulawesi and Philippine Trenches are all intraplate features. The apparent distinction between different crust types, such as Australian continental crust and oceanic crust of the Philippine and Molucca Sea, is partly a boundary inactive since the Early Miocene (east Sulawesi) and partly a younger but now probably inactive boundary of the Sorong Fault. The upper crust of this entire region is deforming in a much more continuous way than suggested by this cartoon.

Here is a map and cross section presented by Waltham et al. (2008). They use a variety of data sources as a basis for their interpretations (seismic reflection data, gravity data). Note how the Molucca Sea plate subducts both to the west and to the east. Below I include the text from the original figure caption in blockquote.

(A) Location and major tectonic features of the Molucca Sea region. Small, black-fi lled triangles are modern volcanoes. Bathymetric contours are at 200, 2000, 4000, and 6000 m. Large barbed lines are subduction zones, and small barbed lines are thrusts. (B) Cross section across the Halmahera and Sangihe Arcs on section line B. Thrusts on each side of the Molucca Sea are directed outward toward the adjacent arcs, although the subducting Molucca Sea plate dips east beneath Halmahera and west below the Sangihe Arc. (C) Inset is the restored cross section of the Miocene–Pliocene Weda Bay Basin of SW Halmahera on section line C, fl attened to the Pliocene unconformity, showing estimated thickness of the section

Early work done in the region was presented by McCaffrey et al. (1980). Here is a map showing seismic refraction lines that they used to constrain the structures in this region. Below I include the text from the original figure caption in blockquote.

Map of the Molucca Sea, eastern Indonesia, showing I~tions of seismic refraction lines (solid straight lines) and gravity traverses (duhed-dotted lines). Thrust faults are shown with teeth on hanging wall. Triangles represent active volcanoes defining the Sangihe and Halmahera magmatic arcs. Isobath interval is 1 km from Mammericks et al. [1976].

Here is a cross section that shows the gravity model they used to interpret this region.

Gravity model for the central Molucca Sea. (II) Crustal model with layers designated by their density contrasts and refraction control points by open circles and vertical bars. (b) Mantle structure used in modeling the gravity profiles in the central Molucca Sea. Figure 124 fits into the small box at the apex of the inverted-V-ehaped lithosphere. Slab dimensions are controlled by earthquake foci (dots) from Hlltherton 11M Dickinaon [1969J, and mantle densities are taken from Grow 11M Rowin [1975J. The column at the left shows assumed densities for the range of depths between the tick marks. The small v pattern represents oceanic crust, and island arc crust is designated by a short parallel line pattern. East is to the right of the figure.

Here is another tectonic map showing the Sorong fault and some splay faults (dashed lines running along Halmahera), one of which may be involved in today’s earthquake.

Location map and active faults of the Molucca Sea region. Fault colours: blue, convergence; red, transvergence; yellow, divergence; grey, uncertain motion. Fault abbreviations: CF, Catabato Fault; GF, Gorontalo Fault; NST, North Sulawesi Trench; PKF, Palu-Koro Fault; SF, Sorong Fault.

This is a geologic map for the islands in the region (Hall et al., 1988).

Sketch geological map of Halmahera based on Apandi & Sudana (1980), Silitonga et al. (1981), Supriatna (1980) & Yasin (1980) and modified after our own observations. Note in particular the absence of thrusting in the NE arm and the major NE-SW fault (the Subaim Fault) running parallel to the south side of Kau Bay.

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

Here is another way to look at these beach balls.

The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj

— AK Earthquake Center (@AKearthquake) August 15, 2018

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy

— Jackie Caplan-Auerbach (@geophysichick) February 23, 2019

Philippines | Western Pacific

Earthquake Reports

2019.07.14 M 7.3 Halmahera
2018.12.29 M 7.0 Philippines
2017.12.08 M 6.5 Caroline Ridge
2017.12.09 M 6.5 Caroline Ridge Update #1
2017.08.11 M 6.2 Philippines
2017.04.28 M 6.9 Philippines
2017.04.08 M 5.9 Philippines
2017.01.10 M 7.3 Celebes Sea
2016.07.29 M 7.7 Mariana
2015.03.17 M 6.2 Molucca Sea
2014.11.26 M 6.8 Molucca Sea
2014.11.21 M 6.5 Molucca Sea
2014.11.15 M 7.1 Molucca Sea

Social Media

Updated #aftershocks of the yesterday Mw 7.3 Halmahera #earthquake, seems the EQ has low aftershocks productivity. The Mc is quite large (~3.5) in this region due to the lack of local (distance <100 km) seismic station, but the regional coverage is good. #seismology #gempa pic.twitter.com/wEzwn9fJvR

— Dimas Sianipar (@SianiparDimas) July 15, 2019

Mw=7.4, HALMAHERA, INDONESIA (Depth: 17 km), 2019/07/14 09:10:50 UTC – Full details here: https://t.co/dNG7xZttM4 pic.twitter.com/BrD8FJ8ofn

— Earthquakes (@geoscope_ipgp) July 14, 2019

Explore the complex region where the M7.3 #Indonesia #earthquake occurred using the IRIS Interactive Earthquake Browser. The size of the dot indicates the magnitude & the color indicates the depth. You can even look at eq hypocenter locations in 3D! https://t.co/Gs3ykBEp0y pic.twitter.com/wEK4g6srok

— IRIS Earthquake Sci (@IRIS_EPO) July 14, 2019

potential #tsunami after #Halmahera #earthquake. Tsunami impact highly depends on a still very uncertain rupture mechanism. Recorded wave amplitude of 10 cm near #Gebe.
Run-up of ~1 m possible around epicenter @ShakingEarth pic.twitter.com/uUHBuf3QkY

— CATnews | Andreas M. Schäfer (@CATnewsDE) July 14, 2019

potentially expected #aftershocks for today's #Halmahera #earthquake, #Indonesia @ShakingEarth pic.twitter.com/R9RUghePs2

— CATnews | Andreas M. Schäfer (@CATnewsDE) July 14, 2019

BMKG: Halmahera Selatan Masuk Wilayah Seismik Aktif dan Kompleks https://t.co/8XcNfJMQEy

— Indonesiainside.id (@indoinsidenews) July 15, 2019

References:

Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
Hall., R., Audley-Charles, M.G., Banner, F.T., Hidayat, S., Tobing, S.L., 1988. Basement rocks of the Halmahera region, eastern Indonesia: a Late Cretaceous-early Tertiary arc and fore-arc in Journal of the Geological Society, v. 145, p. 65-84
Harris, R. and Major, J., 2016. Waves of destruction in the East Indies: the Wichmann catalogue of earthquakes and tsunami in the Indonesian region from 1538 to 1877 in Cummins, P. R. & Meilano, I. (eds) Geohazards in Indonesia: Earth Science for Disaster Risk Reduction. Geological Society, London, Special Publications, 441, http://doi.org/10.1144/SP441.2
Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
Highland, L.M., and Bobrowsky, P., 2008. The landslide handbook—A guide to understanding landslides, Reston, Virginia, U.S. Geological Survey Circular 1325, 129 p.
Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
Horspool, N., Pranantyo, I., Griffin, J., Latief, H., Natawidjaja, D. H., Kongko, W., Cipta, A., Bustaman, B., Anugrah, S. D., and Thio, H. K., 2014. A probabilistic tsunami hazard assessment for Indonesia, Nat. Hazards Earth Syst. Sci., 14, 3105-3122, https://doi.org/10.5194/nhess-14-3105-2014, 2014.
Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
Keefer, D.K., 1984. Landslides Caused by Earthquakes in GSA Bulletin, v. 95, p. 406-421
Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
McCaffrey, R., Silver, E.A., and Raitt, R.W., 1980. Crustal Structure of the Molucca Sea Collision Zone, Indonesia in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands-Geophysical Monograph 23, p. 161-177.
Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013. Seismicity of the Earth 1900–2012 Philippine Sea plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, 1 sheet, scale 1:10,000,000.
Waltham et al., 2008. Basin formation by volcanic arc loading in GSA Special Papers 2008, v. 436, p. 11-26.
Zahirovic et al., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014.
Zulkifli, M., Rudyanto, A., and Sakti, A.P., 2016. The View of Seismic Hazard in The Halmahera Region in proceedings from International Symposium on Earth Hazard and Disaster Mitigation (ISEDM) 2016 AIP Conf. Proc. 1857, 050004-1–050004-7; doi:10.1063/1.4987082

Return to the Earthquake Reports page.

Sorted by Magnitude
Sorted by Year
Sorted By Region

Earthquake Report: Papua New Guinea

Posted on May 6, 2019 by Jay Patton

Earlier today, there was an intermediate depth beneath eastern Papua New Guinea (PNG). With a magnitude M = 7.2, this is one of the largest earthquake so far in 2019. Here is the USGS website for this earthquake.
Today’s earthquake was quite deep, about 130 km. There are several ways that people have interpreted the tectonics here (which is more common than not).
PNG and New Britain are a region of convergence, where the Australia plate to the south is moving northwards to the Pacific plate (and lots of smaller plates are moving around too).
To the east is a subduction zone (convergent plate boundary) where the Solomon Sea plate dives north beneath the South Bismarck plate. I have prepared many earthquake reports for earthquakes in this region, most of them thrust (compressional) earthquakes related to subduction.
To the north of PNG is a transform plate boundary (strike-slip) that begins at the eastern boundary of the New Britain trench and extends along the north side of PNG, eventually turning into the Sorong fault, then the Palu Koro system in Sulawesi. On 28 September 2018 was an interesting earthquake and tsunami, along with some mega landslides. Here is my report for that series of events.
In the center of PNG, running east-west, is a collision zone formed by the north-south compression I mentioned above. There is a series of compressional folds and faults called the Papua Fold Belt. There have been several large quakes recently in this fold belt. Here is a report for one of those thrust earthquakes, much shallower than today’s eq.
The convergent plate boundary faults in this region have been long lived and have an interesting history. Some of the subduction zones that show up on the maps we will look at are fossil subduction zones (they are no longer active). However, just because they are not active does not mean that there are no earthquakes there. Often, earthquakes can happen along pre-existing zones of weakness. Today’s earthquake may be such a quake. It is difficult to really know.
There have been about 4 earthquakes in the area of today’s quake, with magnitudes M > 7.0. Today’s earthquake is extensional, but intermediate depth earthquakes can be of all types. The 2 quakes that have USGS mechanisms were strike-slip, but one was oblique (it was extensional and strike-slip).
Today, there was also a thrust earthquake, associated with the San Cristobal Trench (the subduction zone to the east of the New Britain trench). I did not label this subduction zone in the map below, but here is an earthquake sequence where I describe this fault zone in greater detail.
Today’s M 7.2 temblor is a cool mystery!

Below is my interpretive poster for this earthquake

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
Note the interesting orientation of the slab contours near today’s quake. Along the New Britain trench, they get deeper to the north (red contours near the trench and blue contours to the north). These contours wrap around on the west, so in the region of today’s quake, they get deeper to the south. There may be a subducting slab dipping to the south here, perhaps associated with the Trobriand Trench. This is one interpretation, which suggests today’s temblor was in an oceanic slab dipping to the south. Holm et al. (2015) favor a different interpretation, where the fossil subduction zone (forming the Pockington Trough) left behind a delaminated slab. So, today’s quake would be in the oceanic plate from the Australia plate that used to be dipping to the north. I don’t know if we can tell which is the correct hypothesis.

Magnetic Anomalies

In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
We can see the roughly east-west trends of these red and blue stripes in the Woodlark Basin and Solomon Sea plate. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the overlying plate, so the magnetic anomalies mask the evidence for the downgoing plate.

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

In the upper right corner is a great figure showing the generalized plate tectonic boundaries in this region of the equatorial Pacific Ocean (Holm et al., 2016). I place a blue star in the general location of the M 6.5 earthquake (also plotted in other inset figures). This map shows the major plate boundary faults. Active subduction zones have shaded triangle fault symbols, while inactive subduction zones have un-shaded triangle fault line symbols. I place a blue star in the general location of today’s temblor (as in other inset figures).
In the lower left corner is a map showing the fault systems in the region (Cloos et al., 2005). The legend allows us to distinguish between active and inactive fault systems.
In the lower right corner is a time history of this plate boundary from Holm et al. (2015). This is generalized with south on the left and north on the right. The plate on the left is the Australia plate and the downgoing plate was previously a subduction zone that formed the Pockington Trough.
In the upper left corner is a map that shows the probability (likelihood) for liquefaction (Zhu et al;., 2017). This is a recent addition to the USGS earthquake pages. Read more about this modeling here. These areas that may experience liquefaction are areas that are low lying topography with underlying sediment (as opposed to bedrock) that have the correct properties (e.g. water saturated) to liquefy. I discuss liquefaction in the Donggala-Palu earthquake report here.

Here is the map with a month’s seismicity plotted.

Here is the map with a century’s seismicity plotted.

Here is the map with a month’s seismicity plotted, but i have plotted the active faults in the CCOP database (white lines). These fault lines nicely highlight the Papua fold belt.

Other Report Pages

Some Relevant Discussion and Figures

Here is the Holm et al. (2016) figure, showing the major plate boundary faults (symbols tell us which plate boundaries are no longer active: dark symbols = active, hollow symbols = inactive).

Topography, bathymetry and regional tectonic setting of New Guinea and Solomon Islands. Arrows indicate rate and direction of plate motion of the Australian and Pacific plates (MORVEL, DeMets et al., 2010); Mamberamo thrust belt, Indonesia (MTB); North Fiji Basin (NFB)

This is the Cloos et al. (2005) map from the poster.

Tectonic map of New Guinea, adapted from Hamilton (1979), Cooper and Taylor (1987), Dow et al. (1988), and Sapiie et al. (1999). AFTB—Aure fold and thrust belt, FTB—fold-and-thrust belt, IOB—Irian Ophiolite Belt, TFB—thrust-and-fold belt, POB—Papuan Ophiolite Belt, BTFZ—Bewani-Torricelli fault zone, MDZ—Mamberamo deformation zone, YFZ—Yapen fault zone, SFZ—Sorong fault zone, WO—Weyland overthrust. Continental basement exposures are concentrated along the southern fl ank of the Central Range: BD—Baupo Dome, MA—Mapenduma anticline, DM—Digul monocline, IDI—Idenberg Inlier, MUA—Mueller anticline, KA—Kubor anticline, LFTB—Legguru fold-and-thrust belt, RMFZ—Ramu-Markham fault zone, TAFZ—Tarera-Aiduna fault zone. The Tasman line separates continental crust that is Paleozoic and younger to the east from Precambrian to the west.

This is the Cloos et al. (2005) cross section, showing a different interpretation of the delaminated slab.

Lithospheric-scale cross section at 2 Ma. Plate motion is now focused along the Yapen fault zone in the center of the recently extinct arc. This probably occurred because this zone of weakness had a trend that could accommodate the imposed movements as the corner of the Caroline microplate ruptured, forming the Bismarck plate, and the corner of the Australian plate ruptured, forming the Solomon microplate. The collisional delamination-generated magmatic event ends in the highlands as the lower crustal magma chamber solidifies. Upwelled asthenosphere cools and transforms into lithospheric mantle. This drives a slow regional subsidence of the highlands that will continue for tens of millions of years or until other plate-tectonic movements are initiated. Deep erosion is still concentrated on the fl anks of the mountain belt. RMB—Ruffaer Metamorphic Belt, AUS—Australian plate, PAC—Pacific plate.

Here is the tectonic map figure from Sappie and Cloos (2004). Their work was focused on western PNG, so their interpretations are more detailed there (and perhaps less relevant for us for these eastern PNG earthquakes).

Seismotectonic interpretation of New Guinea. Tectonic features: PTFB—Papuan thrust-and-fold belt; RMFZ—Ramu-Markham fault zone; BTFZ—Bewani-Torricelli fault zone; MTFB—Mamberamo thrust-and-fold belt; SFZ—Sorong fault zone; YFZ—Yapen fault zone; RFZ—Ransiki fault zone; TAFZ—Tarera-Aiduna fault zone; WT—Waipona Trough. After Sapiie et al. (1999).

This is the two panel figure from Holm and Richards (2013) that shows how the New Britain trench megathrust splays into three thrust faults as this fault system heads onto PNG. They plot active thrust faults as black triangles (with the triangles on the hanging wall side of the fault) and inactive thrust faults as open triangles. So, either the NG trench subduction zone extends further east than is presented in earlier work or the Bundi Fault Zone is the fault associated with this deep seismicity.

Topography, bathymetry and major tectonic elements of the study area. (a) Major tectonic boundaries of Papua New Guinea and the western Solomon Islands; CP, Caroline plate; MB, Manus Basin; NBP, North Bismarck plate; NBT, New Britain trench; NGT, New Guinea trench; NST, North Solomon trench; PFTB, Papuan Fold and Thrust Belt; PT, Pocklington trough; RMF, Ramu-Markham Fault; SBP, South Bismarck plate; SCT, San Cristobal trench; SS, Solomon Sea plate; TT, Trobriand trough; WB,Woodlark Basin; WMT,West Melanesian trench. Study area is indicated by rectangle labelled Figure 1b; the other inset rectangle highlights location for subsequent figures. Present day GPS motions of plates are indicated relative to the Australian plate (from Tregoning et al. 1998, 1999; Tregoning 2002; Wallace et al. 2004). (b) Detailed topography, bathymetry and structural elements significant to the South Bismarck region (terms not in common use are referenced); AFB, Aure Fold Belt (Davies 2012); AT, Adelbert Terrane (e.g. Wallace et al. 2004); BFZ, Bundi Fault Zone (Abbott 1995); BSSL, Bismarck Sea Seismic Lineation; CG, Cape Gloucester; FT, Finisterre Terrane; GF, Gogol Fault (Abbott 1995); GP, Gazelle Peninsula; HP, Huon Peninsula; MB, Manus Basin; NB, New Britain; NI, New Ireland; OSF, Owen Stanley Fault; RMF, Ramu-Markham Fault; SS, Solomon Sea; WMR, Willaumez-Manus Rise (Johnson et al. 1979); WT, Wonga Thrust (Abbott et al. 1994); minor strike-slip faults are shown adjacent to Huon Peninsula (Abers & McCaffrey 1994) and in east New Britain, the Gazelle Peninsula (e.g. Madsen & Lindley 1994). Circles indicate centres of Quaternary volcanism of the Bismarck arc. Filled triangles indicate active thrusting or subduction, empty triangles indicate extinct or negligible thrusting or subduction.

Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

Earlier, in other earthquake reports, I have discussed seismicity from 2000-2015 here. The seismicity on the west of this region appears aligned with north-south shortening along the New Britain trench, while seismicity on the east of this region appears aligned with more east-west shortening. Here is a map that I put together where I show these two tectonic domains with the seismicity from this time period (today’s earthquakes are not plotted on this map, but one may see where they might plot).

This map shows plate velocities and euler poles for different blocks. I include the figure caption below as a blockquote.

Tectonic maps of the New Guinea region. (a) Seismicity, volcanoes, and plate motion vectors. Plate motion vectors relative to the Australian plate are surface velocity models based on GPS data, fault slip rates, and earthquake focal mechanisms (UNAVCO, http://jules.unavco.org/Voyager/Earth). Earthquake data are sourced from the International Seismological Center EHB Bulletin (http://www.isc.ac.uk); data represent events from January 1994 through January 2009 with constrained focal depths. Background image is generated from http://www.geomapapp.org. Abbreviations: AB, Arafura Basin; AT, Aure Trough; AyT, Ayu Trough; BA, Banda arc; BSSL, Bismarck Sea seismic lineation; BH, Bird’s Head; BT, Banda Trench; BTFZ, Bewani-Torricelli fault zone; DD, Dayman Dome; DEI, D’Entrecasteaux Islands; FP, Fly Platform; GOP, Gulf of Papua; HP, Huon peninsula; LA, Louisiade Archipelago; LFZ, Lowlands fault zone; MaT, Manus Trench; ML, Mt. Lamington; MT, Mt. Trafalgar; MuT, Mussau Trough; MV, Mt. Victory; MTB, Mamberamo thrust belt; MVF, Managalase Plateau volcanic field; NBT, New Britain Trench; NBA, New Britain arc; NF, Nubara fault; NGT, New Guinea Trench; OJP, Ontong Java Plateau; OSF, Owen Stanley fault zone; PFTB, Papuan fold-and-thrust belt; PP, Papuan peninsula; PRi, Pocklington Rise; PT, Pocklington Trough; RMF, Ramu-Markham fault; SST, South Solomons Trench; SA, Solomon arc; SFZ, Sorong fault zone; ST, Seram Trench; TFZ, Tarera-Aiduna fault zone; TJ, AUS-WDKPAC triple junction; TL, Tasman line; TT, Trobriand Trough;WD, Weber Deep;WB, Woodlark Basin;WFTB, Western (Irian) fold-and-thrust belt; WR,Woodlark Rift; WRi, Woodlark Rise; WTB, Weyland thrust; YFZ, Yapen fault zone.White box indicates the location shown in Figure 3. (b) Map of plates, microplates, and tectonic blocks and elements of the New Guinea region. Tectonic elements modified after Hill & Hall (2003). Abbreviations: ADB, Adelbert block; AOB, April ultramafics; AUS, Australian plate; BHB, Bird’s Head block; CM, Cyclops Mountains; CWB, Cendrawasih block; CAR, Caroline microplate; EMD, Ertsberg Mining District; FA, Finisterre arc; IOB, Irian ophiolite belt; KBB, Kubor & Bena blocks (including Bena Bena terrane); LFTB, Lengguru fold-and-thrust belt; MA, Mapenduma anticline; MB, Mamberamo Basin block; MO, Marum ophiolite belt; MHS, Manus hotspot; NBS, North Bismarck plate; NGH, New Guinea highlands block; NNG, Northern New Guinea block; OKT, Ok Tedi mining district; PAC, Pacific plate; PIC, Porgera intrusive complex; PSP, Philippine Sea plate; PUB, Papuan Ultramafic Belt ophiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen peninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). These deep earthquakes are nearest the cross section D (though are much deeper than these shallow cross sections). I include the figure caption below as a blockquote.

Oblique block diagram of New Guinea from the northeast with schematic cross sections showing the present-day plate tectonic setting. Digital elevation model was generated from http://www.geomapapp.org. Oceanic crust in tectonic cross sections is shown by thick black-and-white hatched lines, with arrows indicating active subduction; thick gray-and-white hatched lines indicate uncertain former subduction. Continental crust, transitional continental crust, and arc-related crust are shown without pattern. Representative geologic cross sections across parts of slices C and D are marked with transparent red ovals and within slices B and E are shown by dotted lines. (i ) Cross section of the Papuan peninsula and D’Entrecasteaux Islands modified from Little et al. (2011), showing the obducted ophiolite belt due to collision of the Australian (AUS) plate with an arc in the Paleogene, with later Pliocene extension and exhumation to form the D’Entrecasteaux Islands. (ii ) Cross section of the Papuan peninsula after Davies & Jaques (1984) shows the Papuan ophiolite thrust over metamorphic rocks of AUS margin affinity. (iii ) Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent. (iv) Across the Bird’s Head, the cross section after Bailly et al. (2009) illustrates deformation in the Lengguru fold-and-thrust belt as a result of Late Miocene–Early Pliocene northeast-southwest shortening, followed by Late Pliocene–Quaternary extension. Abbreviations as in Figure 2, in addition to NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; (U)HP, (ultra)high-pressure.

In 1977, D.B. Dow published a Geological Synthesis of Papua New Guinea. There are some excellent low angle oblique views of tectonic geomorphologic features, including the Papua fold belt. Below are two examples, one with an arc volcano formed in the midst of the fold belt. These images are based on RADAR imagery.

Radar image of Mount Murray stratovolcano (lat. 6°45’S, long. 144°00’E)—of late Pliocene or Quaternary age—surmounting the prominent strike ridges of folded Miocene Darai Limestone. Deep erosion of the crater has exposed the intrusive core of the volcano. (Scale about 1:250 000.)

Side-looking radar image of the eastern end of the Papuan Fold Belt between Mount Murray and Mount Karimui. The prominent ridges are steeply dipping Darai Limestone which has been repeated by folding and thrust-faulting. The karst surface developed on the limestone is evident despite the very heavy jungle cover. This image was obtained with the radar looking from the south, so the image is oriented with north to the bottom of the page to prevent the viewer seeing inverted topography. (Scale about 1:250 000.)

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

Here is another way to look at these beach balls.

The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj

— AK Earthquake Center (@AKearthquake) August 15, 2018

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy

— Jackie Caplan-Auerbach (@geophysichick) February 23, 2019

New Britain | Solomon | Bougainville | New Hebrides | Tonga | Kermadec Earthquake Reports

General Overview

Earthquake Reports

2019.05.06 M 7.2 Papua New Guinea
2018.12.05 M 7.5 New Caledonia
2018.10.10 M 7.0 New Britain, PNG
2018.09.09 M 6.9 Kermadec
2018.08.29 M 7.1 Loyalty Islands
2018.08.18 M 8.2 Fiji
2018.03.26 M 6.9 New Britain
2018.03.26 M 6.6 New Britain
2018.03.08 M 6.8 New Ireland
2018.02.25 M 7.5 Papua New Guinea
2018.02.26 M 7.5 Papua New Guinea Update #1
2017.11.19 M 7.0 Loyalty Islands Update #1
2017.11.07 M 6.5 Papua New Guinea
2017.11.04 M 6.8 Tonga
2017.10.31 M 6.8 Loyalty Islands
2017.08.27 M 6.4 N. Bismarck plate
2017.05.09 M 6.8 Vanuatu
2017.03.19 M 6.0 Solomon Islands
2017.03.05 M 6.5 New Britain
2017.01.22 M 7.9 Bougainville
2017.01.03 M 6.9 Fiji
2016.12.17 M 7.9 Bougainville
2016.12.08 M 7.8 Solomons
2016.10.17 M 6.9 New Britain
2016.10.15 M 6.4 South Bismarck Sea
2016.09.14 M 6.0 Solomon Islands
2016.08.31 M 6.7 New Britain
2016.08.12 M 7.2 New Hebrides Update #2
2016.08.12 M 7.2 New Hebrides Update #1
2016.08.12 M 7.2 New Hebrides
2016.04.06 M 6.9 Vanuatu Update #1
2016.04.03 M 6.9 Vanuatu
2015.03.30 M 7.5 New Britain (Update #5)
2015.03.30 M 7.5 New Britain (Update #4)
2015.03.29 M 7.5 New Britain (Update #3)
2015.03.29 M 7.5 New Britain (Update #2)
2015.03.29 M 7.5 New Britain (Update #1)
2015.03.29 M 7.5 New Britain
2015.11.18 M 6.8 Solomon Islands
2015.05.24 M 6.8, 6.8, 6.9 Santa Cruz Islands
2015.05.05 M 7.5 New Britain

Social Media

References:

Abers, G. and McCaffrey, R., 1988. Active Deformation in the New Guinea Fold-and-Thrust Belt: Seismological Evidence for Strike-Slip Faulting and Basement-Involved Thrusting in JGR, v. 93, no. B11, p. 13,332-13,354
Baldwin, S.L., Monteleone, B.D., Webb, L.E., Fitzgerald, P.G., Grove, M., and Hill, E.J., 2004. Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea in Nature, v. 431, p/ 263-267, doi:10.1038/nature02846.
Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region, Annu. Rev. Earth Planet. Sci., v. 40, pp. 495-520.
Cloos, M., Sapiie, B., Quarles van Ufford, A., Weiland, R.J., Warren, P.Q., and McMahon, T.P., 2005. Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff: Geological Society of America Special Paper 400, 51 p., doi: 10.1130/2005.2400.
Dow, D.B., 1977. A Geological Synthesis of Papua New Guinea, Bureau of Mineral Resources, Geology, and Geophysics, Bulltein 201, Australian Government Publishing Sevice, Canberra, 1977, 58 pp.
Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
Hamilton, W.B., 1979. Tectonics of the Indonesian Region, USGS Professional Paper 1078.
Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
Holm, R. and Richards, S.W., 2013. A re-evaluation of arc-continent collision and along-arc variation in the Bismarck Sea region, Papua New Guinea in Australian Journal of Earth Sciences, v. 60, p. 605-619.
Holm, R.J., Richards, S.W., Rosenbaum, G., and Spandler, C., 2015. Disparate Tectonic Settings for Mineralisation in an Active Arc, Eastern Papua New Guinea and the Solomon Islands in proceedings from PACRIM 2015 Congress, Hong Kong ,18-21 March, 2015, pp. 7.
Holm, R.J., Rosenbaum, G., Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting in Eartth Science Reviews, v. 156, p. 66-81.
Johnson, R.W., 1976, Late Cainozoic volcanism and plate tectonics at the southern margin of the Bismarck Sea, Papua New Guinea, in Johnson, R.W., ed., 1976, Volcanism in Australia: Amsterdam, Elsevier, p. 101-116
Koulali, A., tregoning, P., McClusky, S., Stanaway, R., Wallace, L., and Lister, G., 2015. New Insights into the present-day kinematics of the central and western Papua New Guinea from GPS in GJI, v. 202, p. 993-1004, doi: 10.1093/gji/ggv200
Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
Sapiie, B., and Cloos, M., 2004. Strike-slip faulting in the core of the Central Range of west New Guinea: Ertsberg Mining District, Indonesia in GSA Bulletin, v. 116; no. 3/4; p. 277–293
Tregoning, P., McQueen, H., Lambeck, K., Jackson, R. Little, T., Saunders, S., and Rosa, R., 2000. Present-day crustal motion in Papua New Guinea, Earth Planets and Space, v. 52, pp. 727-730.
Wells, D., l., and Coppersmith, K.J., 1994. New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement in BSSA, vol. 84, no. 4, pp. 974-1002

Return to the Earthquake Reports page.

Sorted by Magnitude
Sorted by Year
Sorted By Region

Earthquake Report: 2018 Summary

Posted on December 31, 2018 by Jay Patton

Here I summarize Earth’s significant seismicity for 2018. I limit this summary to earthquakes with magnitude greater than or equal to M 6.5. I am sure that there is a possibility that your favorite earthquake is not included in this review. Happy New Year.
However, our historic record is very short, so any thoughts about whether this year (or last, or next) has smaller (or larger) magnitude earthquakes than “normal” are limited by this small data set.
Here is a table of the earthquakes M ≥ 6.5.

Here is a plot showing the cumulative release of seismic energy. This summary is imperfect in several ways, but shows how only the largest earthquakes have a significant impact on the tally of energy release from earthquakes. I only include earthquakes M ≥ 6.5. Note how the M 7.5 Sulawesi earthquake and how little energy was released relative to the two M = 7.9 earthquakes.

Here are the annual summaries:

Here are the annual summaries for the Cascadia region.

Below is my summary poster for this earthquake year

I include moment tensors for the earthquakes included in the reports below.
Click on the map to see a larger version.

This is a video that shuffles through the earthquake report posters of the year

2018 Earthquake Report Pages

2018.12.29 M 7.0 Philippines
2018.12.20 M 7.4 Bering Kresla
2018.12.20 M 7.3 Bering Kresla UPDATE #1
2018.12.05 M 7.5 New Caledonia

2018.11.30 M 7.0 Alaska
2018.11.25 M 6.3 Iran/Iraq
2018.11.08 M 6.8 Mid Atlantic Ridge (Jan Mayen fracture zone)

2018.10.25 M 6.8 Greece
2018.10.22 M 6.8 Explorer plate
2018.10.10 M 7.0 New Britain, PNG

2018.09.28 M 7.5 Sulawesi
2018.10.16 M 7.5 Sulawesi UPDATE #1
2018.09.09 M 6.9 Kermadec
2018.09.05 M 6.6 Hokkaido, Japan

2018.08.24 M 7.1 Peru
2018.08.22 M 6.2 Blanco fracture zone
2018.08.21 M 7.3 Venezuela
2018.08.19 M 6.9 Lombok, Indonesia
2018.08.18 M 8.2 Fiji
2018.08.15 M 6.6 Aleutians
2018.08.12 M 6.4 North Alaska
2018.08.12 M 6.4 North Alaska UPDATE #1
2018.08.05 M 6.9 Lombok, Indonesia

2018.05.15 M 5.8 Madagascar
2018.05.04 M 6.9 Hawai’i

2018.04.05 M 5.3 Channel Islands
2018.04.05 M 5.3 Channel Islands Update #1
2018.04.02 M 6.8 Bolivia

2018.02.25 M 7.5 Papua New Guinea
2018.02.26 M 7.5 Papua New Guinea Update #1
2018.02.16 M 7.2 Oaxaca, Mexico

2018.01.25 M 5.8 Mendocino fault
2018.01.23 M 7.9 Gulf of Alaska
2018.01.23 M 7.9 Gulf of Alaska UPDATE #1
2018.01.24 M 7.9 Gulf of Alaska UPDATE #2
2018.01.19 M 6.3 Gulf of California
2018.01.14 M 7.1 Peru
2018.01.15 M 7.1 Peru Update #1
2018.01.11 M 6.0 Burma
2018.01.10 M 7.6 Cayman Trough
2018.01.04 M 4.4 Berkeley

Other Annual Summaries

Here is the USGS website listing the significant earthquakes for 2018.
Here is the USGS website listing the earthquake posters for 2018.
Here are some tweets:

One year of #earthquakes recorded by @INGVterremoti in Italy. About 2500 events with magnitude equal or larger than M2, about seven per day. Data source https://t.co/g1RvR2A989) #Italia #terremoto #Italy #earthquake pic.twitter.com/ft8GAsFjKA

— iunio iervolino (@iuniervo) December 31, 2018

Earthquakes of 2018: a quick post summarising global seismic activity last year (i.e., the figures I showed you yesterday). https://t.co/ahdwpf1OFv pic.twitter.com/S438okD8QQ

— Chris Rowan (@Allochthonous) January 1, 2019

Global #earthquakes by Magnitude (M5+) by year (2000-18), showing remarkable consistency from geologic forcing. Whereas patterns are understood, they do not permit short-term, local predictions; instead, be informed and be prepared. #geohazards @IRIS_EPO @USGS pic.twitter.com/BmtXhhUvWF

— Ben van der Pluijm 🌎 (@vdpluijm) January 2, 2019

The pattern of shallow earthquakes (depth < 33 km) is typical, with much of the country susceptible to regular shallow seismicity, with lower rates in Northland/Auckland and southeast Otago. pic.twitter.com/3jip8Lyje9

— John Ristau 🇨🇦 🇳🇿 (@SinistralSeismo) January 3, 2019

2018 Earthquake Reports

General Overview of how to interact with these summaries

Click on the earthquake “magnitude and location” label (e.g. “M 6.9 Fiji”) to go to the Earthquake Report website for any given earthquake. Click on the map to open a high resolution pdf version of the interpretive poster. More information about the poster is found on the Earthquake Report website.
I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 7.5 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

Background on the Earthquake Report posters

I placed a moment tensor / focal mechanism legend on the posters. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the maps. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>

Magnetic Anomalies

In the maps below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.

2018.01.10 M 7.6 Cayman Trough

Just a couple hours ago there was an earthquake along the Swan fault, which is the transform plate boundary between the North America and Caribbean plates. The Cayman trough (CT) is a region of oceanic crust, formed at the Mid-Cayman Rise (MCR) oceanic spreading center. To the west of the MCR the CT is bound by the left-lateral strike-slip Swan fault. To the east of the MCR, the CT is bound on the north by the Oriente fault.
Based upon our knowledge of the plate tectonics of this region, I can interpret the fault plane solution for this earthquake. The M 7.6 earthquake was most likely a left-lateral strike-slip earthquake associated with the Swan fault.

Plotted with a century’s earthquakes with magnitudes M ≥ 6.5

Plotted with a century’s earthquakes with magnitudes M ≥ 3.5

There were two observations of a small amplitude (small wave height) tsunami recorded on tide gages in the region. Below are those observations.

2018.01.14 M 7.1 Peru

We had a damaging and (sadly) deadly earthquake in southern Peru in the last 24 hours. This is an earthquake, with magnitude M 7.1, that is associated with the subduction zone forming the Peru-Chile trench (PCT). The Nazca plate (NP) is subducting beneath the South America plate (SAP). There are lots of geologic structures on the Nazca plate that tend to affect how the subduction zone responds during earthquakes (e.g. segmentation).
In the region of this M 7.1 earthquake, two large structures in the NP are the Nazca Ridge and the Nazca fracture zone. The Nazca fracture zone is a (probably inactive) strike-slip fault system. The Nazca Ridge is an over-thickened region of the NP, thickened as the NP moved over a hotspot located near Salas y Gomez in the Pacific Ocean east of Easter Island (Ray et al., 2012).
There are many papers that discuss how the ridge affects the shape of the megathrust fault here. The main take-away is that the NR is bull dozing into South America and the dip of the subduction zone is flat here. There is a figure below that shows the deviation of the subducting slab contours at the NR.

Well, I missed looking further into a key update paper and used figures from an older paper on my interpretive poster yesterday. Thanks to Stéphane Baize for pointing this out! Turns out, after their new analyses, the M 7.1 earthquake was in a region of higher seismogenic coupling, rather than low coupling (as was presented in my first poster).
Also, Dr. Robin Lacassin noticed (as did I) the paucity of aftershocks from yesterday’s M 7.1. This was also the case for the carbon copy 2013 M 7.1 earthquake (there was 1 M 4.6 aftershock in the weeks following the M 7.1 earthquake on 2013.09.25; there were a dozen M 1-2 earthquakes in Nov. and Dec. of 2013, but I am not sure how related they are to the M 7.1 then). I present a poster below with this in mind. I also include below a comparison of the MMI modeled estimates. The 2013 seems to have possibly generated more widespread intensities, even though that was a deeper earthquake.

2018.01.23 M 7.9 Gulf of Alaska

2018.01.23 M 7.9 Gulf of Alaska UPDATE #1

2018.01.24 M 7.9 Gulf of Alaska UPDATE #2

This earthquake appears to be located along a reactivated fracture zone in the GA. There have only been a couple earthquakes in this region in the past century, one an M 6.0 to the east (though this M 6.0 was a thrust earthquake). The Gulf of Alaska shear zone is even further to the east and has a more active historic fault history (a pair of earthquakes in 1987-1988). The magnetic anomalies (formed when the Earth’s magnetic polarity flips) reflect a ~north-south oriented spreading ridge (the anomalies are oriented north-south in the region of today’s earthquake). There is a right-lateral offset of these magnetic anomalies located near the M 7.9 epicenter. Interesting that this right-lateral strike-slip fault (?) is also located at the intersection of the Gulf of Alaska shear zone and the 1988 M 7.8 earthquake (probably just a coincidence?). However, the 1988 M 7.8 earthquake fault plane solution can be interpreted for both fault planes (it is probably on the GA shear zone, but I don’t think that we can really tell).
This is strange because the USGS fault plane is oriented east-west, leading us to interpret the fault plane solution (moment tensor or focal mechanism) as a left-lateral strike-slip earthquake. So, maybe this earthquake is a little more complicated than first presumed. The USGS fault model is constrained by seismic waves, so this is probably the correct fault (east-west).
I prepared an Earthquake Report for the 1964 Good Friday Earthquake here.

The USGS updated their MMI contours to reflect their fault model. Below is my updated poster. I also added green dashed lines for the fracture zones related to today’s M 7.9 earthquake (on the magnetic anomaly inset map).

These are the observations as reported by the NTWC this morning (at 4:15 AM my local time).

Large Scale Interpretive Map (from update report)

As a reminder, if the M 7.9 earthquake fault is E-W oriented, it would be left-lateral. The offset magnetic anomalies show right-lateral offset across these fracture zones. This was perhaps the main reason why I thought that the main fault was not E-W, but N-S. After a day’s worth of aftershocks, the seismicity may reveal some north-south trends. But, as a drama student in 7th grade (1977), my drama teacher (Ms. Naichbor, rest in peace) asked our class to go stand up on stage. We all stood in a line and she mentioned that this is social behavior, that people tend to stand in lines (and to avoid doing this while on stage). Later, when in college, professors often commented about how people tend to seek linear trends in data (lines). I actually see 3-4 N-S trends and ~2 E-W trends in the seismicity data.
So, that being said, here is the animation I put together. I used the USGS query tool to get earthquakes from 1/22 until now, M ≥ 1.5. I include a couple inset maps presented in my interpretive posters. The music is copyright free. The animations run through twice.
Here is a screenshot of the 14 MB video embedded below. I encourage you to view it in full screen mode (or download it).

2018.02.16 M 7.2 Oaxaca, Mexico

There was just now an earthquake in Oaxaca, Mexico between the other large earthquakes from last 2017.09.08 (M 8.1) and 2017.09.08 (M 7.1). There has already been a M 5.8 aftershock.Here is the USGS website for today’s M 7.2 earthquake.
The SSN has a reported depth of 12 km, further supporting evidence that this earthquake was in the North America plate.
This region of the subduction zone dips at a very shallow angle (flat and almost horizontal).
There was also a sequence of earthquakes offshore of Guatemala in June, which could possibly be related to the M 8.1 earthquake. Here is my earthquake report for the Guatemala earthquake.
The poster also shows the seismicity associated with the M 7.6 earthquake along the Swan fault (southern boundary of the Cayman trough). Here is my earthquake report for the Guatemala earthquake.

Here is the same poster but with the magnetic anomalies included (transparent).

2018.02.25 M 7.5 Papua New Guinea

2018.02.26 M 7.5 Papua New Guinea Update #1

This morning (local time in California) there was an earthquake in Papua New Guinea with, unfortunately, a high likelihood of having a good number of casualties. I was working on a project, so could not immediately begin work on this report.
This M 7.5 earthquake (USGS website) occurred along the Papua Fold and Thrust Belt (PFTB), a (mostly) south vergent sequence of imbricate thrust faults and associated fold (anticlines). The history of this PFTB appears to be related to the collision of the Australia plate with the Caroline and Pacific plates, the delamination of the downgoing oceanic crust, and then associated magmatic effects (from decompression melting where the overriding slab (crust) was exposed to the mantle following the delamination). More about this can be found in Cloos et al. (2005).

The same map without historic seismicity.

The aftershocks are still coming in! We can use these aftershocks to define where the fault may have slipped during this M 7.5 earthquake. As I mentioned yesterday in the original report, it turns out the fault dimension matches pretty well with empirical relations between fault length and magnitude from Wells and Coppersmith (1994).
The mapped faults in the region, as well as interpreted seismic lines, show an imbricate fold and thrust belt that dominates the geomorphology here (as well as some volcanoes, which are probably related to the slab gap produced by crust delamination; see Cloos et al., 2005 for more on this). I found a fault data set and include this in the aftershock update interpretive poster (from the Coordinating Committee for Geoscience Programmes in East and Southeast Asia, CCOP).
I initially thought that this M 7.5 earthquake was on a fault in the Papuan Fold and Thrust Belt (PFTB). Mark Allen pointed out on twitter that the ~35km hypocentral depth is probably too deep to be on one of these “thin skinned” faults (see Social Media below). Abers and McCaffrey (1988) used focal mechanism data to hypothesize that there are deeper crustal faults that are also capable of generating the earthquakes in this region. So, I now align myself with this hypothesis (that the M 7.5 slipped on a crustal fault, beneath the thin skin deformation associated with the PFTB. (thanks Mark! I had downloaded the Abers paper but had not digested it fully.

Here is the “update” map with aftershocks

2018.03.08 M 6.8 New Ireland

We had an M 6.8 earthquake near a transform micro-plate boundary fault system north of New Ireland, Papua New Guinea today. Here is the USGS website for this earthquake.
The main transform fault (Weitin fault) is ~40 km to the west of the USGS epicenter. There was a very similar earthquake on 1982.08.12 (USGS website).
This earthquake is unrelated to the sequence occurring on the island of New Guinea.
Something that I rediscovered is that there were two M 8 earthquakes in 1971 in this region. This testifies that it is possible to have a Great earthquake (M ≥ 8) close in space and time relative to another Great earthquake. These earthquakes do not have USGS fault plane solutions, but I suspect that these are subduction zone earthquakes (based upon their depth).
This transform system is capable of producing Great earthquakes too, as evidenced by the 2000.11.16 M 8.0 earthquake (USGS website). This is another example of two Great earthquakes (or almost 2 Great earthquakes, as the M 7.8 is not quite a Great earthquake) are related. It appears that the M 8.0 earthquake may have triggered teh M 7.8 earthquake about 3 months later (however at first glance, it seemed to me like the strike-slip earthquake might not increase the static coulomb stress on the subduction zone, but I have not spent more than half a minute thinking about this).

Main Interpretive Poster with emag2

Earthquakes M≥ 6.5 with emag2

2018.03.26 M 6.6 New Britain

The New Britain region is one of the more active regions in the world. See a list of earthquake reports for this region at the bottom of this page, above the reference list.
Today’s M 6.6 earthquake happened close in proximity to a M 6.3 from 2 days ago and a M 5.6 from a couple weeks ago. The M 5.6 may be related (may have triggered these other earthquakes), but this region is so active, it might be difficult to distinguish the effects from different earthquakes. The M 5.6 is much deeper and looks like it was in the downgoing Solomon Sea plate. It is much more likely that the M 6.3 and M 6.6 are related (I interpret that the M 6.3 probably triggered the M 6.6, or that M 6.3 was a foreshock to the M 6.6, given they are close in depth). Both M 6.3 and M 6.6 are at depths close to the depth of the subducting slab (the megathrust fault depth) at this location. So, I interpret these to be subduction zone earthquakes.

2018.03.26 M 6.9 New Britain

Well, those earthquakes from earlier, one a foreshock to a later one, were foreshocks to an earthquake today! Here is my report from a couple days ago. The M 6.6 and M 6.3 straddle today’s earthquake and all have similar hypocentral depths.

2018.04.02 M 6.8 Bolivia

A couple days ago there was a deep focus earthquake in the downgoing Nazca plate deep beneath Bolivia. This earthquake has an hypocentral depth of 562 km (~350 miles).
We are still unsure what causes an earthquake at such great a depth. The majority of earthquakes happen at shallower depths, caused largely by the frictional between differently moving plates or crustal blocks (where earth materials like the crust behave with brittle behavior and not elastic behavior). Some of these shallow earthquakes are also due to internal deformation within plates or crustal blocks.
As plates dive into the Earth at subduction zones, they undergo a variety of changes (temperature, pressure, stress). However, because people cannot directly observe what is happening at these depths, we must rely on inferences, laboratory analogs, and other indirect methods to estimate what is going on.
So, we don’t really know what causes earthquakes at the depth of this Bolivia M 6.8 earthquake. Below is a review of possible explanations as provided by Thorne Lay (UC Santa Cruz) in an interview in response to the 2013 M 8.3 Okhotsk Earthquake.

2018.05.04 M 6.9 Hawai’i

There has been a swarm of earthquakes on the southeastern part of the big island, with USGS volcanologists hypothesizing about magma movement and suggesting that an eruption may be imminent. Here is a great place to find official USGS updates on the volcanism in Hawaii (including maps).
Hawaii is an active volcanic island formed by hotspot volcanism. The Hawaii-Emperor Seamount Chain is a series of active and inactive volcanoes formed by this process and are in a line because the Pacific plate has been moving over the hotspot for many millions of years.
Southeast of the main Kilauea vent, the Pu‘u ‘Ö‘ö crater saw an elevation of lava into the crater, leading to overtopping of the crater (on 4/30/2018). Seismicity migrated eastward along the ERZ. This morning, there was a M 5.0 earthquake in the region of the Hilina fault zone (HFZ). I was getting ready to write something up, but I had other work that I needed to complete. Then, this evening, there was a M 6.9 earthquake between the ERZ and the HFZ.
There have been earthquakes this large in this region in the past (e.g. the 1975.1.29 M 7.1 earthquake along the HFZ). This earthquake was also most likely related to magma injection (Ando, 1979). The 1975 M 7.1 earthquake generated a small tsunami (Ando, 1979). These earthquakes are generally compressional in nature (including the earthquakes from today).
Today’s earthquake also generated a tsunami as recorded on tide gages throughout Hawaii. There is probably no chance that a tsunami will travel across the Pacific to have a significant impact elsewhere.

This version includes earthquakes M ≥ 3.5 (note the seismicity offshore to the south, this is where the youngest Hawaii volcano is).

Below are a series of plots from tide gages installed at several sites in the Hawaii Island Chain. These data are all posted online here and here.

Hilo, Hawaii

Kawaihae, Hawaii

Temblor Reports:

Click on the graphic to see a pdf version of the article.
Click on the html link (date) to visit the Temblor site.

	2018.05.05 Pele, the Hawai’i Goddess of Fire, Lightning, Wind, and Volcanoes
	2018.05.06 Pele, la Diosa Hawaiana del Fuego, los Relámpagos, el Viento y los Volcanes de Hawái

2018.08.05 M 6.9 Lombok, Indonesia

Yesterday morning, as I was recovering from working on stage crew for the 34th Reggae on the River (fundraiser for the non profit, the Mateel Community Center), I noticed on social media that there was an M 6.9 earthquake in Lombok, Indonesia. This is sad because of the likelihood for casualties and economic damage in this region.
However, it is interesting because the earthquake sequence from last week (with a largest earthquake with a magnitude of M 6.4) were all foreshocks to this M 6.9. Now, technically, these were not really foreshocks. The M 6.4 has an hypocentral (3-D location) depth of ~6 km and the M 6.9 has an hypocentral depth of ~31 km. These earthquakes are not on the same fault, so I would interpret that the M 6.9 was triggered by the sequence from last week due to static coulomb changes in stress on the fault that ruptured. Given the large difference in depths, the uncertainty for these depths is probably not sufficient to state that they may be on the same fault (i.e. these depths are sufficiently different that this difference is larger than the uncertainty of their locations).
I present a more comprehensive analysis of the tectonics of this region in my earthquake report for the M 6.4 earthquake here. I especially address the historic seismicity of the region there. This M 6.9 may have been on the Flores thrust system, while the earthquakes from last week were on the imbricate thrust faults overlying the Flores Thrust. See the map from Silver et al. (1986) below. I include the same maps as in my original report, but after those, I include the figures from Koulani et al. (2016) (the paper is available on researchgate).

Here is the map with a month’s seismicity plotted.

2018.08.15 M 6.6 Aleutians

Well, yesterday while I was installing the final window in a reconstruction project, there was an earthquake along the Aleutian Island Arc (a subduction zone) in the region of the Andreanof Islands. Here is the USGS website for the M 6.6 earthquake. This earthquake is close to the depth of the megathrust fault, but maybe not close enough. So, this may be on the subduction zone, but may also be on an upper plate fault (I interpret this due to the compressive earthquake fault mechanism). The earthquake has a hypocentral depth of 20 km and the slab model (see Hayes et al., 2013 below and in the poster) is at 40 km at this location. There is uncertainty in both the slab model and the hypocentral depth.
The Andreanof Islands is one of the most active parts of the Aleutian Arc. There have been many historic earthquakes here, some of which have been tsunamigenic (in fact, the email that notified me of this earthquake was from the ITIC Tsunami Bulletin Board).
Possibly the most significant earthquake was the 1957 Andreanof Islands M 8.6 Great (M ≥ 8.0) earthquake, though the 1986 M 8.0 Great earthquake is also quite significant. As was the 1996 M 7.9 and 2003 M 7.8 earthquakes. Lest we forget smaller earthquakes, like the 2007 M 7.2. So many earthquakes, so little time.

Here is the map with a month’s seismicity plotted.

Here is the map with a centuries seismicity plotted for earthquakes M ≥ 6.6.

2018.08.18 M 8.2 Fiji

We just had a Great Earthquake in the region of the Fiji Islands, in the central-western Pacific. Great Earthquakes are earthquakes with magnitudes M ≥ 8.0.
This earthquake is one of the largest earthquakes recorded historically in this region. I include the other Large and Great Earthquakes in the posters below for some comparisons.
Today’s earthquake has a Moment Magnitude of M = 8.2. The depth is over 550 km, so is very very deep. This region has an historic record of having deep earthquakes here. Here is the USGS website for this M 8.2 earthquake. While I was writing this, there was an M 6.8 deep earthquake to the northeast of the M 8.2. The M 6.8 is much shallower (about 420 km deep) and also a compressional earthquake, in contrast to the extensional M 8.2.
This M 8.2 earthquake occurred along the Tonga subduction zone, which is a convergent plate boundary where the Pacific plate on the east subducts to the west, beneath the Australia plate. This subduction zone forms the Tonga trench.

Here is the map with a centuries seismicity plotted with M ≥ 7.5.

2018.08.19 M 6.9 Lombok, Indonesia

This ongoing sequence began in late July with a Mw 6.4 earthquake. Followed less than 2 weeks later with a Mw 6.9 earthquake.
Today there was an M 6.3 soon followed by an M 6.9 earthquake (and a couple M 5.X quakes).
These earthquakes have been occurring along a thrust fault system along the northern portion of Lombok, Indonesia, an island in the magamatic arc related to the Sunda subduction zone. The Flores thrust fault is a backthrust to the subduction zone. The tectonics are complicated in this region of the world and there are lots of varying views on the tectonic history. However, there has been several decades of work on the Flores thrust (e.g. Silver et al., 1986). The Flores thrust is an east-west striking (oriented) north vergent (dipping to the south) thrust fault that extends from eastern Java towards the Islands of Flores and Timor. Above the main thrust fault are a series of imbricate (overlapping) thrust faults. These imbricate thrust faults are shallower in depth than the main Flores thrust.
The earthquakes that have been happening appear to be on these shallower thrust faults, but there is a possibility that they are activating the Flores thrust itself. Perhaps further research will illuminate the relations between these shallower faults and the main player, the Flores thrust.

Here is the map with a month’s seismicity plotted.

Here is an updated local scale (large scale) map showing the earthquake fault mechanisms for the current sequence. I label them with yellow numbers according to the sequence timing. I outlined the general areas that have had earthquakes into two zones (phases). Phase I includes the earthquakes up until today and Phase II includes the earthquakes from today. There is some overlap, but only for a few earthquakes. In general, it appears that the earthquakes have slipped in two areas of the Flores fault (or maybe two shallower thrust faults).

Here is the interpretive posted from the M 6.4 7/28 earthquake, with historic seismicity and earthquake mechanisms.

2018.08.21 M 7.3 Venezuela

We just had a M 7.3 earthquake in northern Venezuela. Sadly, this large earthquake has the potential to be quite damaging to people and their belongings (buildings, infrastructure).
The northeastern part of Venezuela lies a large strike-slip plate boundary fault, the El Pilar fault. This fault is rather complicated as it strikes through the region. There are thrust faults and normal faults forming ocean basins and mountains along strike.
Many of the earthquakes along this fault system are strike-slip earthquakes (e.g. the 1997.07.09 M 7.0 earthquake which is just to the southwest of today’s temblor. However, today’s earthquake broke my immediate expectations for strike-slip tectonics. There is a south vergent (dipping to the north) thrust fault system that strikes (is oriented) east-west along the Península de Paria, just north of highway 9, east of Carupano, Venezuela. Audenard et al. (2000, 2006) compiled a Quaternary Fault database for Venezuela, which helps us interpret today’s earthquake. I suspect that this earthquake occurred on this thrust fault system. I bet those that work in this area even know the name of this fault. However, looking at the epicenter and the location of the thrust fault, this is probably not on this thrust fault. When I initially wrote this report, the depth was much shallower. Currently, the hypocentral (3-D location) depth is 123 km, so cannot be on that thrust fault.
The best alternative might be the subduction zone associated with the Lesser Antilles.

Here is the map with a month’s seismicity plotted, along with USGS earthquakes M ≥ 6.0.

2018.08.24 M 7.1 Peru

Well, this earthquake, while having a large magnitude, was quite deep. Because earthquake intensity decreases with distance from the earthquake source, the shaking intensity from this earthquake was so low that nobody submitted a single report to the USGS “Did You Feel It?” website for this earthquake.
While doing my lit review, I found the Okal and Bina (1994) paper where they use various methods to determine focal mechanisms for the some deep earthquakes in northern Peru. More about focal mechanisms below. These authors created focal mechanisms for the 1921 and 1922 deep earthquakes so they could lean more about the 1970 deep earthquake. Their seminal work here forms an important record of deep earthquakes globally. These three earthquakes are all extensional earthquakes, similar to the other deep earthquakes in this region. I label the 1921 and 1922 earthquakes a couplet on the poster.
There was also a pair of earthquakes that happened in November, 2015. These two earthquakes happened about 5 minutes apart. They have many similar characteristics, suggest that they slipped similar faults, if not the same fault. I label these as doublets also.
So, there may be a doublet companion to today’s M 7.1 earthquake. However, there may be not. There are examples of both (single and doublet) and it might not really matter for 99.99% of the people on Earth since the seismic hazard from these deep earthquakes is very low.
Other examples of doublets include the 2006 | 2007 Kuril Doublets (Ammon et al., 2008) and the 2011 Kermadec Doublets (Todd and Lay, 2013).

Here is the map with a century’s seismicity plotted, along with USGS earthquakes M ≥ 7.0.

2018.09.05 M 6.6 Hokkaido, Japan

Here is the map with a centuries seismicity plotted.

Temblor Reports:

Click on the graphic to see a pdf version of the article.
Click on the html link (date) to visit the Temblor site.

2018.09.06 Violent shaking triggers massive landslides in Sapporo Japan earthquake

2018.09.09 M 6.9 Kermadec

Today, there was a large earthquake associated with the subduction zone that forms the Kermadec Trench.
This earthquake was quite deep, so was not expected to generate a significant tsunami (if one at all).
There are several analogies to today’s earthquake. There was a M 7.4 earthquake in a similar location, but much deeper. These are an interesting comparison because the M 7.4 was compressional and the M 6.9 was extensional. There is some debate about what causes ultra deep earthquakes. The earthquakes that are deeper than about 40-50 km are not along subduction zone faults, but within the downgoing plate. This M 6.9 appears to be in a part of the plate that is bending (based on the Benz et al., 2011 cross section). As plates bend downwards, the upper part of the plate gets extended and the lower part of the plate experiences compression.

Here is the map with a month’s seismicity plotted.

Here is the map with a centuries seismicity plotted.

2018.09.28 M 7.5 Sulawesi

2018.10.16 M 7.5 Sulawesi UPDATE #1

Well, around 3 AM my time (northeastern Pacific, northern CA) there was a sequence of earthquakes including a mainshock with a magnitude M = 7.5. This earthquake happened in a highly populated region of Indonesia.
This area of Indonesia is dominated by a left-lateral (sinistral) strike-slip plate boundary fault system. Sulawesi is bisected by the Palu-Kola / Matano fault system. These faults appear to be an extension of the Sorong fault, the sinistral strike-slip fault that cuts across the northern part of New Guinea.
There have been a few earthquakes along the Palu-Kola fault system that help inform us about the sense of motion across this fault, but most have maximum magnitudes mid M 6.
GPS and block modeling data suggest that the fault in this area has a slip rate of about 40 mm/yr (Socquet et al., 2006). However, analysis of offset stream channels provides evidence of a lower slip rate for the Holocene (last 12,000 years), a rate of about 35 mm/yr (Bellier et al., 2001). Given the short time period for GPS observations, the GPS rate may include postseismic motion earlier earthquakes, though these numbers are very close.
Using empirical relations for historic earthquakes compiled by Wells and Coppersmith (1994), Socquet et al. (2016) suggest that the Palu-Koro fault system could produce a magnitude M 7 earthquake once per century. However, studies of prehistoric earthquakes along this fault system suggest that, over the past 2000 years, this fault produces a magnitude M 7-8 earthquake every 700 years (Bellier et al., 2006). So, it appears that this is the characteristic earthquake we might expect along this fault.
Most commonly, we associate tsunamigenic earthquakes with subduction zones and thrust faults because these are the types of earthquakes most likely to deform the seafloor, causing the entire water column to be lifted up. Strike-slip earthquakes can generate tsunami if there is sufficient submarine topography that gets offset during the earthquake. Also, if a strike-slip earthquake triggers a landslide, this could cause a tsunami. We will need to wait until people take a deeper look into this before we can make any conclusions about the tsunami and what may have caused it.

There have been tsunami waves recorded on a tide gage over 300 km to the south of the epicenter, at a site called Mumuju. Below is a map and a plot of water surface elevations from this source.

I was interviewed by Henry Fountain (NYT) today. Here is the story he wrote.

Here is the map with a month’s seismicity plotted.

Here is the map with a centuries worth of seismicity plotted.

Here is a map that shows the updated USGS model of ground shaking. The USGS prepared an updated earthquake fault slip model that was additionally informed by post-earthquake analysis of ground deformation. The original fault model extended from north of the epicenter to the northernmost extent of Palu City. Soon after the earthquake, Dr. Sotiris Valkaniotis prepared a map that showed large horizontal offsets across the ruptured fault along the entire length of the western margin on Palu Valley. This horizontal offset had an estimated ~8 meters of relative displacement. InSAR analyses confirmed that the coseismic ground deformation extended through Palu Valley and into the mountains to the south of the valley.

My 2018.10.01 BC Newshour Interview

InSAR Analysis

Synthetic Aperture Radar (SAR) is a remote sensing method that uses Radar to make observations of Earth. These observations include the position of the ground surface, along with other information about the material properties of the Earth’s surface.
Interferometric SAR (InSAR) utilizes two separate SAR data sets to determine if the ground surface has changed over time, the time between when these 2 data sets were collected. More about InSAR can be found here and here. Explaining the details about how these data are analyzed is beyond the scope of this report. I rely heavily on the expertise of those who do this type of analysis, for example Dr. Eric Fielding.

I prepared a map using the NASA-JPL InSAR data. They post all their data online here. I used the tiff image as it is georeferenced. However, some may prefer to use the kmz file in Google Earth.
I include the faults mapped by Wilkinson and Hall (2017), the PGA contours from the USGS model results. More on Peak Ground Acceleration (PGA) can be found here. I also include the spatial extent of the largest landslides that I mapped using post-earthquake satellite imagery provided by Digital Globe using their open source imagery program.

M 7.5 Landslide Model vs. Observation Comparison

Landslides during and following the M=7.5 earthquake in central Sulawesi, Indonesia possibly caused the majority of casualties from this catastrophic natural disaster. Volunteers (citizen scientists) have used satellite aerial imagery collected after the earthquake to document the spatial extent and magnitude of damage caused by the earthquake, landslides, and tsunami.
Until these landslides are analyzed and compared with regions that did not fail in slope failure, we will not be able to reconstruct what happened… why some areas failed and some did not.
There are landslide slope stability and liquefaction susceptibility models based on empirical data from past earthquakes. The USGS has recently incorporated these types of analyses into their earthquake event pages. More about these USGS models can be found on this page.
I prepared some maps that compare the USGS landslide and liquefaction probability maps. Below I present these results along with the MMI contours. I also include the faults mapped by Wilkinson and Hall (2017). Shown are the cities of Donggala and Palu. Also shown are the 2 tide gage locations (Pantoloan Port – PP and Mumuju – M). I also used post-earthquake satellite imagery to outline the largest landslides in Palu Valley, ones that appear to be lateral spreads.

Here is the landslide probability map (Jessee et al., 2018). Below the poster I include the text from the USGS website that describes how this model is prepared.

Nowicki Jessee and others (2018) is the preferred model for earthquake-triggered landslide hazard. Our primary landslide model is the empirical model of Nowicki Jessee and others (2018). The model was developed by relating 23 inventories of landslides triggered by past earthquakes with different combinations of predictor variables using logistic regression. The output resolution is ~250 m. The model inputs are described below. More details about the model can be found in the original publication. We modify the published model by excluding areas with slopes <5° and changing the coefficient for the lithology layer "unconsolidated sediments" from -3.22 to -1.36, the coefficient for "mixed sedimentary rocks" to better reflect that this unit is expected to be weak (more negative coefficient indicates stronger rock).To exclude areas of insignificantly small probabilities in the computation of aggregate statistics for this model, we use a probability threshold of 0.002.

Here is the liquefaction probability (susceptibility) map (Zhu et al., 2017). Note that the regions of low slopes in the valleys and coastal plains are the areas with a high chance of experiencing liquefaction. Areas of slopes >5° are excluded from this analysis.
Note that the large landslides (yellow polygons) are not in regions of high probability for liquefaction.

Zhu and others (2017) is the preferred model for liquefaction hazard. The model was developed by relating 27 inventories of liquefaction triggered by past earthquakes to globally-available geospatial proxies (summarized below) using logistic regression. We have implemented the global version of the model and have added additional modifications proposed by Baise and Rashidian (2017), including a peak ground acceleration (PGA) threshold of 0.1 g and linear interpolation of the input layers. We also exclude areas with slopes >5°. We linearly interpolate the original input layers of ~1 km resolution to 500 m resolution. The model inputs are described below. More details about the model can be found in the original publication.

Temblor Reports:

Click on the graphic to see a pdf version of the article.
Click on the html link (date) to visit the Temblor site.

	2018.09.28 The Palu-Koro fault ruptures in a M=7.5 quake in Sulawesi, Indonesia, triggering a tsunami and likely more shocks
	2018.10.03 Tsunami in Sulawesi, Indonesia, triggered by earthquake, landslide, or both
	2018.10.16 Coseismic Landslides in Sulawesi, Indonesia

2018.10.10 M 7.0 New Britain, PNG

In this region of the world, the Solomon Sea plate and the South Bismarck plate converge to form a subduction zone, where the Solomon Sea plate is the oceanic crust diving beneath the S.Bismarck plate.
The subduction zone forms the New Britain Trench with an axis that trends east-northeast. To the east of New Britain, the subduction zone bends to the southeast to form the San Cristobal and South Solomon trenches. Between these two subduction zones is a series of oceanic spreading ridges sequentially offset by transform (strike slip) faults.
Earthquakes along the megathrust at the New Britain trench are oriented with the maximum compressive stress oriented north-northwest (perpendicular to the trench). Likewise, the subduction zone megathrust earthquakes along the S. Solomon trench compress in a northeasterly direction (perpendicular to that trench).
There is also a great strike slip earthquake that shows that the transform faults are active.
This earthquake was too small and too deep to generate a tsunami.

Here is the map with a century’s seismicity plotted.

Temblor Reports:

Click on the graphic to see a pdf version of the article.
Click on the html link (date) to visit the Temblor site.

2018.10.10 M 7.5 Earthquake in New Britain, Papua New Guinea

2018.10.22 M 6.8 Explorer plate

This region of the Pacific-North America plate boundary is at the northern end of the Cascadia subduction zone (CSZ). To the east, the Explorer and Juan de Fuca plates subduct beneath the North America plate to form the megathrust subduction zone fault capable of producing earthquakes in the magnitude M = 9 range. The last CSZ earthquake was in January of 1700, just almost 319 years ago.
The Juan de Fuca plate is created at an oceanic spreading center called the Juan de Fuca Ridge. This spreading ridge is offset by several transform (strike-slip) faults. At the southern terminus of the JDF Ridge is the Blanco fault, a transtensional transform fault connecting the JDF and Gorda ridges.
At the northern terminus of the JDF Ridge is the Sovanco transform fault that strikes to the northwest of the JDF Ridge. There are additional fracture zones parallel and south of the Sovanco fault, called the Heck, Heckle, and Springfield fracture zones.
The first earthquake (M = 6.6) appears to have slipped along the Sovanco fault as a right-lateral strike-slip earthquake. Then the M 6.8 earthquake happened and, given the uncertainty of the location for this event, occurred on a fault sub-parallel to the Sovanco fault. Then the M 6.5 earthquake hit, back on the Sovanco fault.

Here is the map with a century’s seismicity plotted.

2018.10.25 M 6.8 Greece

Before I looked more closely, I thought this sequence might be related to the Kefallonia fault. I prepared some earthquake reports for earthquakes here in the past, in 2015 and in 2016.
Both of those earthquakes were right-lateral strike-slip earthquakes associated with the Kefallonia fault.
However, today’s earthquake sequence was further to the south and east of the strike-slip fault, in a region experiencing compression from the Ionian Trench subduction zone. But there is some overlap of these different plate boundaries, so the M 6.8 mainshock is an oblique earthquake (compressional and strike-slip). Based upon the sequence, I interpret this earthquake to be right-lateral oblique. I could be wrong.

Here is the map with a century’s seismicity plotted.

Here is the tide gage data from Katakolo, which is only 65 km from the M 6.8 epicenter.

Temblor Reports:

Click on the graphic to see a pdf version of the article.
Click on the html link (date) to visit the Temblor site.

2018.10.26 Greek earthquake in a region of high seismic hazard

2018.11.08 M 6.8 Mid Atlantic Ridge (Jan Mayen fracture zone)

There was a M = 6.8 earthquake along a transform fault connecting segments of the Mid Atlantic Ridge recently.
North of Iceland, the MAR is offset by many small and several large transform faults. The largest transform fault north of Iceland is called the Jan Mayen fracture zone, which is the location for the 2018.11.08 M = 6.8 earthquake.

Here is the map with a century’s seismicity plotted.

Here is the large scale map showing earthquake mechanisms for historic earthquakes in the region. Note how they mostly behave well (are almost perfectly aligned with the Jan Mayen fracture zone). There are a few exceptions, including an extensional earthquake possibly associated with extension on the MAR (2010.06.03 M = 5.6). Also, 2 earthquakes (2003.06.19 and 2005.07.25) are show oblique slip (not pure strike-slip as they have an amount of compressional motion) near the intersection of the fracture zone and the MAR.

2018.11.30 M 7.0 Alaska

Today’s earthquake occurred along the convergent plate boundary in southern Alaska. This subduction zone fault is famous for the 1964 March 27 M = 9.2 megathrust earthquake. I describe this earthquake in more detail here.
During the 1964 earthquake, the downgoing Pacific plate slipped past the North America plate, including slip on “splay faults” (like the Patton fault, no relation, heheh). There was deformation along the seafloor that caused a transoceanic tsunami.
The Pacific plate has pre-existing zones of weakness related to fracture zones and spreading ridges where the plate formed and are offset. There was an earthquake in January 2016 that may have reactivated one of these fracture zones. This earthquake (M = 7.1) was very deep (~130 km), but still caused widespread damage.
The earthquake appears to have a depth of ~40 km and the USGS model for the megathrust fault (slab 2.0) shows the megathrust to be shallower than this earthquake. There are generally 2 ways that may explain the extensional earthquake: slab tension (the downgoing plate is pulling down on the slab, causing extension) or “bending moment” extension (as the plate bends downward, the top of the plate stretches out.

Temblor Report

Here is the map with a century’s seismicity plotted.

Temblor Reports:

Click on the graphic to see a pdf version of the article.
Click on the html link (date) to visit the Temblor site.

	2018.11.30 Exotic M=7.0 earthquake strikes beneath Anchorage, Alaska
	2018.12.11 What the Anchorage earthquake means for the Bay Area, Southern California, Seattle, and Salt Lake City

2018.12.05 M 7.5 New Caledonia

There was a sequence of earthquakes along the subduction zone near New Caledonia and the Loyalty Islands.
This part of the plate boundary is quite active and I have a number of earthquake reports from the past few years (see below, a list of earthquake reports for this region).
But the cool thing from a plate tectonics perspective is that there was a series of different types of earthquakes. At first view, it appears that there was a mainshock with a magnitude of M = 7.5. There was a preceding M 6.0 earthquake which may have been a foreshock.
The M 7.5 earthquake was an extensional earthquake. This may be due to either extension from slab pull or due to extension from bending of the plate. More on this later.
Following the M 7.5, there was an M 6.6 earthquake, however, this was a thrust or reverse (compressional) earthquake. The M 6.6 may have been in the upper plate or along the subduction zone megathrust fault, but we won’t know until the earthquake locations are better determined.
A similar sequence happened in October/November 2017. I prepared two reports for this sequence here and here. Albeit, in 2017, the thrust earthquake was first (2017.10.31 vs. 2017.11.19).
There have been some observations of tsunami. Below is from the Pacific Tsunami Warning Center.

Here is the map with a month’s seismicity plotted.

Here is the map with a century’s seismicity plotted.

2018.12.20 M 7.4 Bering Kresla

2018.12.20 M 7.3 Bering Kresla UPDATE #1

A large earthquake in the region of the Bering Kresla fracture zone, a strike-slip fault system that coincides with the westernmost portion of the Aleutian trench (which is a subduction zone further to the east).
This earthquake happened in an interesting region of the world where there is a junction between two plate boundaries, the Kamchatka subduction zone with the Aleutian subduction zone / Bering-Kresla Shear Zone. The Kamchatka Trench (KT) is formed by the subduction (a convergent plate boundary) beneath the Okhotsk plate (part of North America). The Aleutian Trench (AT) and Bering-Kresla Shear Zone (BKSZ) are formed by the oblique subduction of the Pacific plate beneath the Pacific plate. There is a deflection in the Kamchatka subduction zone north of the BKSZ, where the subduction trench is offset to the west. Some papers suggest the subduction zone to the north is a fossil (inactive) plate boundary fault system. There are also several strike-slip faults subparallel to the BKSZ to the north of the BKSZ.

Here is the map with a month’s seismicity plotted, including the age of the crust.

Here is the map with a century’s seismicity plotted, with earthquakes M ≥ 6.0, including the age of the crust.

UPDATE #1

Here is the map with a month’s seismicity plotted.

Here is the map with a century’s seismicity plotted, with earthquakes M ≥ 6.0.

2018.12.29 M 7.0 Philippines

This magnitude M = 7.0 earthquake is related to the subduction zone that forms the Philippine trench (where the Philippine Sea plate subducts beneath the Sunda plate). Here is the USGS website for this earthquake.
The earthquake was quite deep, which makes it less likely to cause damage to people and their belongings (e.g. houses and roads) and also less likely that the earthquake will trigger a trans-oceanic tsunami.
Here are the tidal data:

Here is the map with a century’s seismicity plotted.

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

Here is another way to look at these beach balls.

The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj

— AK Earthquake Center (@AKearthquake) August 15, 2018

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Return to the Earthquake Reports page.

Sorted by Magnitude
Sorted by Year
Sorted By Region

Earthquake Report: Iran

Posted on November 25, 2018 by Jay Patton

This morning (my time) there was a possibly shallow earthquake in western Iran with a magnitude of M = 6.3. This earthquake occurred in the aftershock zone of the 2017.11.12 M 7.3 earthquake. Here is my report for the M 7.3 earthquake. Here are the USGS webpagea for the M 6.3 and M 7.3 earthquakes.
The M 7.3 earthquake was a reverse/thrust earthquake associated with tectonics of the Zagros fold and thrust belt. This plate boundary fault system is a section of the Alpide belt, a convergent plate boundary that extends from the west of the Straits of Gibraltar, through Europe (causing uplift of the Alps and subduction offshore of Greece), the Middle East, India (causing the uplift forming the Himalayas), then to end in eastern Indonesia (forming the continental collision zone between Australia and Indonesia).
Some of the earthquakes (including this one) are strike-slip earthquakes (see explanation of different earthquake types below in the geologic fundamentals section). So, one might ask why there are strike-slip earthquakes associated with a compressional earthquake?
As pointed out by Baptiste Gombert, these strike-slip earthquakes are are evidence of strain partitioning. Basically, when relative plate motion (the direction that plates are moving relative to each other) is not perpendicular or parallel to a tectonic fault, this oblique motion is partitioned into these perpendicular and parallel directions.
A great example of this type of strain partitioning is the plate boundary offshore of Sumatra where the India-Australia plate subducts beneath the Sunda plate (part of Eurasia). The plate boundary is roughly N45W (oriented to the northwest with an azimuth of 325°) and the relative plate motion direction is oriented closer to a north-south orientation. The relative plate motion perpendicular to the plate boundary is accommodated by earthquakes on the subduction. These earthquakes are oriented showing compression in a northeast direction. Along the axis of Sumatra is a huge strike-slip fault called the Great Sumatra fault. This fault is parallel to the plate boundary and accommodates relative plate motion parallel to the plate boundary. The Great Sumatra fault is a fault called a forearc sliver fault.
There are other examples of this elsewhere, like here in western Iran/eastern Iraq. Relative plate motion between the Arabia and Eurasia plates is oriented north-south, but the plate boundary is oriented northwest-southeast (just like the Sumatra example). So this oblique relative plate motion is partitioned into fault normal compression (the M 7.3 earthquake) and fault parallel shear (today’s M 6.3 earthquake).
There is also a strike-slip fault in the region of today’s M 6.3, the Khanaqin fault. So, given what we know about the tectonics and historic seismicity, I interpret today’s M 6.3 earthquake to have been a strike-slip earthquake associated with the Khanaqin fault, triggered by changes in stress by the M 7.3 earthquake. I could be incorrect and this earthquake could be unrelated to the > 7.3 earthquake.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 5.0 in one version.
I include an inset map showing seismicity from 2016.11.22 through 2018.11.28 showing the aftershocks from the M 7.3 earthquake. Note the cluster of earthquakes to the south of the aftershock zone. This is a swarm with earthquakes in the lower to mid M 5 range. The earthquakes with mechanisms are compressional, oriented the same as the M 7.3.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

In the upper left corner is a map showing the regional plate boundary faults and some information about relative plate motions (Stern and Johnson, 2010). As for other inset figures, I plate a transparent cyan star in the general location of today’s M 6.3 earthquake.
In the lower left corner is a similarly scaled tectonic map from Scharf et al. (2015) showing more information about the amount of plate motion in the Tertiary (post 66 Ma). Note the contrast of the extension (in red) associated with the rifting in east Africa and the convergence (in blue) associated with the Alpide belt in this area.
In the upper right corner is a structural cross section showing the folding of the crust and rocks associated with the convergence at this plate boundary (Verges et al., 2011). I show the general location for this cross section on the map as a cyan line with balls on the ends.
In the lower left center is a map from Emami et al. (2010). This map shows how this convergent plate boundary creates topography (uplift and mountains) with color. Lower elevations are shown as yellow and green and higher elevations are shown as red and brown. Note the location of the Khanaqin fault, a left-lateral strike slip fault..
In the upper left center is a map showing a kinematic interpretation of the faulting in this area (Hessami, 2002). While the focus of this PhD dissertation is for the faulting in the southern Zagros system, they show relative plate motions and how the Khanaqin fault may accommodate this plate motion (oblique to Zagros).
In the lower right corner is a map showing USGS seismicity from 2016.11.22 through 2018.11.25 for earthquakes M ≥ 3.0. The spatial extent of this area is shown in a dashed white rectangle on the main map.
In the lower right center is the USGS seismic hazard map for the region (Jenkins et al., 2014).

Here is the map with a month’s seismicity plotted.

Here is the map with a century’s seismicity plotted for earthquakes M ≥ 5.0.

Other Report Pages

Some Relevant Discussion and Figures

The Alpide Belt, shown in this map, is a convergent plate boundary that extends from Australia to Portugal. This map shows the westernmost extent of this system. The convergence here drives uplift of the Himalayas and the European Alps. Subduction along the Makran and Sunda subduction zones are also part of this system.

Below is the tectonic map from Stern and Johnson (2010).

Simpli”ed map of the Arabian Plate, with plate boundaries, approximate plate convergence vectors, and principal geologic features. Note location of Central Arabian Magnetic Anomaly (CAMA).

Here is the Scharf et al., 2015 map.

Tectonic setting of the Arabian Plate. Red and blue coloured symbols indicate divergence and convergence with overall amount and age, respectively. Green arrows show present-day GPS values with respect to fixed Europa from Iran [21] and white arrow from Oman [22]. a – [23]; b – [20]; c – [18]; d – [19]; e – [14]; f – [15]; g – [8]; h – [16]; i – [17]

This is the Enami et al., 2010 figure.

Tectonic map of the Zagros Fold Belt showing the position and geometry of the Mountain Front Flexure (MFF). Earthquakes of M ≥ 5 are indicated by small black diamonds. Focal mechanisms from Talebian & Jackson (2004) are also shown, in black (Mw ≥ 5.3) and grey (Mw ≥ 5.3). KH, Khavir anticline; SI, Siah Kuh anticline; ZDF, Zagros Deformation Front.

This is the tectonic map from Hessami, 2002.

a) Earthquakes with mb > 5.0 (Jackson and McKenzie, 1984) along seismogenic basement thrusts offset by major strike-slip faults. b) Schematic interpretative map of the main structural features in the Zagros basement. The overall north-south motion of Arabia increases along the belt from NW to SE (arrows with numbers). Central Iran acted as a rigid backstop and caused the strike-slip faults with N-S trends in the west to bulge increasingly eastward. Fault blocks in the north (elongated NW-SE) rotate anticlockwise; while fault blocks in the south (elongated NE-SW) rotate clockwise. c) Simple model involving parallel paper sheets illustrating the observed strike-slip faults in the Zagros. Opening between the sheets (i.e. faults) helped salt diapirs to extrude.

Below are a series of figures from Verges et al., 2011. First is a map that shows the tectonics and locations of the cross section.

Tectonic map of the Zagros showing the location of the previously published cross-sections with the calculated amount of shortening and the extent of major hydrocarbon fields. The balanced cross-section is marked by the thick black line. M – Mand anticline. Dark grey: Naien-Baft ophiolites (Stöklin, 1968).

Here are the cross sections from Verges et al. (2011).

Structural cross-sections showing the style of folding across the studied regional transect (see location in Fig. 3). (a) The front of the Zagros Fold Belt along the Anaran anticline above the Mountain Front Flexure (MFF in Emami et al. 2010); (b) the Kabir Kuh anticline, which represents a multi-detachment fold (Vergés et al. 2010); (c) folds developed in the Upper Cretaceous basinal stratigraphy showing much tighter and upright anticlines (modified from Casciello et al. 2009).

Here is a map that displays an estimate of seismic hazard for the region (Jenkins et al., 2010). This comes from Giardini et al. (1999).

The Global Seismic Hazard Map. Peak ground acceleration (pga) with a 10% chance of exceedance in 50 years is depicted in m/s2. The site classification is rock everywhere except Canada and the United States, which assume rock/firm soil site classifications. White and green correspond to low seismicity hazard (0%-8%g), yellow and orange correspond to moderate seismic hazard (8%-24%g), pink and dark pink correspond to high seismicity hazard (24%-40%g), and red and brown correspond to very high seismic hazard (greater than 40%g).

Just found this as it as posted to the Bertrand tweet (see social media below). This is a figure from Talebian and Jackson (2004) that uses Sumatra as an analogue to the oblique convergence along the Zagros thrust. Pretty cool.

(a) Summary sketch of the tectonic pattern in the Zagros. Overall Arabia–Eurasia motions are shown by the big white arrows, as before. In the NW Zagros (Borujerd-Dezful), oblique shortening is partitioned into right-lateral strike-slip on the Main Recent Fault (MRF) and orthogonal shortening (large gray arrows). In the SE Zagros (Bandar Abbas) no strike-slip is necessary, as the shortening is parallel to the overall convergence. The central Zagros (Shiraz) is where the transition between these two regimes occurs, with anticlockwise rotating strike-slip faults allowing an along-strike extension (black arrows) between Bandar Abbas and Dezful. (b) A similar sketch for the Himalaya (after McCaffrey & N´abˇelek 1998). In this case the overall Tibet-India motion is likely to be slightly west of north. (The India-Eurasia motion is about 020◦, but Tibet moves east relative to both India and Eurasia: Wang et al. 2001). Thrust faulting slip vectors are radially outward around the entire arc (gray arrows). This leads to partitioning of the oblique convergence in the west, where right-lateral strike-slip is prominent on the Karakoram Fault, but no strike-slip in the east, where the convergence and shortening are parallel. The region in between extends parallel to the arc, on normal faults in southern Tibet. (c) A similar sketch for the Java–Sumatra arc, based on McCaffrey (1991). Slip partitioning occurs in the NW, with strike-slip faulting through Sumatra, but not in the SE, near Java. This change along the zone requires the Java–Sumatra forearc to extend along strike.

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

Here is another way to look at these beach balls.

The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj

— AK Earthquake Center (@AKearthquake) August 15, 2018

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Middle East

General Overview

Earthquake Reports

2018.11.25 M 6.3 Iran/Iraq
2017.12.01 M 6.1 Iran
2017.11.12 M 7.3 Iraq
2017.02.08 M 6.3 Makran subduction zone (Pakistan)
2015.10.27 M 7.5 Afghanistan

Social Media

Significant #earthquake in #Iran, likely an aftershock of the M7.3 Ezgeleh earthquake of November 2017. The difference in focal mechanism reveals slip partitionning in the region. 2 other large strike-slip aftershocks were also recorded last summer pic.twitter.com/P2BOzGI625

— Baptiste Gombert (@BaptisteGomb) November 25, 2018

Mw=6.3, IRAN-IRAQ BORDER REGION (Depth: 10 km), 2018/11/25 16:37:31 UTC – Full details here: https://t.co/YoEYOD1agB pic.twitter.com/u54xzgx8ol

— Earthquakes (@geoscope_ipgp) November 25, 2018

strong #earthquake along #Iran #Iraq border, felt #Baghdad, #Kirkuk and #Mosul in Iraq and in #Kermanshah, #Hamadan, #Sulaymaniyah in Iran, even even #Kuwait @LastQuake @Quake_Tracker @JuskisErdbeben @UKEQ_Bulletin pic.twitter.com/NpLVsxxunx

— CATnews (@CATnewsDE) November 25, 2018

GFZ moment tensor solution of M6.3 earthquake on Iran-Iraq border https://t.co/ri4JlRyY3K #earthquake pic.twitter.com/VXAO5EdvNO

— Aram Fathian (@AramFathian) November 25, 2018

Earthquake in Irak Iran border was widely felt more than 500 km away. Local damage close to the epicentre cannot be excluded, but having struck an area of low population, no widespread damage is expected pic.twitter.com/AaxB5X0ZX8

— EMSC (@LastQuake) November 25, 2018

Mwp6.1 #earthquake Iran – Iraq Border Region 2018.11.25-16:37:34UTC https://t.co/kCIw9Vypa6

— Anthony Lomax 🌍🇪🇺 (@ALomaxNet) November 25, 2018

My thoughts and solidarity to the people affected by #IranEarthquake. Deeply proud of our @Iranian_RCS volunteers and staff, who are ready to support their local communities. pic.twitter.com/Axi1dlRFjQ

— Francesco Rocca (@Francescorocca) November 25, 2018

UPDATE: 2018.11.26

An interesting comparison of the latest M6.3 #Iran #Iraq #earthquake aftershocks and the 2013 #Khanaqin earthquake sequence. Epicenters from IRSC & @IRIS_EPO , focal mechanisms from GFZ pic.twitter.com/xTpds1Ke6V

— Sotiris Valkaniotis (@SotisValkan) November 26, 2018

Wrapped interferogram (2.8 cm/1 inch color contours) for M6.3 earthquake near Iran-Iraq border from automatic processing of Copernicus Sentinel-1 SAR by NASA Caltech-JPL ARIA and ESA, with USGS epicenter (star). No sign of surface ruptures, so all fault slip was at depth pic.twitter.com/7eMx6LcpbB

— Eric Fielding (@EricFielding) November 26, 2018

#Sentinel1 #InSAR descending interferogram for the M6.3 #Iran #Iraq #earthquake. No clear indications for surface ruptures, most of the slip occured at depth. Processed with DIAPASON at @esa_gep using @CopernicusEU #Sentinel1 data. pic.twitter.com/2Aj9y1759o

— Sotiris Valkaniotis (@SotisValkan) November 26, 2018

Simulated coseismic ground deformation map of M6.3 earthquake near Iran/Irap border from our "quickdeform" platform: https://t.co/lrLi8Nrbnt.
This website automatically display coseismic deformation maps of recent M >= 6 earthquakes for rapid hazard evaluations. pic.twitter.com/gDqRceAHK7

— Wenbin Xu (@WenbXu) November 27, 2018

References:

Allen, M.B., Saville, C., Blac, E.K-P., Talebian, M., and Nissen, E., 2013. Orogenic plateau growth: Expansion of the Turkish-Iranian Plateau across the Zagros fold-and-thrust belt in Tectonics, v. 32, p. 171-190, doi:10.1002/tect.20025
Emami, H., Verges, J., nalpas, T., Gillespie, P., Sharp, I., Karpuz, R., Blanc, E.P., and Goodarzi, G.H., 2010. Structure of the Mountain Front Flexure along the Anaran anticline in the Pusht-e Kuh Arc (NW Zagros, Iran): insights from sand box models in LETURMY, P. & ROBIN, C. (eds) Tectonic and Stratigraphic Evolution of Zagros and Makran during the Mesozoic–Cenozoic. Geological Society, London, Special Publications, 330, 155–178.
Giardini, D., Grunthal, G., Shedlock, K., Zhang. P., and Global Seismic Hazards Program, 1999. Global seismic hazards map: Accessed on Jan. 9, 2007 at http://www.seismo.ethz.ch/GSHAP.
Hessami, K., 2002. Tectonic History and Present-Day Deformation in the Zagros Fold-Thrust Belt, PhD for the Degree of Doctor of Philosophy in Mineralogy, Petrology, and Tectonics presented at Uppsala University in 2002, ISBN 91-554-5285-5
Jenkins, Jennifer, Turner, Bethan, Turner, Rebecca, Hayes, G.P., Sinclair, Alison, Davies, Sian, Parker, A.L., Dart, R.L., Tarr, A.C., Villaseñor, Antonio, and Benz, H.M., compilers, 2013, Seismicity of the Earth 1900–2010 Middle East and vicinity (ver 1.1, Jan. 28, 2014): U.S. Geological Survey Open-File Report 2010–1083-K, scale 1:7,000,000, https://pubs.usgs.gov/of/2010/1083/k/.
Scharf, A., Mattern, F., and Al Sadi, S., 2016. Kinematics of Post-obduction Deformation of the Tertiary Ridge at Al-Khod Village (Muscat Area, Oman) in SQU Journal for Science, v. 21, no. 1, p. 26-40
Stern, R.J. and Johnson, P., 2010. Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis in Earth-Science Reviews, v. 101, p. 29-67.
Talebian and Jackson, 2004. A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran in GJI, v. 156, no. 3, P. 506–526, https://doi.org/10.1111/j.1365-246X.2004.02092.x
Taymaz, T., Yilmaz, Y., and Dilek, Y., 2007. The geodynamics of the Aegean and Anatolia: introduction in Geological Society, London, Special Publications, v. 291; p. 1-16, doi:10.1144/SP291.1
Verges, J., Saura, E., Casciello, E., Fernandez, M., Villasenor, A., Jimenez-Munt, I., and Garcia-Castellanos, D., 2011. Crustal-scale cross-sections across the NW Zagros belt: implications for the Arabian margin reconstruction in Geol. Mag, v. 148, no. 5-6, p. 739-761, doi:10.1017/S0016756811000331
Woudloper, 2009. Tectonic map of southern Europe and the Middle East, showing tectonic structures of the western Alpide mountain belt.

Return to the Earthquake Reports page.

Sorted by Magnitude
Sorted by Year
Sorted By Region

Earthquake Report: Hokkaido, Japan

Posted on September 5, 2018 by Jay Patton

Following the largest typhoon to strike Japan in a very long time, there was an earthquake on the island of Hokkaido, Japan today. There is lots on social media, including some spectacular views of disastrous and deadly landslides triggered by this earthquake (earthquakes are the number 1 source for triggering of landslides). These landslides may have been precipitated (sorry for the pun) by the saturation of hillslopes from the typhoon. Based upon the USGS PAGER estimate, this earthquake has the potential to cause significant economic damages, but hopefully a small number of casualties. As far as I know, this does not incorporate potential losses from earthquake triggered landslides [yet].
This earthquake is in an interesting location. to the east of Hokkaido, there is a subduction zone trench formed by the subduction of the Pacific plate beneath the Okhotsk plate (on the north) and the Eurasia plate (to the south). This trench is called the Kuril Trench offshore and north of Hokkaido and the Japan Trench offshore of Honshu.
The okhotsk plate is considered part of the North America plate on some maps. The location of the plate boundary of the Okhotsk plate are not well understood (e.g. using GPS plate motion velocities, it is difficult to find the northern boundary with the North America plate).
Many of the earthquakes in this region are related to the subduction zone. Most notably is the 2011 Tohoku-oki M 9.1 tsunamigenic earthquake. More background information about the 2011 earthquake can be found here and information about the tsunami can be found here.
The 2011 earthquake had lots of aftershocks and was quite complicated. One interesting thing that happened is that there was an extensional earthquake in the Pacific plate to the west of the Japan Trench. This M 7.7 earthquake happened along faults formed as the Pacific plate bends near where it meets the trench. Similar subduction zone / outer rise earthquake pairs are known, including some along the New Hebrides Trench in the western equatorial Pacific ocean, as well as further north along the Kuril subduction zone. I spend time discussing the 2006/2007 Kuril earthquake pair in this report.
There was also a subduction zone earthquake in 2003, the Tokachi-oki earthquake, that triggered submarine landslides. These landslides transformed into turbidity currents and these were directly observed with offshore instrumentation.
One of the interesting things about this region is that there is a collision zone (a convergent plate boundary where two continental plates are colliding) that exists along the southern part of the island of Hokkaido. The Hidaka collision zone is oriented (strikes) in a northwest orientation as a result of northeast-southwest compression. Some suggest that this collision zone is no longer very active, however, there are an abundance of active crustal faults that are spatially coincident with the collision zone.
Today’s M 6.6 earthquake is a thrust or reverse earthquake that responded to northeast-southwest compression, just like the Hidaka collision zone. However, the hypocentral (3-D) depth was about 33 km. This would place this earthquake deeper than what most of the active crustal faults might reach. The depth is also much shallower than where we think that the subduction zone megathrust fault is located at this location (the fault formed between the Pacific and the Okhotsk or Eurasia plates). Based upon the USGS Slab 1.0 model (Hayes et al., 2012), the slab (roughly the top of the Pacific plate) is between 80 and 100 km. So, the depth is too shallow for this hypothesis (Kuril Trench earthquake) and the orientation seems incorrect. Subduction zone earthquakes along the trench are oriented from northwest-southweast compression, a different orientation than today’s M 6.6.
So today’s M 6.6 earthquake appears to have been on a fault deeper than the crustal faults, possibly along a deep fault associated with the collision zone. Though I am not really certain. This region is complicated (e.g. Kita et al., 2010), but there are some interpretations of the crust at this depth range (Iwasaki et al., 2004) shown in an interpreted cross section below.
I present more about the basics behind ground shaking, triggered landslides, and possible earthquake triggering on Temblor here:

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
I also include active crustal faults from the Coordinating Committee for Geoscience Programmes in East and Southeast Asia (CCOP). Note the abundance of north-northwest oriented yellow lines to the east of today’s earthquakes. While today’s earthquake was not on those crustal faults, the earthquakes and these faults are responding to similarly oriented tectonic stresses.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

Magnetic Anomalies

In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
Note the parallel magnetic anomalies to the east of Japan. These were formed about 150 million years ago at the spreading center where this portion of the Pacific plate was created. More can be found about the creation of the Pacific plate in Boschman and van Hinsbergen, 2(016).

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

In the upper right corner is a low angle oblique view of the tectonic configuration in this region. Note how many subduction zones are that are interacting in different ways. This is from the AGU blog, “Trembling Earth.” I place a blue star in the general location of today’s earthquakes (same for other figures in this poster).
In the lower right corner is a plate tectonic map of this part of the world (Liu et al., 2013). The major plate boundary faults are shown, along with the volcanoes in the magmatic arcs. Also, seismicity is shown (the 2011 earthquake as a small blue star) and the slab contours for the Pacific and Philippine Sea plates. Color shows the age of the oceanic crust. These authors place the southern boundary of the Okhotsk plate further to the south (dashed black line), where the Izu Collision Zone intersects Japan (near the intersection of the magmatic arc associated with the Izu-Bonin Trench, with Japan).
In the lower left corner is a geologic map of Japan (van Horne et al., 2016). Note the orientation of the rocks in Hokkaido as they are oriented in a northwest-southeast direction in the area labeled Hidaka Collision. These rocks are oriented this way due to the northeast-southwest convergence. This map places the southern boundary of the Okhotsk plate near where the Hidaka Collision is. Compare this with the Liu map to the right.
In the upper left corner is a large scale portion of a figure from NUMO (Kurikami et al., 2009), a publication put together by the N to evaluate the suitability of sites for high level radioactive waste. They considered various geologic hazards in this report. This map shows some key tectonic features and geologic data. I include the legend to the right of the map. The magmatic arc is shown as a red line. The Hidaka Collision Zone is shown as a dashed blue line with arrows showing the direction of collision. The blue arrows show the direction of maximum stress, the stress field. These arrows are pointed in the direction of compression. The convergence direction along the collision zone is oriented well with today’s earthquakes, but the stress field data are not perfectly oriented.

Here is the map with a month’s seismicity plotted.

Here is the map with a centuries seismicity plotted.

Some Relevant Discussion and Figures

This map shows the current tectonic configuration of this region, along with some inherited features from the tectonic past (e.g. green lines). This is from NUMO’s report: “Evaluating Site Suitability for a HLW Repository (Scientific Background and Practical Application of NUMO’s Siting Factors), NUMO-TR-04-04.”

Also from the NUMO report, this shows the Niigata-Kobe fold and thrust belt. In addition, this map shows a northwest striking convergent plate boundary along the southeastern boundary of Hokkaido. However, it cannot explain the interesting orientation of the M 6.2 deep (240 km) earthquake.

Here is a great figure from Itoh et al. (2005) that shows how they interpret the Hidaka Collision Zone.

Maps showing tectonic context around the Japanese Islands (a) and geologic belts in Hokkaido (b; after Kato et al., 1990).

This map (also from Itoh et al., 2005) shows the active faults and folds mapped in the region, along with the geology.

Geologic map around the Umaoi anticline redrawn from Geological Survey of Japan (2002). Location of active fault and/or fold scarps (after Ikeda et al., 2002) are also shown. buQ and bdQ attached on fault traces are upthrown and downthrown sides of faults, respectively. Sampling points of surface paleomagnetic data is after Kodama et al. (1993).

Here is more evidence for the thrust faults associated with the Hidaka Collision Zone (Iwasaki et al., 2004). These authors used seismic refraction and seismic reflection experiments to interpret the deep crustal structures associated with the collision here. The profile shown in the next figure is denoted by the east-west oriented black arrows in the lower part of this figure.

Geological map of Central Hokkaido with our seismic refraction/wide-angle reflection profiles and shot points (stars). Seismic reflection lines of the Hokkaido Transect were laid out from shot L-2 to M-5 on the wide-angle line. Reflection lines carried out from 1994 to 1997 in the southernmost part of the HCZ and refraction/wide-angle reflection lines in 1984 and 1992 are also shown. SYB: Sorachi-Yezo Belt; KMB: Kamuikotan Metamorphic Belt; IB: Idon’nappu Belt; HMB: Hidaka Metamorphic Belt; HB: Hidaka Belt; YB: Yubetsu Belt; TB: Tokoro Belt; HMT: Hidaka Main Trust.

Here is the interpreted cross section from Iwasaki et al. (2004). Note (1) the thrust faults and (2) the depths for these different structures. There are still regions that are poorly understood. Recall the depth of the M 6.6 earthquake is about 33 km.

Geological interpretation of the seismic model. KMB: Kamuikotan Metamorphic Belt; IB: Idon’nappu Belt; HMB: Hidaka Metamorphic Belt; Yz: Yezo Super Group; Sr: Sorachi Group; HMT: Hidaka Main Thrust.

Here is the cool tectonic map from Liu et al. (2013). We all like cool maps! (right?)

Tectonic settings of the study region (black box). The solid sawtooth lines and the black dashed line denote the plate boundaries (Bird 2003). The red triangles denote the active volcanoes. The blue dashed lines and the pink lines denote the depth contours to the upper boundary of the subducting Pacific slab and that of the subducting Philippine Sea slab, respectively (Hasegawa et al. 2009; Zhao et al. 2012). The topography data are derived from the GEBCO_08 Grid, version 20100927, http://www.gebco.net. The ages of oceanic plates are from M¨uller et al. (2008).

This is a very cool figure (also from Liu et al., 2013) that shows a plot of earthquakes from 3 different perspectives. First is the map view. To the right of the map is a plot of earthquakes shown as viewed from the east of the map and this shows the hypocenters. The profile below the map shows a cross section of seismicity as viewed from the south looking north. The original figure includes more maps (A and B).

(c) Distribution of the 4803 earthquakes used in
this study. The black crosses denote 3818 events (Group-1) that occurred under the seismic network. The green dots show 228 events (Group-2) that occurred outside the seismic network, selected from the events relocated by Gamage et al. (2009) using sP depth phases. The red dots denote 757 suboceanic earthquakes (Group-3) that are newly relocated in this work using P-wave, S-wave and sP depth-phase data. (d) East–west and (e) north–south vertical cross-sections of the earthquakes shown in (c).

Here is a map from the recent update of the Japan National Seismic Hazard Maps, resulting from knowledge gained following the 2011 M 9.1 earthquake (Fujiwara et al., 2012). The color represents the chance that a region will experience ground shaking at or greater that Japan Meteorological Agency (JMA) seismic intensity 6 in the next 30 years. JMA intensity is a scale of shaking intensity similar to the Modified Mercalli Intensity (MMI) Scale. The numbers are different, so they are difficult to compare. The JMA intensity 6 is similar to MMI X. Today’s earthquakes are in a region of slightly elevated chance of ground shaking (between 6-26%). Today’s M 6.6 earthquake may have reached

This is a map from the National Research Institute for Earth Science and Disaster Resilience, where data from the Strong-motion Seismograph Networks in Japan are located. This shows measurements of JMA intensity. It appears that a site near the epicenter (red star) reached JMA intensity 7.

This is an animation from the same source showing observations of JMA intensity recorded at the surface throughout Japan. h/t to Jascha Polet for sharing this on twitter.

Here is the upper figure showing the tectonic setting (Kurikami et al., 2009). Note how the Okhotsk plate has a strike-slip fault that terminates near the Hidaka Collision Zone (called a forearc-sliver fault, formed because the plate convergence is oblique to the subduction zone fault). I include their figure caption as a blockquote.

Tectonic setting of Kyushu within the Japanese island arc. The locations of active faults and volcanoes that have been active in the last 10,000 years are also shown.

This is a fantastic educational video from IRIS that discusses the plate tectonics and mentions some earthquakes in the region of Japan.

Here is a USGS poster than summarizes the earthquake history and plate geometry for this region. This is the USGS Open File Report 2010-1083-D (Rhea et al., 2010).

I put together an animation that shows the earthquake epicenters in Japan from 1900-2016/04/01. I include earthquakes with magnitude ≥ 6.0. Below is a screenshot of all these earthquakes, followed by the video. Here is the kml that I made using a USGS earthquake query. Here is the query that I used. The animation has an additional cross section showing the Japan trench, where the 2011/03/11 Tohoku-Oki M 9.0 subduction zone earthquake occurred. Here is a summary of the observations made following that 2011 earthquake.

Here is a link to the embedded video below (for download). (20 MB mp4)

Earthquake Triggered Landslides

Here is the aerial video from NHK that shows some of the landslides triggered by this sequence of earthquakes today. This comes from a tweet below.

Well, here is a great figure from Keefer (1984) that shows that the larger the magnitude of an earthquake, the larger an area can be subject to triggering of landslides from the ground shaking from that earthquake.

Area affected by landslides in earthquakes of different magnitudes. Numbers beside data points are earthquakes listed in Table 1. Dots = onshore earthquakes; x = offshore earthquakes. Horizontal bars indicate range in reported magnitudes. Solid line is approximate upper bound enclosing all data.

In 2008 there was an earthquake in China with a magnitude M 7.9. Unfortunately this earthquake caused many deaths. Using satellite imagery, geologists identified about 60,000 individual landslides (Gorum et al., 2011). Below is a map that shows the faults in the region, as well as epicenters from the earthquakes from this sequence.

Location and 12May 2008Wenchuan earthquake fault surface rupturemap, and focalmechanisms of the main earthquake (12May) and two of the major aftershocks (13 May and 25 May). Also the epicenters of historic earthquakes are indicated. The following faults are indicated: WMF: Wenchuan–Maowen fault; BF: Beichuan–Yingxiu fault; PF: Pengguan fault; JGF: Jiangyou–Guanxian fault; QCF: Qingchuan fault; HYF: Huya fault;MJF:Minjian fault based on the following sources: (Surface rupture: Xu et al., 2009a,b; Epicenter and aftershocks: USGS 2008; Historic earthquakes: Kirby et al., 2000; Li et al., 2008; Xu et al., 2009a,b).

This map shows the region where there was a high density of landslides (Fan et al., 2012). Note how the majority of landslides are located near the larger earthquakes (the larger circles in the above map).

Distribution of landslide dams triggered by the Wenchuan earthquake, China. The high landslide density zone is defined by a landslide area density >0.1 km−2; also shown are epicenters of historical earthquakes (USGS, 2008) and the historical Diexi landslide dams (Dahaizi, Xiaohaizi and Diexi). White polygons are unmapped due to the presence of clouds and shadows in post-earthquake imagery. WMF: Wenchuan–Maowen fault; YBF: Yingxiu–Beichuan fault; PF: Pengguan fault; JGF: Jiangyou–Guanxian fault; QCF: Qingchuan fault; HYF: Huya fault; MJF: Minjiang fault (after X. Xu et al., 2009). MJR: Minjiang River; MYR: Mianyuan River; JJR: Jianjiang River; QR: Qingjiang River.

Many of these landslides dammed rivers, causing an additional hazard. These earthen dams block rivers, leading to a large lake forming upstream of these dams. The dams can be overtopped when the lakes fill with water. once the water reaches the top of the dam, they can overflow and rapidly down cut back to the level of the river prior to the dam emplacement. If this happens too rapidly, a flood can occur, putting those downstream at risk of flooding.

Comparison of densities of blocking and non-blocking landslides. (a) Landslide density. (b) Landslide dam point density. White dashed lines are 240-km by 25-km swath profiles. (c). Mean normalized landslide and landslide dam densities along the SW–NE profile. Red lines are Yingxiu-Beichuan fault (YBF) and Pengguan fault (PF). Yellow dash lines are the boundary of the P1–P7 watersheds in the Pengguan Massif. YX, WC, HW, BC, and QC are the cities of Yingxiu, Wenchuan, Hanwang, Beichuan and Qingchuan, respectively. MJR, JJR, FJR, and QR represent Minjiang, Jianjiang, Fujiang and Qingjiang rivers, respectively.

In 1959, there was an earthquake in southwestern Montana, the M 7.2 Hebgen Lake Earthquake. This earthquake triggered a landslide that dammed the Madison River. This dam created a lake now called “Earthquake Lake.” I was actually driving on a road trip following my graduation from Oregon State University in 2014. I drove to this area and arrived the day that the Earthquake Lake Visitor Center opened. Pretty cool.
Here is a view of the lake as it was in May, 2014. Note the dead trees. The landslide is the bare looking mountainside in the distance on the left. We are looking to the West.

Here is a view of the landslide from my truck.

Here are all the people waiting to go into the visitor center on opening day.

Here is another cool view of the ghost forest.

Here is an educational display near the lake. Click on the image and one may zoom in within their browser, or save the image and zoom in that way. The text is readable if one wants to follow along.

This is from the poster and shows the landslide dam after it formed.

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

Here is another way to look at these beach balls.

The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj

— AK Earthquake Center (@AKearthquake) August 15, 2018

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Japan | Izu-Bonin | Mariana

Earthquake Reports

2018.09.05 M 6.6 Hokkaido, Japan
2016.07.29 M 7.7 Mariana
2016.11.21 M 6.9 Japan
2016.10.19 M 6.2 Japan
2016.08.20 M 6.0 Japan
2016.04.14 M 6.2 Japan
2016.04.01 M 6.0 Japan
2015.05.30 M 7.8 Izu Bonin
2015.05.31 M 7.8 Izu Bonin Update #1: triggered earthquakes
2015.05.31 M 7.8 Izu Bonin Update #2: Historic Seismicity
2015.06.09 M 7.8 Izu Bonin Update #3: seismic wave animations
2015.02.25 M 6.3 Japan (Sanriku Coast Update #5)
2015.02.21 M 6.7 Japan (Sanriku Coast Update #4)
2015.02.20 M 6.7 Japan (Sanriku Coast Update #3)
2015.02.16 M 6.7 Japan (Sanriku Coast Update #2)
2015.02.16 M 6.7 Japan (Sanriku Coast Update #1)
2015.02.16 M 6.7 Japan (Sanriku Coast)
2013.10.25 M 7.1 Japan (Honshu)
2011.03.11 M 9.0 Japan (Tohoku-Oki) Earthquake
2011.03.11 M 9.0 Japan (Tohoku-Oki) Tsunami

Social Media

If you need information in English, you can call to Hokkaido Disaster Prevention Information (available 24 hours).

Please refer to the link.#Japanearthquake #HokkaidoEarthquake

Emergency information for foreigners – News – NHK WORLD – English https://t.co/5mXIWbuYzU

— へニキ藤山 (@He2ki) September 6, 2018

Nice example of basin effects around Tokyo! https://t.co/r2UHEixJgK

— Emily Wolin (@GeoGinger) September 6, 2018

Japan has more measurable #earthquakes than any other country and has over 100 active volcanoes. These both result from Japan being wedged among four major tectonic plates. Learn more – https://t.co/KGI16OduAI #JapanEarthquake pic.twitter.com/ADbi2T8kGv

— IRIS Earthquake Sci (@IRIS_EPO) September 5, 2018

Landslides that seemed be happened by the 6th Sept 2018 M6.7 Hokkaido earthquake pic.twitter.com/eZAiculsHX

— Deepa Mele Veedu (@deepameleveedu) September 6, 2018

今、NHKでも中継見てるけど、信じられない光景……。https://t.co/6aOXDKqWtq pic.twitter.com/drZBljla0a

— よんます (@yonmas) September 6, 2018

NHK News stream – massive landslides, probably assisted by heavy rain in the previous 30 hours. Some houses were in the wrong place. Hopefully there wasn't anybody home, but at 3:08 am there probably was #Earthquake @LastQuake @TTremblingEarth @Ambassador_SR pic.twitter.com/p5fLJyNEfN

— Jamie Gurney (@UKEQ_Bulletin) September 5, 2018

#Sapporo #Hokkaido Massive landslide due M6.6 earthquake @davepetley pic.twitter.com/pOsnQAPVaK

— Luis Donoso (@Geo_Risk) September 6, 2018

釧路が停電し、街の明かりが消えていく様子#北海道地震 pic.twitter.com/ySa8Rg1kei

— saimon98 (@saimon98se) September 5, 2018

It took 11min 18sec for the first seismic waves from today's M6.6 quake in Japan to reach my @raspishake in Turlock, CA. Picking up quakes from 4,860mi away… NBD. pic.twitter.com/g5y5fsD0Qk

— Ryan Hollister (@phaneritic) September 6, 2018

Aerial video shows a landslide burying homes in Hokkaido after a strong magnitude 6.6 earthquake struck the northern Japan island 🎥: @nhk_news pic.twitter.com/pEXYLxnQ5m

— BuzzFeed Storm (@BuzzFeedStorm) September 5, 2018

Mw=6.6, HOKKAIDO, JAPAN REGION (Depth: 30 km), 2018/09/05 18:07:58 UTC – Full details here: https://t.co/IS1AVC6Enn pic.twitter.com/WngxJgHQkp

— Earthquakes (@geoscope_ipgp) September 5, 2018

This shows bedded marine sediments (turbidites) on plane with sliding. Likely explains it. pic.twitter.com/Ayr9Fp9VDy

— Patrick Williams (@quake_science) September 6, 2018

Hundreds of landslides reported after 6.6 magnitude Japan quakehttps://t.co/MvqYg5kEez

— Carlo Meletti (@CarloMeletti) September 5, 2018

After A strong 6.6 #earthquake #Terremoto #Temblor a Lightning #storm over Hokkaido #Japón #Japan right now ⚡ pic.twitter.com/nGqf8TKb5o

— Teacher From PR 🌧️🌀🌩️ (@MaestroDEPR) September 5, 2018

今朝未明の北海道の地震は、当初は最大震度6強と見られましたが、最大震度7に修正されています。
震度7を観測した厚真町では土砂崩れが相次ぎ、新千歳空港は閉鎖、札幌市内で液状化現象、全道で停電など広範囲で影響が出ています。
今後も大きな余震に警戒が必要です。https://t.co/Tfi8PeI3gr pic.twitter.com/ZeVKpjduA5

— ウェザーニュース (@wni_jp) September 6, 2018

Helicopter rescues for those who authorities can reach – now the challenge is getting to those trapped inside the mud (currently nearly 20 missing). Currently 100 injured from the #japanearthquake in Hokkaido. Pics via NHK pic.twitter.com/Sp3bC49H4B

— Jake Sturmer (@JakeSturmer) September 6, 2018

3 million without power, all flights to New Chitose Airport cancelled today as 6.7 quake hits Hokkaido (Shindo 6+ in some parts) #Japanearthquake pic.twitter.com/w5SssxpyEd

— Jake Sturmer (@JakeSturmer) September 5, 2018

Bullet trains suspended too from the #Japanearthquake pic.twitter.com/dbfdTosIvM

— Jake Sturmer (@JakeSturmer) September 5, 2018

Tomari nuclear plant using emergency generators – News – NHK WORLD – English https://t.co/GT1FJ4bIvP

— patton_cascadia (@patton_cascadia) September 6, 2018

Near the epicenter, landslides wiped out homes in Atsuma. All of the missing are from this town.Helicopter crews are carrying out rescue operations. pic.twitter.com/MLubmtDTO4

— NHK WORLD News (@NHKWORLD_News) September 6, 2018

震度７を観測した北海道厚真町 NHKがドローンで撮影した映像です
　#nhk_news　#ドローン　 #地震　 #震度７　 #厚真町　
　 #土砂崩れ pic.twitter.com/jaSVW78yup

— NHKニュース (@nhk_news) September 6, 2018

UPDATE 2018.09.06

More details are emerging about the landslides triggered by 6th September 2018 Hokkaido earthquake. The high landslide density may reflect recent rainfall from typhoon Jebi:- https://t.co/tbg1zba6Za pic.twitter.com/wYWid5PZDg

— Dave Petley (@davepetley) September 6, 2018

Pre- and post-seismic image of 2018 Hokkaido earthquake. Phenomenal landslides.
Credit: https://t.co/Cu6UjVmIpQ pic.twitter.com/sZd7E4MTDT

— Jay Tung (@jaytung_earth) September 6, 2018

ALOS-2 InSAR interferogram of #HokkaidoEarthquake .
Source: https://t.co/7K2hfGwqM0
master and slave: 2018/08/23 & 2018/09/06 pic.twitter.com/zuLUM8nlbs

— Sadra Karimzadeh (@Sadra_Krmz) September 6, 2018

Liquefaction probability map after #HokkaidoEarthquake M 6.6 https://t.co/Or2K7xnZIB pic.twitter.com/Qz5uVGYL0m

— Sadra Karimzadeh (@Sadra_Krmz) September 6, 2018

UPDATE 2018.09.07

https://t.co/27q2xKv3Al

— temblor (@temblor) September 7, 2018

Actualización terremoto #Hoakkaido, Japón🇯🇵.

Asciende a 20 cifra de decesos; aún reportan personas desaparecidas.

Se observa licuefacción: suelos saturados de agua, que suben a superficie, pierden firmeza por la sacudida del sismo desestabilizando el suelo.

Créditos: NHK pic.twitter.com/zHOr1HR1tx

— SkyAlert (@SkyAlertMx) September 7, 2018

UPDATE 2018.09.08

Death toll rises to 30 in the aftermath of Japan's Hokkaido earthquakehttps://t.co/z9b8LdSvhZ

— TIME (@TIME) September 8, 2018

UPDATE 2018.09.09

NASA JPL-Caltech ARIA preliminary deformation map from Copernicus Sentinel-1 data for 5 September 2018 Hokkaido earthquake. Total motion is approximate due to very high noise level (low coherence), but deformation signal is between 9 and 14 cm of motion up and east. pic.twitter.com/YoL2IvdjSO

— Eric Fielding (@EricFielding) September 9, 2018

UPDATE 2018.09.10

Comparison view between 2015 (up) and post-earthquake (below) reveals the extent of co-seismic #landslides from the M6.6 #earthquake near Atsuma, Hokkaido, #Japan. Point cloud data for 2015 & 2018 extracted from aerial imagery provided by Japan Geographical Survey Institute. pic.twitter.com/lriXZt10Za

— Sotiris Valkaniotis (@SotisValkan) September 10, 2018

UPDATE 2018.09.11

Slippery volcanic soils blamed for deadly landslides during #Hokkaido earthquake, reports @guardianeco https://t.co/BhB0sm8kHW pic.twitter.com/sX8BFAXZfJ

— EGU (@EuroGeosciences) September 11, 2018

【地殻変動情報】だいち2号のSARデータを使用した解析による、 #平成30年北海道胆振東部地震に伴う地殻変動分布図を公開しました。
今後も地殻変動の監視を続けていきます。
詳細はこちら→https://t.co/vDFA0hYtOd pic.twitter.com/ivFreV1bFL

— 国土地理院 (@GSI_chiriin) September 10, 2018

High quality drone footage has been posted on Facebook providing detailed views of the landslides from the 2018 Hokkaido Eastern Iburi earthquake:- https://t.co/pjMXJOAxkK pic.twitter.com/LZJ1zUzwk2

— Dave Petley (@davepetley) September 11, 2018

Potential liquefaction damage map in urban areas based on LiquickMap, slope map, differential InSAR coherence and weighted overlay analysis (WOA). #hokkaidoearthquake pic.twitter.com/rJejYS2D8Y

— Sadra Karimzadeh (@Sadra_Krmz) September 12, 2018

UPDATE 2018.09.12

GNSS and ALOS-2 InSAR observations, and fault model for Mj6.7 #HokkaidoEarthquake on Sep 6 by GSI. The depth of the upper edge of the fault is ~15km, much shallower than the hypocenter depth (>30km). https://t.co/iQBE4GwPtL pic.twitter.com/0MX9PhWG5h

— Yu Morishita (@Yu__Morishita) September 12, 2018

UPDATE 2018.09.16

Slippery volcanic soils blamed for deadly landslides during Hokkaido earthquake https://t.co/s3WAAygPOr

— temblor (@temblor) September 16, 2018

References:

Chapman et al., 2009. Development of Methodologies for the Identification of Volcanic and Tectonic Hazards to Potential HLW Repository Sites in Japan –The Kyushu Case Study-, NUMO-TR-09-02, NOv. 2009, 192 pp.
Fan, X., et al., 2012. Transient water and sediment storage of the decaying landslide dams induced by the 2008 Wenchuan earthquake, China in Geomorphology, v. 171-172, p. 58-68, doi:10.1016/j.geomorph.2012.05.003
Fujiwara, H., Morikawa, N., Okumura, T., Ishikawa, Y., and Nojima, N., 2012. Revision of Probabilistic Seismic Hazard Assessment for Japan after the 2011 Tohoku-oki Mega-thrust Earthquake (M9.0) in Proceedings of the 15th World Conference on Earthquake Engineering, 15th World Conference on Earthquake Engineering, Lisbon.
Gorum, T., Fan, X., van Westen, C.J., Huang, R., Xu, Q., Tang, C., Wang, G., 2011. Distribution pattern of earthquake-induced landslides triggered by the 12 May 2008 Wenchuan earthquake in Geomorphology, v. 133, p. 152-167, doi:10.1016/j.geomorph.2010.12.030
Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
Itoh, Y., Ishiuyama, T., and Nagasaki, Y., 2005. Deformation mode in the frontal edge of an arc–arc collision zone: subsurface geology, active faults and paleomagnetism in southern central Hokkaido, Japan in Tectonophysics, v. 395, p. 81-97 doi:10.1016/j.tecto.2004.09.003
Iwasaki, T., et al., 2004. Upper and middle crustal deformation of an arc–arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments in Tectonophysics, v. 388, p. 59-73, doi:10.1016/j.tecto.2004.03.025
Keefer, D.K., 1984. Landslides caused by earthquakes in Geological Society of America Bulletin, v. 95, p. 406-421, doi: 10.1130/0016-7606(1984)95<406:LCBE>2.0.CO;2
Kurikami et al., 2009. Study on strategy and methodology for repository concept development for the Japanese geological disposal project, NUMO-TR-09-04, Sept. 20-09, 101 pp.
Lay, T., and Kanamori, H., 1980, Earthquake doublets in the Solomon Islands: Physics of the Earth and Planetary Interiors, v. 21, p. 283-304.
Lay, T., Ammon, C.J., Kanamori, H., Kim, M.J., and Xue, L., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
Lay, T., H. Kanamori, C. J. Ammon, A. R. Hutko, K. Furlong, and L. Rivera, 2009. The 2006 – 2007 Kuril Islands great earthquake sequence in J. Geophys. Res., 114, B11308, doi:10.1029/2008JB006280.
Liu, X., Zhao, D., and Li, DS., 2013. Seismic heterogeneity and anisotropy of the southern Kuril arc: insight into megathrust earthquakes in Geophysical Journal International, Volume 194, Issue 2, 1 August 2013, Pages 1069–1090, https://doi.org/10.1093/gji/ggt150
Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., Furlong, K.P., and Benz, H.M., 2010. Seismicity of the Earth 1900-2007, Kuril-Kamchatka arc and vicinity: U.S. Geological Survey Open-File Report 2010-1083-C, 1 map sheet, scale 1:5,000,000.
Van Horne, A., Sato, H., Ishiyama, T., 2017. Evolution of the Sea of Japan back-arc and some unsolved issues in Tectonophysics, v. 710-711, p. 6-20, http://dx.doi.org/10.1016/j.tecto.2016.08.020

Return to the Earthquake Reports page.

Sorted by Magnitude
Sorted by Year
Sorted By Region

°
≥
ñ

Earthquake Report: Lombok, Indonesia: Update #1

Posted on August 7, 2018 by Jay Patton

UPDATE 2018.08.08

Based on Eric Fielding and JD Dianala’s interpretation of the InSAR data, the M 6.4 and M 6.9 earthquakes could possibly have a similar hypocentral depth. See Social Media update below.
Find out more about InSAR (Interferometric Synthetic Aperture Radar) here.
In addition, as Dr. Anthony Lomax pointed out, the USGS depth uncertainty is large enough for these earthquakes that they may be along the same fault.
Dr. Fielding uses the InSAR data (see update below) to interpret the fault geometry.

UPDATE 2018.08.12

People have been asking me if we might expect another large or larger earthquake in this region. So, here is what I have told them:

It is difficult to say if there will be a larger or another large earthquake or not.
Based upon historic seismicity, the M 6.9 is probably the mainshock in this sequence. But the historic record is short (100 yrs +-), so may not be a perfect sample of what could happen.
The M 6.9 probably ruptured the Flores thrust fault, a back thrust to the subduction zone.
There is probably a small chance that the Flores thrust fault (east west fault dipping to the south) to the east and west of the M 6.9 has an increased amount of stress imparted upon it from the M 6.9 (small amount, so if the fault was almost ready to go, this change might make it go). but this is a small possibility (but still possible). (i.e. Bali).
There is also a small chance that the subduction zone (south of the islands, dipping to the north) also has an increased amount of stress from this M 6.9 earthquake. but this is probably less likely than the other example (due to the distance between the M .6.9 and the subduction zone fault.
Though there will probably be earthquakes up to M 5 or mid M 5 as aftershocks… and as time passes, the chance of a larger earthquake diminish to the background risk of such an earthquake. by the time it is Sept through Dec, we will probably have passed the increased risk due to the M 6.9 sequence.
But we must always remember, we cannot absolutely know what will happen. our observational history is only a few centuries and seismometers are only a century old (and modern ones, with a global network, maybe 50 years). so it is challenging to think that we know about how this region (or any region) behaves tectonically.

UPDATE 2018.08.19

I prepared an updated report given the new M 6.9 earthquake from today (8/19/2018) here.

Below is my interpretive poster for this earthquake

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

Magnetic Anomalies

In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Australia plate.

I include some inset figures.

In the upper right corner is a low angle oblique view of the Sunda subduction zone beneath Java, Bali, Lombok, and Sumbawa (from Earth Observatory Singapore). I place a blue star in the general location of today’s earthquake’s epicenter (as for all figures here). The India-Australia plate is subducting northwards beneath the Sunda plate (part of the Eurasia plate).
In the upper left corner is a map from Koulali et al. (2016) that presents a plate tectonic map for this region. They present earthquake mechanisms for some historic earthquakes. These authors favor the interpretation that the Flores thrust fault system extends to eastern Java.
To the right of the Koulali et al. (2016) map is a cross section of seismicity presented by Hengresh and Whitney (2016). These authors argue for a north vergent Flores thrust in this region, though most of their work was on the subduction/collision zone.
In the lower right corner is another Koulali et al. (2016) map that shows the relative amount of motion across these plate boundary fault systems as modeled in their analysis. Based on their modeling, there is about 10-20 mm/yr of strain accumulating on the Flores thrust system north of Lombok, Indonesia.
In the lower left corner is a Koulali et al. (2016) that shows their estimate of this strain accumulation (via fault slip deficit) for the Flores thrust fault.

Here is the map with a month’s seismicity plotted.

Here is the map with a centuries’ seismicity plotted.

Here is the interpretive posted from last week, with historic seismicity and earthquake mechanisms.

Other Report Pages

Some Relevant Discussion and Figures

Below is a map showing historic seismicity (Jones et al., 2014). Cross sections B-B’ and C-C’ are shown. The seismicity for the cross sections below are sourced from within each respective rectangle.

Here are the seismcity cross sections.

Here is the map from McCaffrey and Nabelek (1987). They used seismic reflection profiles, gravity modeling along these profiles, seismicity, and earthquake source mechanism analyses to support their interpretations of the structures in this region.

Tectonic and geographic map of the eastern Sunda arc and vicinity. Active volcanoes are represented by triangles, and bathymetric contours are in kilometers. Thrust faults are shown with teeth on the upper plate. The dashed box encloses the study area.

Here is the Audley (2011) cross section showing how the backthrust relates to the subduction zone beneath Timor. I include their figure caption in blockquote below.

Cartoon cross section of Timor today, (cf. Richardson & Blundell 1996, their BIRPS figs 3b, 4b & 7; and their fig. 6 gravity model 2 after Woodside et al. 1989; and Snyder et al. 1996 their fig. 6a). Dimensions of the filled 40 km deep present-day Timor Tectonic Collision Zone are based on BIRPS seismic, earthquake seismicity and gravity data all re-interpreted here from Richardson & Blundell (1996) and from Snyder et al. (1996). NB. The Bobonaro Melange, its broken formation and other facies are not indicated, but they are included with the Gondwana mega-sequence. Note defunct Banda Trench, now the Timor TCZ, filled with Australian continental crust and Asian nappes that occupy all space between Wetar Suture and the 2–3 km deep deformation front north of the axis of the Timor Trough. Note the much younger decollement D5 used exactly the same part of the Jurassic lithology of the Gondwana mega-sequence in the older D1 decollement that produced what appears to be much stronger deformation.

This are the seismicity cross sections from Hangesh and Whitney (2016). These are shown to compare the subduction zone offshore of Java and the collision zone in the Timor region.

Comparison of hypocentral profiles across the (a) Java subduction zone and (b) Timor collision zone (paleo-Banda trench). Catalog compiled from multiple reporting agencies listed in Table 1. Events of Mw>4.0 are shown for period 1815 to 2015.

Here is a map of the same general area from Silver et al. (1986), used here to locate the following large scale map.

Location of SeaMARC II survey (Plate 1 and Figures 2) and geographic features discussed in text. Triangles on upper plates of thrust zones.

This is the large scale map showing the detailed thrust fault mapping (Silver et al., 1986).

Bathymetry, faults, and mud diapirs of the central Flores thrust zone, based on interpretation of SeaMARC II data and seismic reflection profiles. Shown also are locations (circled numbers) of all seismic profiles. Mud diapirs are solid black. Triangles on upper plates of thrust faults.

Here is the tectonic map from Hangesh and Whitney (2016).

Illustration of major tectonic elements in triple junction geometry: tectonic features labeled per Figure 1; seismicity from ISC-GEM catalog [Storchak et al., 2013]; faults in Savu basin from Rigg and Hall [2011] and Harris et al. [2009]. Purple line is edge of Australian continental basement and fore arc [Rigg and Hall, 2011]. Abbreviations: AR = Ashmore Reef; SR = Scott Reef; RS = Rowley Shoals; TCZ = Timor Collision Zone; ST = Savu thrust; SB = Savu Basin; TT = Timor thrust; WT =Wetar thrust; WASZ = Western Australia Shear Zone. Open arrows indicate relative direction of motion; solid arrows direction of vergence.

Here are some focal mechanisms from earthquakes in the region from Hangesh and Whitney (2016). Symbol color represents depth.

(a) Focal mechanism solutions for the study region. The focal mechanisms are classified based on depth intervals to illustrate the style of faulting within the different structural domains. Note (b) sinistral reverse motion along Timor trough, (c) subduction related pattern along Java trench, and dextral solutions along the western Australia extended margin (Figure 4a) north of 20°S. Centroid moment tensor (CMT) solutions [Dziewonski et al., 1981] are from the CMT project [Ekström et al., 2012; http://www.globalcmt.org/CMTcite.html] for events of Mw>5.0 for the period 1976 onward.

Here is a figure showing the regional geodetic motions (Bock et al., 2003). I include their figure caption below as a blockquote.

Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, New Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals.

This map from Hangesh and Whitney (2016) shows the GPS velocities in this region. Note the termination of the Flores thrust and the north-northeast striking (oriented) cross fault between Lombok and Sumbawa.

GPS velocities of Sunda and Banda arc region. Large black and grey arrow shows motion of Australia relative to Eurasia [DeMets et al., 1994]. Thin black arrows show GPS velocities of Sunda and Banda arc regions relative to Australia [Nugroho et al., 2009]. Seismicity from ISC-GEM catalog [Storchak et al., 2013]. Note reduction of station velocities from west to east indicating progressive coupling of the Banda arc to the Australian plate compared to the area along the Sunda arc.

Below are the 4 figures from Koulani et al., 2016. First is the plate tectonic map. I include their figure captions in block quote.

Seismotectonic setting of the Sunda-Banda arc-continent collision, East Indonesia. Major faults (thick black lines) [Hamilton, 1979]. Topography and bathymetry are from Shuttle Radar Topography Mission (http://topex.ucsd.edu/www_html/srtm30_plus.html). Focal mechanisms are from the Global Centroid Moment Tensor. Blue mechanisms correspond to earthquakes with Mw>7 (brown transparent ellipses are the corresponding rupture areas for Flores 1992 and Alor 2004 earthquakes), while the green focal mechanism shows the highest magnitude recorded in Sumbawa. Red dots indicate the locations of major historical earthquakes [Musson, 2012].

This figure shows their estimates for plate motion relative velocities as derived from GPS data, constrained by the fault geometry in their block modeling.

GPS velocities determined in this study with respect to Sunda Block. Uncertainty ellipses represent 95% confidence level. The inset figure corresponds to the area of the dashed rectangle in the map. Light blue arrows show the velocities for East and West Makassar Blocks.

This figure shows their estimates of slip rate deficit along all the plate boundary faults in this region.

Relative slip vectors across block boundaries, derived from our best fit model. Arrows show motion of the hanging wall (moving block) relative to the footwall (fixed block) with 95% confidence ellipses. The tails of arrows is located within the “moving” block. Black thick lines show well-defined boundaries we use as active faults in our model and dashed lines show less well-defined boundaries (green : free-slipping boundaries and black: fixed locked faults) . Principal axes of the horizontal strain tensor estimated for the SUMB, EMAK, and EJAV are shown in pink. The thick pink arrow shows the relative motion of Australia with respect to Sunda (AUST/SUND). Abbreviations are Sumba Block (SUMB), West Makassar Block (WMAK), East Makassar Block (EMAK), East Java Block (EJAV), and Timor Block (TIMO). The background seismicity is from the International Seismological Centre catalog with magnitudes ≥5.5 and depths <40 km.

Here is their figure that shows the slip deficit along the plate boundary faults.

Fault slip rate components: (a) fault normal (extension positive) and (b) fault parallel (right-lateral positive).

UPDATE 2018.08.08

NASA InSAR

Here is the InSAR result from Eric Fielding at NASA, the files are available here.
These data are from a change in position between 2018.07.30 and 2018.08.05, so they compare the ground motion of only the M 6.9 earthquake (generally speaking).

From Dr. Fielding

Deformation of Lombok Island, Indonesia due to 5 August 2018 earthquake shows uplift of northwest corner due to fault slip at depth, measured with #InSAR of Copernicus Sentinel-1 radar images processed by Caltech-JPL ARIA project. Data at https://go.nasa.gov/2OlbxY6
Black contours are 5 cm (2 inches). Copernicus Sentinel-1 data acquired on 30 July and 5 August 2018. White areas where measurement not possible, largely due to dense forests.
Measurements with #InSAR are in direction towards satellite, so not purely vertical or horizontal. Mostly vertical in this case.
My preliminary interpretation is that uplift is due to a north-dipping blind thrust fault that would project to the surface near the “zero” level of the interferogram, but a south-dipping thrust fault is also possible with down-dip end of rupture beneath the “zero” line

Rusi P InSAR

These two InSAR images allow us to compare ground deformation from these two earthquakes. Rusi P presents these results on twitter here. This tweet is also posted below in the Social Media section.
This is the analysis for the M 6.4 earthquake. This interferogram is made from SAR data collected on 7/18 and 7/30.

This is the analysis for the M 6.9 earthquake. This interferogram is made from SAR data collected on 7/30 and 8/05.

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Indonesia | Sumatra Earthquake Reports

General Overview

M 9.2 Andaman-Sumatra subduction zone 2014 Earthquake Anniversary
M 9.2 Andaman-Sumatra subduction zone SASZ Fault Deformation
M 9.2 Andaman-Sumatra subduction zone 2016 Earthquake Anniversary

Earthquake Reports

2018.08.05 M 6.9 Lombok, Indonesia
2018.07.28 M 6.4 Lombok, Indonesia
2017.12.15 M 6.5 Java
2017.08.31 M 6.3 Mentawai, Sumatra
2017.08.13 M 6.4 Bengkulu, Sumatra, Indonesia
2017.05.29 M 6.8 Sulawesi, Indonesia
2017.03.14 M 6.0 Sumatra
2017.03.01 M 5.5 Banda Sea
2016.10.19 M 6.6 Java
2016.03.02 M 7.8 Sumatra/Indian Ocean
2015.07.22 M 5.8 Andaman Sea
2015.11.08 M 6.4 Nicobar Isles
2012.04.11 M 8.6 Sumatra outer rise
2004.12.26 M 9.2 Andaman-Sumatra subduction zone

Social Media

Expert warns of strong aftershocks in Indonesia following Lombok quakehttps://t.co/sEJyziJO1b pic.twitter.com/LtwwKLcsF7

— BBC News (World) (@BBCWorld) August 6, 2018

Mw=6.9, SUMBAWA REGION, INDONESIA (Depth: 18 km), 2018/08/05 11:46:34 UTC – Full details here: https://t.co/jws7oFBoaM pic.twitter.com/AtlFwTFCZa

— Earthquakes (@geoscope_ipgp) August 5, 2018

M6.9 #earthquake #Lombok, #Indonesia: High-frequency seismogram (lower; station JAGI on Java ~250km W of epicenter) suggests up to ~50sec of rupture duration. Geoscope and USGS long-period waveform analyses give ~20sec duration.https://t.co/F8EkNdVkfY https://t.co/0fVh9lO59J pic.twitter.com/enJn9nYwUF

— Anthony Lomax 🌍🇪🇺 (@ALomaxNet) August 7, 2018

Do you believe in seismic gaps ?!! the location of the Mw 6.9 Lombok earthquake 2018 added to fig1 of our GRL 2016 paper pic.twitter.com/VXHWC7Mzop

— Achraf (@KoulaliAchraf) August 6, 2018

Great @ESA_EO #Sentinel1 coverage of M6.9 #Loloan quake, Indonesia. Automatic @UAFGI #SARVIEWS InSAR 🛰️ data shows significant deformation. Download free InSAR data products for this event @ https://t.co/OfHXvqNlu6.@InSARinfo @Ak_Satellite @NASAEarthData pic.twitter.com/SkQ17k5gAT

— Franz J Meyer (@SARevangelist) August 6, 2018

Watch the waves from the M6.9 #LombokEarthquake roll across the USArray seismic network (https://t.co/RIcNz4bgWq). Red means the ground is going up; blue means down. The waves are too small to be felt but can be detected by these sensitive instruments. https://t.co/SoZMmJHvCU pic.twitter.com/G66CUjeZqE

— IRIS Earthquake Sci (@IRIS_EPO) August 6, 2018

2018-08-05 Mw6.9 Indonesia earthquake interferogram#insar #earthquake pic.twitter.com/al2ahJJ4pJ

— R P (@rusi_p) August 6, 2018

Here are high-frequency estimates of apparent rupture duration for large earthquakes 1992-2012 compared to Global CMT (Lomax & Michelini, 2012).
Much scatter due to different earthquake types, but 50sec is on the high end for M7.https://t.co/po5va0kmlX https://t.co/y2FptSd3YU pic.twitter.com/fgg6EGeOPk

— Anthony Lomax 🌍🇪🇺 (@ALomaxNet) August 7, 2018

Q: What is the probability that an #earthquake is a foreshock to a larger earthquake?
A: "Worldwide the probability that an earthquake will be followed within 3 days by a large earthquake nearby is somewhere just over 6%." – @USGS https://t.co/pgaXc03xsT

— IRIS Earthquake Sci (@IRIS_EPO) August 7, 2018

Here is a coseismic interferogram of the Mw6.9 earthquake in Lombok island on Aug. 5. Analysis of Sentinel-1A/B images with Gamma(R) pic.twitter.com/tlJJT7Lx6i

— 橋本学 (@manabu0131dpri) August 7, 2018

Mechanism & epicenter of today’s M6.9 quake in Lombok, Indonesia, similar to that of M6.4 week earlier (for which map & crosssection of historical seismicity attached), although bit deeper. Event likely did not occur on subduction interface, but on backthrust behind it. pic.twitter.com/2yMPIrwFcX

— Jascha Polet (@CPPGeophysics) August 5, 2018

close-up view on #Lombok #earthquake, also covering #Bali, still expecting extensive damage and fatalities. Be prepared for various (strong) aftershocks. pic.twitter.com/1prVkTzNOr

— CATnews (@CATnewsDE) August 5, 2018

#RT Several major #earthquakes have struck the Indonesian island of #Lombok in the past week. #Indonesia sits along the “Pacific Ring of Fire” where several #tectonicplates collide but there are other unique conditions around Lombok @ConversationUS ➡️ https://t.co/a4a6cCXnxl pic.twitter.com/Ir1YRFijHX

— Raspberry Shake (@raspishake) August 7, 2018

Earthquake: Aug-06 M6.9 Pulau Lombok, Indonesia | depth 31 Km, ~100 people dead. Damage as far away as Bali. Mass evacuation 'chaos' from the Gili "tourist" Islands.#Lombokquake #ahemQUAKES https://t.co/b1P3wdq8nd

— aHEMagain ❌ (@aHEM_again) August 6, 2018

#Indonesia’s National Disaster Management Agency released video of thousands of tourists trying to get off #Gili Islands after #Lombokquake. pic.twitter.com/ZXOW1aqpuQ

— Jon Williams (@WilliamsJon) August 6, 2018

All trapped tourists have been evacuated from #Lombok's nearby Gili Meno Island after deadly earthquake, according to Indonesia's Tourism Ministry #Lombokquake pic.twitter.com/I2AofMLe3v

— CGTN (@CGTNOfficial) August 7, 2018

Some of the damage at Teluk Nara bus station #Lombokquake pic.twitter.com/x4EkCLaFxP

— David Lipson (@davidlipson) August 6, 2018

#Lombokquake M 7.0 which struck Lombok, Indonesia on 5 August 2018, caused small tsunamis. Death toll: 82
The earthquake was the main shock following its foreshocks, a nearby M 6.4 earthquake on the morning of 29 July 2018. 91 people confirmed killed ,over 100 confirmed injured. pic.twitter.com/2wFW8FrAgq

— Bali Promotion Center💅 (@translatorbali) August 6, 2018

The shoes of those trapped under this collapsed mosque #Lombokquake pic.twitter.com/mKNIAWJz9y

— David Lipson (@davidlipson) August 7, 2018

#Lombokquake: Thousands evacuated after dozens die on #Indonesia island https://t.co/lIXD7vBENe

— Ekanem Etim-Offiong (@akemmapapa) August 6, 2018

Tourists flee Indonesia's Lombok island after earthquake kills 98 #Lombokquake #Indonesia https://t.co/DxFlg7Mch3

— RangerRick ن (@sacreole) August 6, 2018

UPDATE 2018.08.08

There's no scale, and actual modeling would show more precisely, but just comparing the number fringes from the InSAR by @rusi_p , it seems that there shouldn't be a very big difference in the depths of both earthquakes? https://t.co/xM1heCiXyg

— JD Dianala (@geoloJD) August 7, 2018

USGS closest station JAGI is ~250km from epicenter (need stations at distance of approx true depth to resolve depth well) and residuals at nearer stations are very large (>1sec). So event depth is likely very poorly constrained.https://t.co/gU9lOgE4vp pic.twitter.com/g2gsVsgA1s

— Anthony Lomax 🌍🇪🇺 (@ALomaxNet) August 8, 2018

Displacement map for 5 August 2018 M6.9 #Lombok #earthquake from Copernicus Sentinel-1 radar data #InSAR processed by NASA Caltech-JPL ARIA released on @NASAJPL news page https://t.co/YXv2BzVHgu
NASA Disasters data portal https://t.co/v7V1sNFqq4

— Eric Fielding (@EricFielding) August 9, 2018

Updated – More details of death and destruction emerge from Lombok after the #IndonesiaEarthQuake . This graphic explains the tragedy https://t.co/MDscEPWc3A

— Reuters Graphics (@ReutersGraphics) August 7, 2018

UPDATE 2018.08.09

Liquifaksi (luquefaction) yaitu tanah yang kaku berubah menjadi gembur dan muncul lumpur akibat tekanan gempa 7 SR terjadi di Desa Selengen Kecamatan Kayangan Lombok Utara. Liquifaksi banyak menyebabkan bangunan roboh karena bangunan berdiri diatas tanah gembur dan pondasi patah. pic.twitter.com/Wfu1NhSkJW

— Sutopo Purwo Nugroho (@Sutopo_PN) August 9, 2018

References:

Audley-Charles, M.G., 1986. Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology in: Journal of fhe Geological Society, London, Vol. 143, 1986, pp. 161-175.
Audley-Charles, M.G., 2011. Tectonic post-collision processes in Timor, Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 241–266.
Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region in Annu. Rev. Earth Planet. Sci., v. 41, p. 485-520.
Benz, H.M., Herman, Matthew, Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 New Guinea and vicinity: U.S. Geological Survey Open-File Report 2010–1083-H, scale 1:8,000,000.
Darman, H., 2012. Seismic Expression of Tectonic Features in the Lesser Sunda Islands, Indonesia in Berita Sedimentologi, Indonesian Journal of Sedimentary Geology, no. 25, po. 16-25.
Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
Hangesh, J. and Whitney, B., 2014. Quaternary Reactivation of Australia’s Western Passive Margin: Inception of a New Plate Boundary? in: 5th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 21-27 September 2014, Busan, Korea, 4 pp.
Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
Jones, E.S., Hayes, G.P., Bernardino, Melissa, Dannemann, F.K., Furlong, K.P., Benz, H.M., and Villaseñor, Antonio, 2014. Seismicity of the Earth 1900–2012 Java and vicinity: U.S. Geological Survey Open-File Report 2010–1083-N, 1 sheet, scale 1:5,000,000, https://dx.doi.org/10.3133/ofr20101083N.
Koulali, A., S. Susilo, S. McClusky, I. Meilano, P. Cummins, P. Tregoning, G. Lister, J. Efendi, and M. A. Syafi’i, 2016. Crustal strain partitioning and the associated earthquake hazard in the eastern Sunda-Banda Arc in Geophys. Res. Lett., 43, 1943–1949, doi:10.1002/2016GL067941
McCaffrey, R., and Nabelek, J.L., 1984. The geometry of back arc thrusting along the Eastern Sunda Arc, Indonesia: Constraints from earthquake and gravity data in JGR, Atm., vol., 925, no. B1, p. 441-4620, DOI: 10.1029/JB089iB07p06171
Okal, E. A., & Reymond, D., 2003. The mechanism of great Banda Sea earthquake of 1 February 1938: applying the method of preliminary determination of focal mechanism to a historical event in EPSL, v. 216, p. 1-15.
Silver, E.A., Breen, N.A., and Prastyo, H., 1986. Multibeam Study of the Flores Backarc Thrust Belt, Indonesia, in JGR., vol. 91, no. B3, p. 3489-3500
Zahirovic, S., Seton, M., and Müller, R.D., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014

Earthquake Report: Lombok, Indonesia

Posted on July 28, 2018 by Jay Patton

Earlier today there was a shallow M 6.4 earthquake with an epicenter on the island of Lombok, Indonesia. With a hypocentral depth of about 7.5 km, this size of an earthquake can be quite damaging. The USGS PAGER estimate of impact suggests that there is about a 10% chance that there are more than 10 fatalities. Hopefully there are none. There have been several aftershocks, two M > 5.
This earthquake is probably along a thrust fault associated with the Flores thrust fault, a north vergent (dipping into the earth in a southerly direction) back thrust fault to the Sunda subduction zone fault. The Flores thrust possibly extends from east of Timor on the east to the northern shore of Java (McCaffrey and Nabelek, 1987). Others suggest that the Flores thrust ends at a cross fault just east of Lombok (Hengresh and Whitney, 2016). However, the seismic profiles from Silver et al. (1986) are convincing that there are east-west compressional structures extending between the northern shore of Java to where the Flores thrust is mapped.
Detailed mapping of the seafloor to the east of Lombok, north of the island of Sumbawa, reveals that there are imbricate (overlapping) thrust faults (Silver et al., 1986). I think that it is reasonable to presume that there are similar structures on the northern flank of Lombok.
Lombok is also a volcano complex as part of the Sunda magmatic arc. There may be fault systems associated with the volcanic activity. I include tectonic faults that are included in the global scale fault data set from the Coordinating Committee for Geoscience Programme in East and Southeast Asia. The most active volcano on Lombok is the Rinjani volcano. Here is a great place to learn about this volcano (the Volcano Discovery website).
If the M 6.4 earthquake was on the Flores fault, it would need to dip at about 10°. The Flores thrust fault proposed by Hengesh and Whitney (2016) has a much steeper dip. So this sequence is probably in the upper plate somewhere.
There was a M 6.0 earthquake to the east of the M 6.4, but it was much deeper (almost 600 km), so is unlikely to be genetically related to the M 6.4 sequence.

Magnetic Anomalies

In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Australia plate.

Historic Seismicity

Below I discuss analogues to today’s M 6.4 earthquake.
To the west, between Lombok and Bali, there was a series of earthquakes all in 1979. They happened several months apart, but had a similar magnitude and orientation. The hypocentral depths were in the 25-40 km depth range, so some of these may have been on the Flores thrust system. These alone suggest that the Flores thrust extends at least this far west.
To the east, along the eastern part of Sumbawa, there was a series of earthquakes in the first decade of the 21st century, from 2002-2009. These also all share a similar magnitude range and orientation. These earthquakes all happened within a narrow range of depths (18-20 km; though the 2002 earthquake has a default depth on 10 km).
Based on earthquakes in the regions to the east and to the west, it is possible that this M 6.4 is the first of a series of mid M 6 earthquakes (either within a year like in Bali or over several years like Sumbawa).

Below is my interpretive poster for this earthquake

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

I include some inset figures.
In the upper right corner is a low angle oblique view of the Sunda subduction zone beneath Java, Bali, Lombok, and Sumbawa (from Earth Observatory Singapore). I place a blue star in the general location of today’s earthquake’s epicenter (as for all figures here). The India-Australia plate is subducting northwards beneath the Sunda plate (part of the Eurasia plate).
In the upper left corner is a plate tectonic map showing the major fault systems, volcanic arc islands, and oceanic plateaus and basins of the region (Darman, 2012). The map shows the Flores thrust extending as far west as Lombok. Compare the complicated tectonics in the eastern portion of this region compared to the western portion of this region.
To the right of the Darman (2012) map is a cross section of seismicity presented by Hengresh and Whitney (2016). These authors argue for a north vergent Flores thrust in this region, though most of their work was on the subduction/collision zone.
In the lower right corner is another, earlier, tectonic map from Silver et al. (1986). These authors use seismic reflection and multibeam bathymetry data to map the Flores thrust as far as Java, west of Bali. The location for the map in the lower left corner of this interpretive poster is outlined here as a dashed line rectangle.
In the lower left corner is a map from Silver et al. (1986) that shows the detailed mapping of imbricate north (and some south) vergent thrust faults.

Here is the same map but with seismicity from the past month.

Here is the same map but with historic seismicity.

USGS Earthquake Pages

These are from this current sequence

2018-07-28 17:07:23 UTC M 6.0
2018-07-28 22:47:37 UTC M 6.4
2018-07-28 23:06:49 UTC M 5.4
2018-07-29 01:50:32 UTC M 5.3

These are from earlier

Western – Bali
1979-05-30 M 6.1
1979-12-17 M 6.3
1979-10-20 M 6.2

Eastern – Sumbawa
2002-10-06 M 6.3
2006-12-01 M 6.3
2007-11-25 M 6.5 A
2007-11-25 M 6.5 B
2009-11-08 M 6.6

Other Report Pages

Earthquake Report dot com

Some Relevant Discussion and Figures

Below is a map showing historic seismicity (Jones et al., 2014). Cross sections B-B’ and C-C’ are shown. The seismicity for the cross sections below are sourced from within each respective rectangle.

Here are the seismcity cross sections.

Here is the map from McCaffrey and Nabelek (1987). They used seismic reflection profiles, gravity modeling along these profiles, seismicity, and earthquake source mechanism analyses to support their interpretations of the structures in this region.

Tectonic and geographic map of the eastern Sunda arc and vicinity. Active volcanoes are represented by triangles, and bathymetric contours are in kilometers. Thrust faults are shown with teeth on the upper plate. The dashed box encloses the study area.

Here is the Audley (2011) cross section showing how the backthrust relates to the subduction zone beneath Timor. I include their figure caption in blockquote below.

Cartoon cross section of Timor today, (cf. Richardson & Blundell 1996, their BIRPS figs 3b, 4b & 7; and their fig. 6 gravity model 2 after Woodside et al. 1989; and Snyder et al. 1996 their fig. 6a). Dimensions of the filled 40 km deep present-day Timor Tectonic Collision Zone are based on BIRPS seismic, earthquake seismicity and gravity data all re-interpreted here from Richardson & Blundell (1996) and from Snyder et al. (1996). NB. The Bobonaro Melange, its broken formation and other facies are not indicated, but they are included with the Gondwana mega-sequence. Note defunct Banda Trench, now the Timor TCZ, filled with Australian continental crust and Asian nappes that occupy all space between Wetar Suture and the 2–3 km deep deformation front north of the axis of the Timor Trough. Note the much younger decollement D5 used exactly the same part of the Jurassic lithology of the Gondwana mega-sequence in the older D1 decollement that produced what appears to be much stronger deformation.

This are the seismicity cross sections from Hangesh and Whitney (2016). These are shown to compare the subduction zone offshore of Java and the collision zone in the Timor region.

Comparison of hypocentral profiles across the (a) Java subduction zone and (b) Timor collision zone (paleo-Banda trench). Catalog compiled from multiple reporting agencies listed in Table 1. Events of Mw>4.0 are shown for period 1815 to 2015.

Here is a map of the same general area from Silver et al. (1986), used here to locate the following large scale map.

Location of SeaMARC II survey (Plate 1 and Figures 2) and geographic features discussed in text. Triangles on upper plates of thrust zones.

This is the large scale map showing the detailed thrust fault mapping (Silver et al., 1986).

Bathymetry, faults, and mud diapirs of the central Flores thrust zone, based on interpretation of SeaMARC II data and seismic reflection profiles. Shown also are locations (circled numbers) of all seismic profiles. Mud diapirs are solid black. Triangles on upper plates of thrust faults.

Here is the tectonic map from Hangesh and Whitney (2016).

Illustration of major tectonic elements in triple junction geometry: tectonic features labeled per Figure 1; seismicity from ISC-GEM catalog [Storchak et al., 2013]; faults in Savu basin from Rigg and Hall [2011] and Harris et al. [2009]. Purple line is edge of Australian continental basement and fore arc [Rigg and Hall, 2011]. Abbreviations: AR = Ashmore Reef; SR = Scott Reef; RS = Rowley Shoals; TCZ = Timor Collision Zone; ST = Savu thrust; SB = Savu Basin; TT = Timor thrust; WT =Wetar thrust; WASZ = Western Australia Shear Zone. Open arrows indicate relative direction of motion; solid arrows direction of vergence.

Here are some focal mechanisms from earthquakes in the region from Hangesh and Whitney (2016). Symbol color represents depth.

(a) Focal mechanism solutions for the study region. The focal mechanisms are classified based on depth intervals to illustrate the style of faulting within the different structural domains. Note (b) sinistral reverse motion along Timor trough, (c) subduction related pattern along Java trench, and dextral solutions along the western Australia extended margin (Figure 4a) north of 20°S. Centroid moment tensor (CMT) solutions [Dziewonski et al., 1981] are from the CMT project [Ekström et al., 2012; http://www.globalcmt.org/CMTcite.html] for events of Mw>5.0 for the period 1976 onward.

Here is a figure showing the regional geodetic motions (Bock et al., 2003). I include their figure caption below as a blockquote.

Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, New Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals.

This map from Hangesh and Whitney (2016) shows the GPS velocities in this region. Note the termination of the Flores thrust and the north-northeast striking (oriented) cross fault between Lombok and Sumbawa.

GPS velocities of Sunda and Banda arc region. Large black and grey arrow shows motion of Australia relative to Eurasia [DeMets et al., 1994]. Thin black arrows show GPS velocities of Sunda and Banda arc regions relative to Australia [Nugroho et al., 2009]. Seismicity from ISC-GEM catalog [Storchak et al., 2013]. Note reduction of station velocities from west to east indicating progressive coupling of the Banda arc to the Australian plate compared to the area along the Sunda arc.

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Indonesia | Sumatra Earthquake Reports

General Overview

M 9.2 Andaman-Sumatra subduction zone 2014 Earthquake Anniversary
M 9.2 Andaman-Sumatra subduction zone SASZ Fault Deformation
M 9.2 Andaman-Sumatra subduction zone 2016 Earthquake Anniversary

Earthquake Reports

2018.07.28 M 6.4 Lombok, Indonesia
2017.12.15 M 6.5 Java
2017.08.31 M 6.3 Mentawai, Sumatra
2017.08.13 M 6.4 Bengkulu, Sumatra, Indonesia
2017.05.29 M 6.8 Sulawesi, Indonesia
2017.03.14 M 6.0 Sumatra
2017.03.01 M 5.5 Banda Sea
2016.10.19 M 6.6 Java
2016.03.02 M 7.8 Sumatra/Indian Ocean
2015.07.22 M 5.8 Andaman Sea
2015.11.08 M 6.4 Nicobar Isles
2012.04.11 M 8.6 Sumatra outer rise
2004.12.26 M 9.2 Andaman-Sumatra subduction zone

Social Media

Map and cross-section of historical seismicity of the area around the deadly M6.4 earthquake on Lombok Island, Indonesia, that occurred a few days ago. The Flores Backarc Thrust has been suggested as a possible causative fault. pic.twitter.com/sxxLMT4Kep

— Jascha Polet (@CPPGeophysics) July 30, 2018

The latest update about our response in #Lombok earthquake (29/7/2018) #PMISiapBantu pic.twitter.com/apZyv9dSH7

— Indonesian Red Cross (@palangmerah) July 29, 2018

#UPDATE Death toll rises to 13, with hundreds injured, after a powerful M6.4 #earthquake struck popular tourist island of #Lombok in Indonesia early morning
pic.twitter.com/drvvxcZAis

— Bali Promotion Center💅 (@translatorbali) July 29, 2018

A 6.4 magnitude earthquake had hit Lombok and Sumbawa Islands today, killing at least 10, including a Malaysian. Here is a footage from Mt Rinjani. Our thoughts and prayers are with them. pic.twitter.com/hZ2zc4tV70

— | outdoorian (@twt_outdoor) July 29, 2018

BBC News – Indonesia earthquake: 10 dead on tourist island Lombok https://t.co/mC1ytJrsfV

— Anthony Lomax 🌍🇪🇺 (@ALomaxNet) July 29, 2018

Deadly #earthquake in #Lombok, #Indonesia – Link to our in-depth article – https://t.co/Dex2gm12Wh pic.twitter.com/KgiqYvUHSO

— Armand Vervaeck 🎗️ (@ArmandVervaeck) July 29, 2018

Surveying the damage in Lombok after a powerful earthquake hit the popular tourist destination in Indonesiahttps://t.co/WIi1D8YuK5 pic.twitter.com/myRoTCMguy

— BBC News (World) (@BBCWorld) July 29, 2018

Mw=6.5, SUMBAWA REGION, INDONESIA (Depth: 14 km), 2018/07/28 22:47:37 UTC – Full details here: https://t.co/3sCPDO5xqv pic.twitter.com/3yeO7KZrnk

— Earthquakes (@geoscope_ipgp) July 28, 2018

Korban gempa Sembalun Lombok Timur pic.twitter.com/wdug832T8M

— #2019GantiPresiden (@IpungLombok) July 28, 2018

Beberapa kondisi bangunan setelah Gempa di Lombok. Semoga tidak ada korban jiwa 🙏🏻 pic.twitter.com/kz4E9gm98O

— baiq tanisya ellya m (@tanisyaEM) July 28, 2018

🇮🇩💢#Indonesia #Earthquake Mw 6.4 depth 10 km Lombok Región.
-El Sismo ha destruido viviendas como era probable , especialmente las de Construcción no asísmica. #PrayforIndonesia 🙏 pic.twitter.com/jWDzUSAUiB

— 🌎 (@Geo__Data) July 28, 2018

Estimated population in the felt area: 3.7 millions inhabitants pic.twitter.com/GcP4108N8u

— EMSC (@LastQuake) July 28, 2018

strong #earthquake on #Lombok, #Indonesia, well felt on #Bali, #Sumbawa, damage expected pic.twitter.com/8yft7KEZmN

— CATnews (@CATnewsDE) July 28, 2018

Wake up in the morning with the shaking bed !

Got called from farm and so many facility broke down..#agendafreetv #lombok #earthquake pic.twitter.com/GbANUMLpxW

— Reyn (@suharja_reynard) July 28, 2018

#Earthquake_alert – The impact of 7/29/18 – 06:47 local time Mag 6.4 SR quake at 28 km NW of East Lombok w/ 10 km hypocenter, some structures & houses are damaged in Sambelia, East Lombok. The BPBD is still collecting the data. Source: @Sutopo_PN #Lombok pic.twitter.com/EewC7iPDQ4 https://t.co/bH7VHxVjFL

— Desianto F. Wibisono (@TDesiantoFW) July 29, 2018

#UPDATE Death toll rises to 10, with 40 injured, after a powerful M6.4 #earthquake struck popular tourist island of #Lombok in Indonesia early morning pic.twitter.com/gpojRbC5iI

— CGTN (@CGTNOfficial) July 29, 2018

At least 3 killed after powerful 6.4-magnitude earthquake strikes off Indonesian island of #Lombok https://t.co/rgma2EEZe9 pic.twitter.com/3sOESVHmjM

— The Straits Times (@STcom) July 29, 2018

Sorry to say that reports of casualties are starting to filter through from the 6.4 magnitude #Lombok #Earthquake this morning, updates to come #LombokEarthquake https://t.co/w93y8aIOvj

— Alison Bevege (@AlisonBevege) July 29, 2018

Thinking of many friends in #Lombok … never a dull moment https://t.co/8eaPNQljQr

— Krista Slade (@krista_slade) July 29, 2018

#Update Indonesia's disaster response agency spokesman says 10 were dead, 40 injured after a M6.4 earthquake struck popular tourist island of #Lombok in Indonesia early morning. pic.twitter.com/CTjub5UG7A

— China Daily (@ChinaDailyUSA) July 29, 2018

Powerful #Earthquake hits #Indonesia's #Lombok, 10 killed, houses damagedhttps://t.co/6ae6C4mNuB — https://t.co/qIz2A1IP5X pic.twitter.com/fwgcXfjeBX

— Z9 (@Z9Network) July 29, 2018

References:

Audley-Charles, M.G., 1986. Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology in: Journal of fhe Geological Society, London, Vol. 143, 1986, pp. 161-175.
Audley-Charles, M.G., 2011. Tectonic post-collision processes in Timor, Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 241–266.
Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region in Annu. Rev. Earth Planet. Sci., v. 41, p. 485-520.
Benz, H.M., Herman, Matthew, Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 New Guinea and vicinity: U.S. Geological Survey Open-File Report 2010–1083-H, scale 1:8,000,000.
Darman, H., 2012. Seismic Expression of Tectonic Features in the Lesser Sunda Islands, Indonesia in Berita Sedimentologi, Indonesian Journal of Sedimentary Geology, no. 25, po. 16-25.
Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
Hangesh, J. and Whitney, B., 2014. Quaternary Reactivation of Australia’s Western Passive Margin: Inception of a New Plate Boundary? in: 5th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 21-27 September 2014, Busan, Korea, 4 pp.
Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
Jones, E.S., Hayes, G.P., Bernardino, Melissa, Dannemann, F.K., Furlong, K.P., Benz, H.M., and Villaseñor, Antonio, 2014. Seismicity of the Earth 1900–2012 Java and vicinity: U.S. Geological Survey Open-File Report 2010–1083-N, 1 sheet, scale 1:5,000,000, https://dx.doi.org/10.3133/ofr20101083N.
McCaffrey, R., and Nabelek, J.L., 1984. The geometry of back arc thrusting along the Eastern Sunda Arc, Indonesia: Constraints from earthquake and gravity data in JGR, Atm., vol., 925, no. B1, p. 441-4620, DOI: 10.1029/JB089iB07p06171
Okal, E. A., & Reymond, D., 2003. The mechanism of great Banda Sea earthquake of 1 February 1938: applying the method of preliminary determination of focal mechanism to a historical event in EPSL, v. 216, p. 1-15.
Silver, E.A., Breen, N.A., and Prastyo, H., 1986. Multibeam Study of the Flores Backarc Thrust Belt, Indonesia, in JGR., vol. 91, no. B3, p. 3489-3500
Zahirovic, S., Seton, M., and Müller, R.D., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014

Earthquake Report: Channel Islands Update #1

Posted on April 7, 2018 by Jay Patton

Well well.
There was lots of interest in this M 5.3 earthquake offshore of Ventura/Los Angeles, justifiably so. Southern California is earthquake country.
Here is an update. There was lots of information that I was trying to incorporate and I needed an additional report to cover some of this material. That being said, there is still some mystery about this earthquake. My favored interpretation is that this EQ was a left-lateral strike-slip earthquake. There is still room to interpret this as a right-lateral strike-slip (llss) earthquake however.
Below I have prepared some figures that provide additional information that helps us learn about the faulting and basin development in the CA Borderlands here. There is lots of work that has been done here and this is far from a comprehensive analysis.
As I mentioned before (here is my initial Earthquake Report for this EQ), due to the big bend in the San Andreas fault (SAF) in southern CA, there is evidence for compression in the form of thrust faults and uplifted mountains (e.g. Sierra Madre fault and the San Gabriel Mtns). One of these thrust faults (which may also have some strike-slip motion) is the Hollywood fault (recently highlighted by the recent work by the CA Geological Survey).
Also part of the development of the SAF involved the clockwise rotation of a crustal block where the Transverse Ranges are (the mtns to the north of Ventura/Santa Barbara). Along the southern boundary of the Transverse Ranges formed left-lateral strike slip faults. The Santa Cruz Island fault just happens to be a left-lateral strike-slip fault.
The CA Borderlands is a complex region of faulting, inheriting structures from the Tertiary, overprinted by modern tectonics and everything in between. The Hollywood fault trends towards (and turns into?) the Malibu Coast fault, which may turn into the Santa Cruz Island fault (SCIF), a vertical left-lateral strike-slip fault (but may have some vertical motion on it, based upon offsets in vertical uplift rates from marine terrace profiles).
Schindler used seismic reflection profiles in the Santa Cruz Basin area to interpret the tectonic history here. I placed the faults interpreted by them as orange lines in the interpretive poster (labeled as the Ferrelo fault and the East Santa Cruz (ECS) Basin fault system). The ESCBFS is a thrust fault system, with possible oblique motion (strike-slip). My initial interpretation was that this M 5.3 was a llss earthquake associated with this fault. There are some interesting problems that arise considering this fault. To the south, the fault is oriented similar to the San Clemente fault (which may have had a M 5.5 right-lateral strike-slip (rlss) earthquake on 1981.09.04). Due to this, the simple interpretation is that the ESCBFS is right lateral oblique at the southern part of the Santa Cruz Basin. However, along the northern boundary of this basin, the ESCBFS rotates to an east-west strike (orientation). The simple interpretation would be that this part of the fault system would be llss, similar to the SCIF. So, clearly, things are not so simple here. See the Chaytor et al. (2008) figure below.
That being said, if this M 5.3 earthquake was on an east-west fault, it would be llss. There is no evidence for a north-south oriented fault on the western boundary of the Santa Cruz Basin (see Schindler (2007) seismic profile below), supporting the left-lateral interpretation.

Below is my interpretive poster for this earthquake

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

I include some inset figures.
In the upper left corner is a cross section from Shaw and Suppe (1994). This cross section location is shown on the interpretive poster as a blue line labeled X-Y. This cross section (from interpretations of offshore seismic reflection profiles) shows the major player here is a thrust fault, the Channel Islands Thrust. Note the SCIF is also shown to rip right through Santa Cruz Island.
In the upper right corner is a map that shows the area of this fault ramp of the Channel Islands Thrust (Shaw and Suppe, 1994). Note that this fault ramp area is also shown on the interpretive poster, outlined in light orange.
In the center left is a figure from Fuis et al. (2001) that shows a block diagram revealing how the north-south convergence (from the bend in the San Andreas) is accommodated by thrust/reverse faults. The Sierra Madre fault is also labeled on the interpretive poster. A recent earthquake in La Habra is an example of this north-south compression. Here are my report and report update for this M 5.1 La Habra earthquake.
In the lower left corner is a seismic reflection profile from Schindler (2007), from her Master’s Thesis. The profile A-A’ is shown on the map as a green line labeled A-A’. Note that there is no faulting on the western boundary of the Santa Cruz Basin. When I first looked at this section, I thought that the ESCBFS were either normal (extensional) or strike-slip faults. After reading her thesis, I learned that these faults did have normal offset (in the Miocene Epoch, part of the Tertiary Period), but have been reactivated as thrust faults in post-Miocene time. The San Clemente fault (labeled on the interpretive poster) turns into the Santa Cruz-Catalina Ridge fault (labeled on this cross section).
In the lower right corner is a figure that shows how these faults interact in a complicated manner (Sorlien et al., 2006). This figure was prepared after they interpreted seismic reflection profile data. The upper panel is a low-angle oblique view of the faults in 3-D view. The lower two panels are the cross sections B-B’ and E-E’ (also shown on the interpretive poster as orange lines). These cross sections show how the Malibu Coast fault is more deeply dipping (more close to vertical) compared to the Santa Monica-Dume fault (a shallow dipping thrust fault). Both of these faults appear to join in some way near the coast, where they turn into the Hollywood fault. There are probably some inaccuracies in how I am interpreting how these faults interact beyond the limit of the figures I present here.

Here is the same map including the magnetic anomaly data (the red and blue shades).

USGS Earthquake Pages

These are from this current sequence

2018.04.05 M 5.3 Channel Islands
2018.04.06 M 2.4 Channel Islands

Some Relevant Discussion and Figures

Here is a map that shows where the seismic profile was acquired (Shaw and SUppe, 1994).

Epicenters from an earthquake swarm in 1984 (Henyey and Teng, 1985) define the active axial surface (A) of the Offshore Oak Ridge trend. Single-event (C and D) and composite (E and F) focal mechanism solutions from the 1984 seismicity have gentle north dipping (C, D, and E) and horizontal (F) nodal planes (Henyey and Teng, 1985) consistent with folding through the active axial surfaces by bedding parallel slip (see Figure 10B). Cross section traces: X-X’ (Fig. 7); X-Y (Fig. 11). SCIF = Santa Cruz Island fault.

Here is the cross section. The upper panel shows the modern configuration and the lower panel shows their interpretation during the Tertiary (Shaw and Suppe, 1994).

A balanced geologic cross section across the eastern Santa Barbara Channel and Santa Cruz Island combines subsurface seismic reflection and well-log data (the section trace is in Figs. 1 and 10A). The Channel Islands thrust ramps beneath the Offshore Oak Ridge trend and approaches the surface south of Santa Cruz Island. The kink-band width (A-A’) of the Offshore Oak Ridge trend represents dip slip on the underlying Channel Islands thrust. The shallow fold and fault geometry along the Offshore Oak Ridge and Blue Bottle trends is depicted in Figure 7. Strike-slip motion out of the section plane may occur on the Santa Cruz Island fault; however, moderate displacements on this fault should not significantly effect our area balance and restoration, because the strike-slip fault trace is perpendicular to the section plane (Fig. 10A). SCIF = Santa Cruz Island fault. Horizontal equals vertical scale.

For background, here is a timeline for the tectonics along the Pacific-North America plate boundary (Schindler, 2007). The Transverse Ranges block is shown as a green bleb labeled WTR. Note how this block is rotating in a clockwise fasion, and see that there are strike-slip faults that form along the block edge to accommodate this rotation.

A simple tectonic model of the evolution of the Pacific-North American plate boundary that includes the Inner and Outer Borderland (IB, OB) and rotation of the western Transverse Ranges (WTR) province (from Nicholson et al, 1994). The model assumes a constant rate and direction of Pacific plate motion and constant rate of western Transverse Ranges rotation. As each partially subducted microplate is captured by the Pacific plate (Monterey, ~19 Ma; Arguello, ~17.5 Ma; Guadalupe and Magdalena, ~12 Ma), this results in a transfer of part of the over-riding North American upper plate to the Pacific plate. The fine gray lines provide a reference grid fixed to North America. ArP-Arguello plate; GP-Guadalupe plate; MtP-Monterey plate; SG-San Gabriel block; JdFP-Juan de Fuca plate; SLB-San Lucia Bank; SMB-Santa Maria basin; SB-southern Borderland;T-AFTosco- Arbreojos fault; MP-Magdalena plate. Red areas are regions of transtension; Purple areas are captured or soon to be captured microplates.

Here is the seismic reflection profile from Schindler (2010).

Regional seismic line WC82-108 showing the ~50 km wide Santa Rosa Ridge anticlinorium. Parallel bedding of pre-Pliocene strata indicates that this anticlinal structure formed post Miocene. The Cretaceous-Paleogene sedimentary rocks are eroded by the early Miocene unconformity (green) and truncate against basement (black arrows). Mapped reference horizons and faults are shown in color and in black, respectively.

This is a fantastic low-angle oblique view of the topography and bathymetry of this region (and the Santa Cruz Basin) from Schindler (2010). The figure caption is embedded in the figure.

This is the figure from Schindler (2010) that shows the geometry of the ESCBFS and Ferrlo faults. Red shows the upper part of the faults. These faults dip to the north, northeast, and east.

A map view of 3D fault surfaces surrounding Santa Cruz basin in the northern Borderland. Depths down-dip along fault surfaces are shown as changing colors at even kilometer levels. The ESCB fault system is observed to be a gently east- to northeast-dipping, right stepping, en echelon reactivated reverse or oblique-reverse fault that bends to become more northerly dipping as it approaches Santa Cruz Island.

There has been lots of work here. Jason Chaytor (now at USGS in Woods Hole) worked on submerged marine terraces in this region. These marine terraces were formed when sea level was lower and are a result of erosion from ocean waves at that time. Dr. Chaytor used radiometric ages and sea level curve data to evaluate the tectonic uplift in the region. Here is a map that shows Jason’s interpretation of the seismic profiles for this region (same seismic data used by Schindler).

Preliminary map of geologic structures currently mapped using multichannel sparker, and recently released WesternGeco multichannel seismic-reflection profiles (modified from Chaytor, 2006). SCIF—Santa Cruz Island fault.

Geologic Fundamentals

For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

Social Media

139. Why the spattering of felt reports at ~100 km? Shaking is amplified by sediments, but also, waves travel down, hit a discontinuity known as the Moho, and bounce back up at distances of 80-100 km #200EQFacts pic.twitter.com/aJEgo859YM

— Susan Hough (@SeismoSue) April 7, 2018

140. Not to be outdone by California, Oklahoma dishes up a M4.6 near Perry. But whereas intensities for the M5.3 CA quake are bang on expectations, intensities for the M4.6 are low relative to central US expectations. Induced earthquakes = low stress drop?! #200EQFacts pic.twitter.com/hZKJwXgoCb

— Susan Hough (@SeismoSue) April 7, 2018

Watch the #earthquake waves from the M5.3 earthquake in southern California roll across our network of seismic stations! https://t.co/SoZMmJpUek #ChannelIslandsEarthquake pic.twitter.com/W2XefDeIE0

— IRIS Earthquake Sci (@IRIS_EPO) April 6, 2018

137. Tsunamis might not be a major hazard along most of the California coast, but they are possible…as demonstrated by the 21 Dec. 1812 earthquake #200EQFacts pic.twitter.com/D0xa67RXC0

— Susan Hough (@SeismoSue) April 6, 2018

https://t.co/6EvPXYh5VX https://t.co/bxn4MK1HCV

— Volkan Sevilgen (@volkansevilgen) April 5, 2018

San Andreas fault

General Overview

Earthquake Reports

Northern CA

2015.08.30 M 3.6 Mendocino County, CA
2015.07.27 M 3.5 Point Arena, CA

Central CA

2018.01.04 M 4.4 Berkeley

Southern CA

2016.02.23 M 4.9 Bakersfield

2014.04.01 M 5.1 La Habra p-3
2014.03.29 M 5.1 La Habra p-2
2014.03.28 M 5.1 La Habra p-1

Eastern CA

2016.08.04 M 4.5 Honey Lake, CA

Southern CA

Earthquake Reports

2018.04.05 M 5.3 Channel Islands
2018.04.05 M 5.3 Channel Islands Update #1
1994.11.17 M 6.7 Northridge, CA
1971.02.09 M 6.7 Sylmar, CA

References:

Chaytor, J.D., Goldfinger, C., Meiner, M.A., Huftile, G.J., Romsost, C.G., Legg, M.R., 2008. Measuring vertical tectonic motion at the intersection of the Santa Cruz–Catalina Ridge and Northern Channel Islands platform, California Continental Borderland, using submerged paleoshorelines in GSA Bulletin, v. 120, no. 7/8, p. 1053-1071, doi: 10.1130/B26316.1
Du, X., Hendy, I., Schimmelmann, 2018. A 9000-year flood history for Southern California: A revised stratigraphy of varved sediments in Santa Barbara Basin in Marine Geology, v. 397, p. 29-42, https://doi.org/10.1016/j.margeo.2017.11.014
Fuis, G.S., Ryberg, T., Godfrey, N.J., Okaya, D.A., Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1, p. 15-18
Legg, M. R., M. D. Kohler, N. Shintaku, and D. S. Weeraratne, 2015. Highresolution mapping of two large-scale transpressional fault zones in the California Continental Borderland: Santa Cruz-Catalina Ridge and Ferrelo faults, J. Geophys. Res. Earth Surf., 120, 915–942, doi:10.1002/2014JF003322.
Pinter, N., Lueddecke, S.B., Keller, E.A., Simmons, K.R., 1998. Late Quaternary slip on the Santa Cruz Island fault, California in GSA Bulletin, v. 110, no. 6, p. 711-722
Pinter, N., Johns, B., Little, B., Vestal, W.D., 2001. Fault-Related Folding in California’s Northern Channel Islands Documented by Rapid-Static GPS Positioning in GSA Today, May, 2001
Schindler, C.S., 2010. 3D Fault Geometry and Basin Evolution in the Northern Continental Borderland Offshore Southern California Catherine Sarah Schindler, B.S. A Thesis Submitted to the Department of Physics and Geology California State University Bakersfield In Partial Fulfillment for the Degree of Masters of Science in Geology
Shaw, J.H., Suppe, J., 1994. Active faulting and growth folding in the eastern Santa Barbara Channel, California in GSA Bulletin, v. 106, p. 607-626
Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [https://pubs.er.usgs.gov/publication/pp1515].