Earthquake Report: Indonesia

Posted on June 24, 2019 by Jay Patton

I had been making an update to an earthquake report on a regionally experienced M 5.6 earthquake from coastal northern California when I noticed that there was a M 7.3 earthquake in eastern Indonesia.
https://earthquake.usgs.gov/earthquakes/eventpage/us600044zz/executive
This earthquake is in a region of strike-slip faulting (if in downgoing plate for example) or subduction thrusting, so I thought it may or may not produce a tsunami. There are also intermediate depth quakes here (deeper than subduction zone megathrust events), like this earthquake (which reduces the chance of a tsunami). While we often don’t think of strike-slip earthquakes as those that could cause a tsunami, they can trigger tsunami, albeit smaller in size than those from subduction zone earthquakes or locally for landslides. But, I checked tsunami.gov just in case (result = no tsunami locally nor regionally). I also took a look at the tide gages in the region here and here (result = no observations).
South of this earthquake is a convergent plate boundary, where the Australia plate dives northwards beneath a part of the Sunda plate (Eurasia) forming the Java and Timor trenches (subduction zones). Far to the west, on 2 June 1994 there was a subduction zone megathrust earthquake along the Java Trench. Earlier, on 19 August 1977 there was an M 8.3 earthquake, but it was not a subduction zone thrust event, but an extensional earthquake in the downgoing Australia plate (Given and Kanamori, 19080). Both 1977 and 1994 events are shown on one of the maps below. The 1977 earthquake was tsunamigenic, creating a wave observed on tide gages at Damier, Hampton, and Port Hedland in Australia (Gusman et al., 2009).
To the north of the subduction zone, there is a parallel fault system that dips in the opposite direction as the subduction zone. This is referred to as a backthrust fault (it is a thrust fault and “backwards” to the main fault). The Wetar and Flores faults are both part of this backthrust system. In JUly and August of 2018 there was a series of earthquakes near the Island of Flores associated with this backthrust. Here is my final of 3 reports on those earthquakes.
The Timor trough wraps around to the north on its eastern end and eventually forms the Seram Trench, which dips to the south. The shape of these linked trenches forms a “U” shape with the open part of the U pointing to the west. Recently it has been published that the basin formed by these fault systems is the deepest forearc basin on Earth (Pownall et al., 2016). There was a subduction zone earthquake in 1938, called the Great Banda Sea Earthquake. Okal and Reymond (2003) prepared an earthquake mechanism for this M 8.5 earthquake.
To complicate matters, there is a large strike-slip system that comes into the area from the east (Papua New Guinea) and bisects the crest of the “U” shape. This strike slip system feeds into the backthrust so that the backthrust is both a thrust fault and a strike-slip fault. There are probably separate faults that accommodate these different senses of motion. There have been a series of strike-slip earthquakes in the 20th century associated with the strike-slip motion along this boundary. For example, Osada and Abe (1981) uses seismologic records (e.g. from seismometers) to prepare an earthquake mechanism for this M 8.1 earthquake. They found that it was an oblique strike-slip earthquake. The depth was pretty shallow compared to the M 7.3 earthquake I am reporting about today.
On 17 June 1987 there was another relatively shallow M 7.1 strike-slip earthquake on this strike-slip fault system.
However, there is also a deeper strike-slip fault within the Australia plate. This fault is probably what ruptured on 2 March 2005 (M 7.1) and 10 December 2012 (M 7.1). The M 7.3 earthquake from a day ago had a similar magnitude, depth, mechanism, and location as these earlier quakes. These may have all ruptured the same fault (or not).

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 1919-2019 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. Some earthquakes have older focal mechanisms plotted in black and white.

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.

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 Caroline and Australia plates. 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.

Global Strain

In a map below, I include a transparent overlay of the Global Strain Rate Map (Kreemer et al., 2014).
The mission of the Global Strain Rate Map (GSRM) project is to determine a globally self-consistent strain rate and velocity field model, consistent with geodetic and geologic field observations. The overall mission also includes:

contributions of global, regional, and local models by individual researchers
archive existing data sets of geologic, geodetic, and seismic information that can contribute toward a greater understanding of strain phenomena
archive existing methods for modeling strain rates and strain transients

The completed global strain rate map will provide a large amount of information that is vital for our understanding of continental dynamics and for the quantification of seismic hazards.
The version used in the poster(s) below is an update to the original 2004 map (Kreemer et al., 2000, 2003; Holt et al., 2005).

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 from Benz et al. (2011) that shows historic earthquake locations (epicenters) along with some of the plate boundary faults. Note the strike slip fault (with the opposing black arrows) that cross the location of the 1938 earthquake (labeled in yellow on that map). I placed a blue star in the location of the M 7.3 quake. There is a cross section to the right of the map that shows how earthquakes dive down with a westward trend (following the plate down the subduction zone). The cross section location is shown on the map (B-B’).
In the upper right corner is a larger scale tectonic map from Audley (2011) showing the major thrust faults and the large forearc basin is labeled “Weber Deep.”
Hangesh and Whitney (2016) did lots of work on the faulting in the region to the south of the M 7.3. They show block boundaries and relative plate motion arrows in white. Note how they extend strike-slip motion along the Timor trough. This may be in addition to the strike-slip along the backthrust.

Here is the map with a month’s seismicity plotted. I included MMI contours from a recent M 6.3 earthquake in PNG, which led to a sequence of additional M~6 quakes to the southeast of that main shock. I won’t be writing a report for those quakes, even though it is interesting (check it out!). Sorry to have misspelled Hengesh as Hangesh.

Here is the map with a century’s seismicity (M ≥ 7.0) plotted.

Here is the map with a month’s seismicity (M ≥ 0.5) plotted with the Global Strain data plotted. We can see the 2018 Flores swarm show up here.

Other Report Pages

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).

This is a great visualization showing the Australia plate and how it formed the largest forearc basin on Earth (Pownall et al., 2014).
The maps on the left show a time history of the tectonics. The low angle oblique view on the right shows the dipping crust (north is not always up, as in this figure).
In the lower right, they show how there is strike-slip faulting along the Seram trough also (I left out the figure caption for E).

Reconstructions of eastern Indonesia, adapted from Hall (2012), depict collision of Australia with Southeast Asia and slab rollback into Banda Embayment. Yellow star indicates Seram. Oceanic crust is shown in purple (older than 120 Ma) and blue (younger than 120 Ma); submarine arcs and oceanic plateaus are shown in cyan; volcanic island arcs, ophiolites, and material accreted along plate margins are shown in green. A: Reconstruction at 15 Ma. B: Reconstruction at 7 Ma. C: Reconstruction at 2 Ma. D: Visualization of present-day slab morphology of proto–Banda Sea based on earthquake hypocenter distribution and tomographic models

Here is a map and some cross sections showing seismic tomography (like C-T scans into the Earth using seismic waves instead of X-Rays). The map shows the location of the cross sections (Spakman et al., 2010).

The Banda arc and surrounding region. 200 m and 4,000 m bathymetric contours are indicated. The numbered black lines are Benioff zone contours in kilometres. The red triangles are Holocene volcanoes (http://www.volcano.si.edu/world/). Ar=Aru, Ar Tr=Aru trough, Ba=Banggai Islands, Bu=Buru, SBS=South Banda Sea, Se=Seram, Sm=Sumba, Su=Sula Islands, Ta=Tanimbar, Ta Tr=Tanimbar trough, Ti=Timor, W=Weber Deep.

Tomographic images of the Banda slab. Vertical sections through the tomography model along the lines shown in Fig. 1. Colours: P-wave anomalies with reference to velocity model ak135 (ref. 30). Dots: earthquake hypocentres within 12 km of the section. The dashed lines are phase changes at ~410 km and ~660 km. The sections are plotted without vertical exaggeration; the horizontal axis is in degrees. The labelled positive anomalies are the Sunda (Su) and Banda (Ba) slabs: BuDdetached slab under Buru, FlDslab under Flores, SDslab under Seram, TDslab under Timor. a, The Sunda slab enters the lower mantle whereas the Banda embayment slab is entirely in the upper mantle with the change under Sulawesi. b–e, Banda slab morphology in sections parallel to Australia plate motion shows a transition from a steep slab with a flat section (fs) (b) to a spoon shape shallowing eastward (c–e).

Here is the map from Benz et al. (2011).

Here is the tectonic map from Hengesh 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 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.

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.

Whitney and Hengesh (2015) used GPS modeling to suggest a model of plate blocks. Below are their model results.

Plate boundary segments in the Banda Arc region from Nugroho et al (2009). Numbers inside rectangles show possible micro-plate blocks near the Sumba Triple Junction (colored) based on GPS velocities (black arrows) with in a stable Eurasian reference frame.

Here is the conceptual model from Whitney and Hengesh (2015) that shows how left-lateral strike-slip faulting can come into the region.

Schematic map views of kinematic relations between major crustal elements in the Sumba Triple Junction region. CTZ= collisional tectonic zone. Red arrow size designates schematic plate motion relations based on geological data relative to a fixed Sunda shelf reference frame (pin).

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

Indonesia | Sumatra

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

2019.06.23 M 7.3 Banda Sea
2019.04.12 M 6.8 Sulawesi, Indonesia
2018.09.28 M 7.5 Sulawesi
2018.10.16 M 7.5 Sulawesi UPDATE #1
2018.08.19 M 6.9 Lombok, Indonesia
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

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.
Given, J. W., and H. Kanamori (1980). The depth extent of the 1977 Sumbawa, Indonesia, earthquake, in EOS Trans. AGU., v. 61, p. 1044.
Gusnman, A.R., Tanioka, Y., Matsumoto, H., and Iwasakai, S.-I., 2009. Analysis of the Tsunami Generated by the Great 1977 Sumba Earthquake that Occurred in Indonesia in BSSA, v. 99, no. 4, p. 2169-2179, https://doi.org/10.1785/0120080324
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.
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>
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
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.
Osada, M. and Abe, K., 1981. Mechanism and tectonic implications of the great Banda Sea earthquake of November 4, 1963 in Physics of the Earth and Plentary Interiors, v. 25, p. 129-139
Pownall, J.M., Hall, R., Armstrong,, R.A., and Forster, M.A., 2014. Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia in Geology, v. 42, no. 4, p. 379-282, https://doi.org/10.1130/G35230.1
Spakman, W. and Hall, R., 2010. Surface deformation and slab–mantle interaction during Banda arc subduction rollback in Nature Geosceince, v. 3, p. 562-566, https://doi.org/10.1038/NGEO917
Whitney, B.B. and Hengesh, J.V., 2015. A new model for active intraplate tectonics in western Australia in Proceedings of the Tenth Pacific Conference on Earthquake Engineering Building an Earthquake-Resilient Pacific 6-8 November 2015, Sydney, Australia, paper number 82
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

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Earthquake Report: Mendocino triple junction

Posted on June 23, 2019 by Jay Patton

Well, I was on the road for 1.5 days (work party for the Community Village at the Oregon Country Fair). As I was driving home, there was a magnitude M 5.6 earthquake in coastal northern California.
https://earthquake.usgs.gov/earthquakes/eventpage/nc73201181/executive
I didn’t realize this until I was almost home (finally hit the sack around 4 am).
This earthquake follows a sequence of quakes further to the northwest, however their timing is merely a coincidence. Let me repeat this. The M 5.6 earthquake is not related to the sequence of earthquakes along the Blanco fracture zone.
Contrary to what people have posted on social media, there was but a single earthquake. This earthquake happened beneath the area of Petrolia, nearby the 1991 Honeydew Earthquake. More about the Honeydew Earthquake can be found here.
This region also had a good sized shaker in 1992, the Cape Mendocino Earthquake, which led to the development of the National Tsunami Hazard Mitigation Program. More about the Cape Mendocino Earthquake can be found on the 25th anniversary page here and in my earthquake report here.
The regional tectonics in coastal northern California are dominated by the Pacific-North America plate boundary. North of Cape Mendocino, this plate boundary is convergent and forms the Cascadia subduction zone (CSZ). To the south of Cape Mendocino, the plate boundary is the right-lateral (dextral) San Andreas fault (SAF). Where these 2 fault systems meet, there is another plate boundary system, the right-lateral strike-slip Mendocino fault (don’t write Mendocino fracture zone on your maps!). Where these 3 systems meet is called the Mendocino triple junction (MTJ).
The MTJ is a complicated region as these plate boundaries overlap in ways that we still do not fully understand. Geologic mapping in the mid- to late-20th century provides some basic understanding of the long term history. However, recent discoveries have proven that this early work needs to be revisited as there are many unanswered questions (and some of this early work has been demonstrated to be incorrect). Long live science!
Last night’s M 5.6 temblor happened where one strand of the MF trends onshore (another strand bends towards the south). But, it also is where the SAF trends onshore. At this point, I am associating this earthquake with the MF (so, a right-lateral strike-slip earthquake). The mechanism suggest that this is not a SAF related earthquake. However, it is oriented in a way that it could be in the Gorda plate (making it a left-lateral strike-slip earthquake). However, this quake is at the southern edge of the Gorda plate (sedge), so it is unlikely this is a Gorda plate event.

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 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 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.

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 ~north-south trends of these red and blue stripes in the Pacific 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 North America plate, so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Juan de Fuca and Gorda plates.

Global Strain

In a map below, I include a transparent overlay of the Global Strain Rate Map (Kreemer et al., 2014).
The mission of the Global Strain Rate Map (GSRM) project is to determine a globally self-consistent strain rate and velocity field model, consistent with geodetic and geologic field observations. The overall mission also includes:

contributions of global, regional, and local models by individual researchers
archive existing data sets of geologic, geodetic, and seismic information that can contribute toward a greater understanding of strain phenomena
archive existing methods for modeling strain rates and strain transients

The completed global strain rate map will provide a large amount of information that is vital for our understanding of continental dynamics and for the quantification of seismic hazards.
The version used in the poster(s) below is an update to the original 2004 map (Kreemer et al., 2000, 2003; Holt et al., 2005).

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

n the upper left corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004)
Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes.
In the lower right corner is a map that shows a comparison between the USGS Did You Feel It? reports and the USGS Modified Mercalli Intensity shakemap model. This comparison shows that the model is a decent fit for the reports from real people. If you felt the earthquake, please submit a report to the USGS here.
In the upper right corner I include a larger scale view of seismicity for this area. I highlight the important historic events (e.g. the 1991 Honeydew Earthquake and the 1992 Cape Mendocino Earthquake sequence.

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 century’s seismicity plotted along with the Global Strain Map with a 30% transparency.

Here is the educational interpretive poster from the 1992 Cape Mendocino Earthquake (report here).

The USGS has been increasing the list of products that are produced in association with their earthquake pages. One of these products is an earthquake forecast (not a prediction as nobody can predict earthquakes yet) that lists the chance of an earthquake with a given magnitude over a certain period of time. The forecast for the M 5.6 earthquake is found here. These forecasts are updated periodically, so the information will change with time. Below is a table where I present the forecast as it was when I checked the page this morning (would be nice if the USGS would produce an easy to read table).

From the USGS:

Be ready for more earthquakes

More earthquakes than usual (called aftershocks) will continue to occur near the mainshock.
When there are more earthquakes, the chance of a large earthquake is greater which means that the chance of damage is greater.
The USGS advises everyone to be aware of the possibility of aftershocks, especially when in or around vulnerable structures such as unreinforced masonry buildings.
This earthquake could be part of a sequence. An earthquake sequence may have larger and potentially damaging earthquakes in the future, so remember to: Drop, Cover, and Hold on.

What we think will happen next

According to our forecast, over the next 1 Week there is a < 1 % chance of one or more aftershocks that are larger than magnitude 5.6. It is likely that there will be smaller earthquakes over the next 1 Week, with 0 to 11 magnitude 3 or higher aftershocks. Magnitude 3 and above are large enough to be felt near the epicenter. The number of aftershocks will drop off over time, but a large aftershock can increase the numbers again, temporarily.

About our earthquake forecasts

No one can predict the exact time or place of any earthquake, including aftershocks. Our earthquake forecasts give us an understanding of the chances of having more earthquakes within a given time period in the affected area. We calculate this earthquake forecast using a statistical analysis based on past earthquakes.
Our forecast changes as time passes due to decline in the frequency of aftershocks, larger aftershocks that may trigger further earthquakes, and changes in forecast modeling based on the data collected for this earthquake sequence.

Gosh, almost forgot to include this photo of the seismic waves recorded on the Humboldt State University Department of Geology Baby Benioff seismometer. Photo Credit: Amanda Admire.

USGS Landslide and Liquefaction Ground Failure data products

Below I present a series of maps that are intended to address the excellent ‘new’ products included in the USGS earthquake pages: landslide probability and liquefaction susceptibility (a.k.a. the Ground Failure data products).
First I present the landslide probability model. This is a GIS data product that relates a variety of factors to the probability (the chance of) landslides as triggered by this earthquake. There are a number of assumptions that are made in order to be able to produce this model across such a large region, though this is still of great value (like other aspects from teh USGS, e.g. the PAGER alert). Learn more about all of these Ground Failure products here.
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). I spend more time discussing landslides and liquefaction in this recent earthquake report.
This model, like all landslide computer models, uses similar inputs. I review these here:

Some information about ground shaking. Often, people use Peak Ground Acceleration, though in the past decade+, it has been recognized that the parameter “Arias Intensity” is a better measure of the energy imparted by the earthquake across the land and seascape. Instead of simply accounting for the peak accelerations, AI integrates the entire energy (duration) during the earthquake. That being said, PGA is a more common parameter that is available for people to use. For example, when I was modeling slope stability for the 2004 Sumatra-Andaman subduction zone earthquake, the only model that was calibrated to observational data were in units of PGA. The first order control to shaking intensity (energy observed at any particular location) is distance to the earthquake fault that slipped.
Some information about the strength of the materials (e.g. angle of internal friction (the strength) and cohesion (the resistance).
Information about the slope. Steeper slopes, with all other things being equal, are more likely to fail than are shallower slopes. Think about skiing. Beginners (like me) often choose shallower slopes to ski because they will go down the slope slower, while experts choose steeper slopes.

Areas that are red are more likely to experience landslides than areas that are colored blue. I include a coarse resolution topographic/bathymetric dataset to help us identify where the mountains are relative to the coastal plain and continental shelf (submarine).

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.
The liquefaction susceptibility map for the M 5.6 earthquake did not suggest that there would be possibly much liquefaction from this earthquake (probably due to the small magnitude). I discuss liquefaction more in my earthquake report on the 28 September 20018 Sulawesi, Indonesia earthquake, landslide, and tsunami here.

Here is a map that shows shaking intensity using the MMI scale (mentioned and plotted in the main earthquake poster maps). I present this here in the same format as the ground failure model maps so we can compare these other maps with the ground shaking model (which is a first order control on slope failure).

Other Report Pages

Some Relevant Discussion and Figures

Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).

This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes.

I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.

Hemphill-Haley, E., 1995. Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern coastal Washington in GSA Bulletin, v. 107, p. 367-378.
Nelson, A.R., Shennan, I., and Long, A.J., 1996. Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America in Journal of Geophysical Research, v. 101, p. 6115-6135.
Atwater, B.F. and Hemphill-Haley, E., 1997. Recurrence Intervals for Great Earthquakes of the Past 3,500 Years at Northeastern Willapa Bay, Washington in U.S. Geological Survey Professional Paper 1576, Washington D.C., 119 pp.

This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.

Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.

Here is a link to the embedded video below, showing the week-long seismicity in April 1992.

This is the map used in the animation below. Earthquake epicenters are plotted (some with USGS moment tensors) for this region from 1917-2017 with M ≥ 6.5. I labeled the plates and shaded their general location in different colors.
I include some inset maps.

In the upper right corner is a map of the Cascadia subduction zone (Chaytor et al., 2004; Nelson et al., 2004).
In the upper left corner is a map from Rollins and Stein (2010). They plot epicenters and fault lines involved in earthquakes between 1976 and 2010.

Here is a link to the embedded video below, showing these earthquakes.

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

Cascadia subduction zone

General Overview

1700.09.26 M 9.0 Cascadia’s 315th Anniversary 2015.01.26
1700.09.26 M 9.0 Cascadia’s 316th Anniversary 2016.01.26 updated in 2017 and 2018
1992.04.25 M 7.1 Cape Mendocino 25 year remembrance
1992.04.25 M 7.1 Cape Mendocino 25 Year Remembrance Event Page
Earthquake Information about the CSZ 2015.10.08

Earthquake Reports

Explorer plate

Uncertain

2017.06.11 M 3.5 Gorda or NAP?

2016.07.21 M 4.7 Gorda or NAP? p-1
2016.07.21 M 4.7 Gorda or NAP? p-2

Social Media

References:

Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gràcia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2012 a. Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper # 1661F. U.S. Geological Survey, Reston, VA, 184 pp.
Dengler, L.A., and McPherson, R.C., 1993. The 17 August 1991 Honeydew Earthquake, North Coast California: A Case for Revising the Modified Mercalli Scale in Sparsely Populated Areas in BSSA, v. 83, no. 4, pp. 1081-1094
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>
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.
McCrory, P.A., 2000, Upper plate contraction north of the migrating Mendocino triple junction, northern California: Implications for partitioning of strain: Tectonics, v. 19, p. 11441160.
McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006, Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey
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
Nelson, A.R., Kelsey, H.M., Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone. Quaternary Research 65, 354-365.
Oppenheimer, D., Beroza, G., Carver, G., Dengler, L., Eaton, J., Gee, L., Gonzalez, F., Jayko, A., Ki., W.H., Lisowski, M., Magee, M., Marshall, G., Murray, M., McPherson, R., Romanowicz, B., Satake, K., Simpson, R., Somerille, P., Stein, R., and Valentine, D., The Cape Mendocino, California, Earthquakes of April, 1992: Subduction at the Triple Junction in Science, v. 261, no. 5120, p. 433-438.
Patton, J. R., Goldfinger, C., Morey, A. E., Romsos, C., Black, B., Djadjadihardja, Y., and Udrekh, 2013. Seismoturbidite record as preserved at core sites at the Cascadia and Sumatra–Andaman subduction zones, Nat. Hazards Earth Syst. Sci., 13, 833-867, doi:10.5194/nhess-13-833-2013, 2013.
Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
Stein, R.S., Marshall, G.A., Murray, M.H., Balazs, E., Carver, G.A., Dunklin, T.A>, McLaughlin, R.J., Cyr, K., and Jayko, A., 1993. Permanent Ground Movement Associate with the 1992 M=7 Cape Mendocino, California, Earthquake: Implications for Damage to Infrastructure and Hazards to navigation, U.S. Geological Survey Open-File Report 93-383.
Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003, A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.

Return to the Earthquake Reports page.

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Earthquake Report: Kermadec Trench

Posted on June 15, 2019 by Jay Patton

There was just an earthquake associated with the plate boundary that forms the Kermadec Trench, a deep oceanic trench that extends north from New Zealand, towards the Fiji Islands.
https://earthquake.usgs.gov/earthquakes/eventpage/us6000417i/executive
A minor tsunami (~25 cm in size) has been recorded at Raoul Island, due west of the earthquake, the closest gage to the temblor. Tide gages in New Zealand just began recording a small tsunami the moments I started writing this report (about an hour ± after the earthquake).
This tsunami is small enough that it probably won’t cause much damage. However, tidal inlets and harbors can have currents that are higher in response to even small tsunami, if the shape of the seafloor/harbor is optimal for this. However, further away from the earthquake, the tsunami will be even smaller; so small that it may not be observable in tide gage data.

These are the tide gage data from Raoul Island.
These are data from 15 Jun 22:30 UTC until 16 Jun 02:48 UTC.

Here are the tsunami observations as reported by the Pacific Tsunami Warning Center found at http://tsunami.gov

In this part of the world, there is a convergent plate boundary where the Pacific plate dives westward beneath the Australia plate forming the Kermadec megathrust subduction zone fault. This fault has a history of earthquakes with magnitudes commonly exceeding M 7 and some exceeding M 8.
There was recently an M 6.9 earthquake in this same area and here is my earthquake report for that shaker.

While we cannot predict earthquakes, based on the historic record, this earthquake may be all that happens right now. But our historic record is incredibly short, so people must remain vigilant at all times.

Below is my interpretive poster for this earthquake

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes (including a M 6.1 earthquake that happened about an hour prior to the M 7.2. This is very close in time. The M 6.1 is too small of a magnitude to change the static coulomb stress significantly. It seems possible that there was dynamic triggering though (???). I will need to think about this a little more (check out the literature on dynamic triggering, to see what time window that may be a relevant trigger).

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.

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.

Global Strain

In a map below, I include a transparent overlay of the Global Strain Rate Map (Kreemer et al., 2014).
The mission of the Global Strain Rate Map (GSRM) project is to determine a globally self-consistent strain rate and velocity field model, consistent with geodetic and geologic field observations. The overall mission also includes:

contributions of global, regional, and local models by individual researchers
archive existing data sets of geologic, geodetic, and seismic information that can contribute toward a greater understanding of strain phenomena
archive existing methods for modeling strain rates and strain transients

The completed global strain rate map will provide a large amount of information that is vital for our understanding of continental dynamics and for the quantification of seismic hazards.
The version used in the poster(s) below is an update to the original 2004 map (Kreemer et al., 2000, 2003; Holt et al., 2005).

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

In the lower right corner is a map that shows the major islands, the major plate tectonic boundaries (the faults, the volcanoes), and the location of two profiles shown above (Ballance et al., 1999. I place a blue star in the general location of the earthquake.
In the upper right corner are these two profiles (17-1 & 17-2). These profiles show how the elevation changes (solid line) and how the geomagnetic properties intensity, declination, inclination (dashed) vary across the plate boundary.
In the lower left corner is a map from Benz et al. (2010) that shows earthquakes with circles that represent magnitude (diameter) and depth (color). Deeper = blue & shallower = red. There is a cross section (cut into the earth) profile through this seismicity that uses a source area as shown by a rectangle (the green line J-J’).
In the upper left corner is cross section J-J’ that shows earthquake hypocenters (3-D locations) in the region of the M 7.2 earthquake.
there is a cross section of the Kermadec trench that includes bathymetry of the region (topography of the sea floor). This graphic was created by scientists at Woods Hole. I label the Louisville Seamount Chain for reference to compare with the main map.

Here is the map with a month’s seismicity M ≥ 0.5 plotted (and magnetic anomalies).

Here is the map with a years’s seismicity M ≥ 2.0 plotted (and magnetic anomalies).

Here is the map with a century’s seismicity M ≥ 6.0 plotted (and strain).

Here is the map with a century’s seismicity M ≥ 7.0 plotted (and strain).

Other Report Pages

Tide Gage Data

First I present a tide gage summary map with the earthquakes from the past month shown transparently. Below are some of the tide gage data plots. These are all available from the International Oceanographic Commission.

here is a map that shows cross sections of seismicity, along with the tide gage data from the nearest station.

Here I have congregated all the tide gage data onto a single figure, each aligned relative to GMT time. Note which sites have up-first tsunami waves, relative to those that have down-first waves. Can you make sense of this?

Some Relevant Discussion and Figures

Here is the tectonic map from Ballance et al., 1999.

Map of the Southwest Pacific Ocean showing the regional tectonic setting and location of the two dredged profiles. Depth contours in kilometres. The presently active arcs comprise New Zealand–Kermadec Ridge–Tonga Ridge, linked with Vanuatu by transforms associated with the North Fiji Basin. Colville Ridge–Lau Ridge is the remnant arc. Havre Trough–Lau Basin is the active backarc basin. Kermadec–Tonga Trench marks the site of subduction of Pacific lithosphere westward beneath Australian plate lithosphere. North and South Fiji Basins are marginal basins of late Neogene and probable Oligocene age, respectively. 5.4sK–Ar date of dredged basalt sample (Adams et al., 1994).

Here is a great visualization of the Kermadec Trench from Woods Hole.

Kermadec Trench from Woods Hole Oceanographic Inst. on Vimeo.

Here is another map of the bathymetry in this region of the Kermadec trench. This was produced by Jack Cook at the Woods Hole Oceanographic Institution. The Lousiville Seamount Chain is clearly visible in this graphic.

I put together an animation of seismicity from 1965 – 2015 Sept. 7. Here is a map that shows the entire seismicity for this period. I plot the slab contours for the subduction zone here. These were created by the USGS (Hayes et al., 2012).

Here is the animation. Download the mp4 file here. This animation includes earthquakes with magnitudes greater than M 6.5 and this is the kml file that I used to make this animation.

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

2019.06.15 M 7.2 Kermadec
2019.05.14 M 7.5 New Ireland
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

Mw=6.2, TONGA ISLANDS (Depth: 15 km), 2019/06/15 21:56:09 UTC – Full details here: https://t.co/5PNNCtfejY pic.twitter.com/xIF3FBF4Ba

— Earthquakes (@geoscope_ipgp) June 15, 2019

Mw=7.4, KERMADEC ISLANDS, NEW ZEALAND (Depth: 37 km), 2019/06/15 22:55:02 UTC – Full details here: https://t.co/NtMjNEXgZ1 pic.twitter.com/lacrYnNpHi

— Earthquakes (@geoscope_ipgp) June 15, 2019

Mw=6.4, KERMADEC ISLANDS REGION (Depth: 25 km), 2019/06/16 05:17:16 UTC – Full details here: https://t.co/zq4F7wwAvo pic.twitter.com/oLX1Ep8Bp1

— Earthquakes (@geoscope_ipgp) June 16, 2019

Preliminary Magnitude 7.3 #earthquake in KermadecIslandsRegion, 2019-06-15 22:54 UTC time by #RaspberryShake #CitizenScience #seismograph network. Join the largest citizen #seismic community – https://t.co/bkUOY6dBm2. https://t.co/Ten7tGxaTS pic.twitter.com/FzthIcYzHE

— Raspberry Shake (@raspishake) June 15, 2019

Meanwhile, in other news that's much nicer to think about, a large earthquake deep below the Pacific that didn't hurt people showed up beautifully in my central London flat on the @raspishake pic.twitter.com/i2F3fWGrjP

— Austin Elliott (@TTremblingEarth) June 16, 2019

Check out large amplitude long period waves from morning M7.2 kermadec's earthquake at GRZ (geonet.bottom plot) compared to more distant 2011 M7.4 but similar depth& rupture mechanism (top plot). Similarity of path, mechanism&depth points towards slow rupture mechanism. Thoughts? pic.twitter.com/6iJP4OKOdu

— Caroline Holden (@Carolinenz59) June 16, 2019

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. Lavall ee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
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

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Earthquake Report: Chile

Posted on June 14, 2019 by Jay Patton

This morning (my time) there was a magnitude M 6.4 earthquake offshore of Chile. While it was in the correct location to possibly cause a tsunami, the magnitude was too small.
https://earthquake.usgs.gov/earthquakes/eventpage/us600040ja/executive
The major plate boundary here is the megathrust subduction zone that forms the Peru-Chile trench. Here, the Nazca plate dives eastwards beneath the South America plate.
Many people are familiar with subduction zone earthquakes which are responsible for the largest size temblors possible, as well as tsunami capable of travelling across the entire Pacific Ocean. The largest earthquake recorded on modern instruments is the 22 May 1960 M 9.5 Chile earthquake. There have been 2 large transoceanic tsunami caused by subduction zone earthquakes in 2010 and 2015. At the bottom of this report is a list of other earthquakes in this region.
A few months ago, there was an earthquake with a magnitude of M 6.7. However, this earthquake was an extensional earthquake, instead of a compressional earthquake that we typically associate with subduction zones.
This M 6.7 was down-dip (east) of today’s quake. It is possible that the M 6.7 terremoto caused “static coulomb” stress changes in the surrounding region that may have led to today’s earthquake. Someone would need to conduct some numerical analyses to test this hypothesis (I don’t currently have a matlab license, so cannot run Coulomb software to do this analysis myself). I wrote about the M 6.7 earthquake in an earthquake report, as well as for a Temblor article.
There have been several sequences in this same area of the subduction zone that people have used to suggest other types of stress changes from earlier quakes that led to later quakes (e.g. a sequence in 1997, e.g. Leyton et al., 2009 and Gardi et al., 2006).
There are a number of examples at other subduction zones where extensional and compressional earthquakes in different regions can trigger earthquakes of the opposite type. In 2009 earthquakes along the Kuril subduction zone and in 2011 earthquakes east of Japan are good examples.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past year, with color representing depth and diameter representing magnitude (see legend), for earthquakes M ≥ 4.0. I include earthquake epicenters from some specific historic earthquakes with magnitudes M ≥ 4.0 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.

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.

Magnetic Anomalies

In one 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 northwest-southeast 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 South America plate, so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Nazca 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 I include a figure that includes a variety of interesting information (Horton, 2018). From left to right are (a) the tectonic features, (b) the topography, and (c) features the South America plate that reflect the response to changes in the subduction zone over time. I include a blue star in the general location of today’s earthquake.
In the lower right corner is a map that shows the relative seismic hazard for this plate boundary (Rhea et al., 2010). I plot both 2019 earthquakes.< The numbers (“80”) indicate the rate at which the Nazca Plate is subducting beneath South America. 80 mm/yr = 3 in/yr.
In the upper left corner is a profile slicing into the Earth showing earthquakes as they get deeper as the Nazca plate dives deeper beneath the South America plate (Leyton et al., 2009). This cross section is located just to the south of today’s earthquake. I plot both M 6.7 and M 6.4 earthquakes on this section.
This is an illustration showing some locations where earthquakes may happen along subduction zones in general. The M 6.4 earthquake is probably a megathrust subduction zone earthquake, while the M 6.7 is probably in the downgoing oceanic crust of the Nazca plate.
This is a composite figure from several figures from Metois et al., 2016. On the left is a panel that shows the latitudinal range of earthquake ruptures (I fixed it in places as the original figure did not extend the 2010 rupture sufficiently to the north). The panel on the right shows how much the subduction zone fault is “locked” (or, seismically coupled). Darker colors represent parts of the fault that are storing more energy over time and are possibly places where the fault will slip (compared to parts of the fault that are white or yellow, which may be places where the fault is currently slipping and would not generate earthquakes in the future). This is simply a model and there is not way to really know where an earthquake will happen until there is an earthquake.

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

Here is the map with a seismicity plotted that is associated with specific earthquakes. I plot earthquakes for the 3 months following the mainshock listed for these example earthquakes (e.g. 1960, 1985, 2007, 2014, and 2015.

Other Report Pages

Some Relevant Discussion and Figures

Here is the overview figure from Horton, 2018.

Maps of (A) tectonic framework, (B) topography, and (C) sedimentary basin configuration of South America. (A) Map of plate boundaries, Andean magmatic arc (including the northern, central, and southern volcanic zones), regions of flat slab subduction, modern stress orientations from earthquake focal mechanisms, eastern front of Andean fold-thrust belt, and key segments of the retroarc foreland basin system. Plate velocities are shown relative to stable South American plate (DeMets et al., 2010). (B) DEM topographic map showing the Andes mountains and adjacent foreland region, including the Amazon, Parana, Orinoco, and Magdalena (Mag) river systems. (C) Map of Andean retroarc basins, showing isopach thicknesses (in km) of Cretaceous-Cenozoic basin fill, forebulge axis (from Chase et al., 2009), and locations of 13 sites (8 foreland basins, 5 hinterland basins) considered in this synthesis

Here is the seismic hazard map is from Rhea et al. (2010).

Here is the seismicity map and space time diagram from Métois et al. (2016). The subduction zone fault in the region of Coquimbo, Chile changes geometry, probably because of the Juan Fernandez Ridge (this structure controls the shape of the subduction zone). This figure shows a map and cross section for two parts of the subduction zone (Marot et al., 2014). The example on the left is the in the region of both the M 6.7 and M 6.4 earthquakes. Note how the subduction zone flattens out with depth here. The M=6.7 quake was shallower than this, but the shape of the downgoing slab does affect the amount of slab pull (tension in the down-dip direction) is exerted along the plate.

Left estimated extent of large historical or instrumental ruptures along the Chilean margin adapted from ME´ TOIS et al. (2012). Gray stars mark major intra-slab events. The recent Mw[8 earthquakes are indicated in red. Gray shaded areas correspond to LCZs defined in Fig. 3. Right seismicity recorded by the Centro Sismologico Nacional (CSN) during
interseismic period, color-coded depending on the event’s depth. Three zones have been defined to avoid including aftershocks and preshocks associated with major events: (1) in North Chile, we plot the seismicity from 2008 to january 2014, i.e., between the Tocopilla and Iquique earthquakes; (2) in Central Chile, we plot the seismicity on the entire 2000–2014 period; (3) in South-Central Chile, we selected events that occurred between 2000 and 2010, i.e., before the Maule earthquake.

This figure is the 3 panel figure in the interpretive poster showing how seismicity is distributed along the margin, how historic earthquake slip was distributed, and how the fault may be locked (or slipping) along the megathrust fault.

a Histogram depicts the rate of Mw>3 earthquakes registered by the CSN catalog during the interseismic period defined for each zone (see Fig. 2) on the subduction interface, on 0.2° of latitude sliding windows. Stars are swarm-like sequences detected by HOLTKAMP et al. (2011) depending on their occurrence date. Swarms located in the Iquique LCZ and Camarones segment are from RUIZ et al. (2014). Empty squares are significant intraplate earthquakes. b Red curve variations of the average coupling coefficient on the first 60 km of depth calculated on 0.2° of latitude sliding windows for our best model including an Andean sliver motion. Dashed pink curves are alternative models with different smoothing options that fit the data with nRMS better than 2 (see supplementary figure 6): the pink shaded envelope around our best model stands for the variability of the coupling along strike. Green curves coseismic distribution for Maule (VIGNY et al. 2011), Iquique (LAY et al. 2014) and Illapel earthquakes (RUIZ et al. 2016). Gray shaded areas stand for the identified low coupling zones (LCZs). LCZs and high coupling segments are named on the left. The apparent decrease in the average coupling North of 30°S is considered as an artifact of the Andean sliver motion (see Sect. 5.2). c Best coupling distribution obtained inverting for Andean sliver motion and coupling amount simultaneously. The rupture zones for the three major earthquakes are indicated as green ellipses. White shaded areas are zones where we lack resolution.

This is a figure that shows details about the coupling compared to some slip models for the 2010, 2014, and 2015 earthquakes. Today’s M=6.4 earthquake happened near the city of La Serena. Notice the location of this city compared to the slip on the subduction zone during the 20015 M=8.4 [8.43] earthquake.

Left coupling maps (color coded) versus coseismic slip distributions (gray shaded contours in cm) for the last three major Chilean earthquakes (epicenters are marked by white stars). From top to bottom Iquique area, white squares are pre-seismic swarm event in the month before the main shock, green star is the 2005, Tarapaca´ intraslab earthquake epicenter, blue star is the Mw 6.7 Iquique aftershock; Illapel area, green squares show the seismicity associated with the 1997 swarm following the Punitaqui intraslab earthquake (green star); Maule area, green star is the epicenter of the 1939 Chillan intraslab earthquake. Right interseismic background seismicity in the shallow part of the subduction zone (shallower than 60 km depth) for each region (red dots) together with 80 and 90 % coupling contours. White dots are events identified as mainshock after a declustering procedure following GARDNER and KNOPOFF (1974). Yellow areas extent of swarm sequences identified by HOLTKAMP et al. (2011) for South and Central Chile, and RUIZ et al. (2014) for North Chile.

This is the fault locking figure from Saillard et al. (2017), showing the percent coupling (how much of the plate convergence contributes to deformation of the plate boundary, which may tell us places on the fault that might slip during an earthquake. We are still learning about why this is important and what it means.

Comparison between the uplift rates, interseismic coupling, major bathymetric features, and peninsulas along the Andean margin (10°S–40°S). (a) Uplift rates of marine terraces reported in the literature (we present the average rate since terrace abandonment; Table S1 in the supporting information [Jara-Muñoz et al., 2015]). Each color corresponds to a marine terrace assigned to a marine isotopic stage (MIS). Gray dots are the uplift rates of the central Andean rasa estimated from a numerical model of landscape evolution [Melnick, 2016]. (b) Major bathymetric features and peninsulas and pattern of interseismic coupling of the Andean margin from GPS data inversion (this study). Gray shaded areas correspond to the areas where the spatial resolution of inversion is low due to the poor density of GPS observations (see text and supporting information for more details). The Peru-Chile trench (thick black line), the coastline (thin black line), and the convergence direction (black arrows) are indicated. We superimposed the curve obtained by shifting the trench geometry eastward by 110 km (trench-coast distance of 110 km; blue line) with the curve reflecting the 40 km isodepth of the subducting slab (red line; Slab1.0 from Hayes and Wald [2009]), a depth which corresponds approximately with the downdip end of the locked portion of the Andean seismogenic zone (±10 km) [Ruff and Tichelaar, 1996; Khazaradze and Klotz, 2003; Chlieh et al., 2011; Ruegg et al., 2009; Moreno et al., 2011; Métois et al., 2012]. The two curves are spatially similar in the erosive part of the Chile margin (north of 34°S), whereas they diverge along the shallower slab geometry in the accretionary part of the Chile margin (south of 34°S), where the downdip end of the locked zone may be shallower (Figure 4b). Red arrows indicate the low interseismic coupling associated with peninsulas and marine terraces and evidence of aseismic afterslip (after Perfettini et al. [2010] below the Pisco-Nazca Peninsula; Pritchard and Simons [2006], Victor et al. [2011], Shirzaei et al. [2012], Bejar-Pizarro et al. [2013], and Métois et al. [2013] for the Mejillones Peninsula; Métois et al. [2012, 2014] below the Tongoy Peninsula; and Métois et al. [2012] and Lin et al. [2013] for the Arauco Peninsula). FZ: Fracture zone. Horizontal blue bands are the areas where coastline is less than 110 km (light blue) or 90 km (dark blue) from the trench (see Figure 1).

The following figures from Leyton et al. (2009) are great analogies, showing examples of interplate earthquakes (e.g. subduction zone megathrust events) and intraplate earthquakes (e.g. slab quakes, or events within the downgoing plate). The first figures are maps showing these earthquakes, then there are some seismicity cross sections.

Maps showing the location of the study and the events used ((a)–(c)). In red we present interplate earthquakes, while in blue, the intermediate depth, intraplate ones. We used beach balls to plot those events with known focal and circles for those without. White triangles mark the position of the Chilean Seismological Network used to locate the events; those with names represent stations used in the waveform analysis (either accelerometers or broadbands with known instrumental response). Labels over beach balls correspond to CMT codes.

Here are 2 cross sections showing the earthquakes plotted in the maps above (Leyton et al., 2009).

Cross-section at (a) 33.5◦S and (b) 36.5◦S showing the events used in this study. In red we present interplate earthquakes, while in blue, the intermediate depth, intraplate ones.We used beach balls (vertical projection) to plot those events with knownfocal and circles for those without. In light gray is shown the background seismicity recorded from 2000 to 2006 by the Chilean Seismological Service

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

Chile | South America

General Overview

2010.02.27 M 8.8 Earthquake Review

Earthquake Reports

2019.06.14 M 6.4 Chile
2019.05.26 M 8.0 Peru
2019.05.12 M 6.1 Panama
2019.03.01 M 7.0 Peru
2019.02.22 M 7.5 Ecuador
2019.01.20 M 6.7 Chile
2018.08.21 M 7.3 Venezuela
2018.08.24 M 7.1 Peru
2018.04.02 M 6.8 Bolivia
2018.01.14 M 7.1 Peru
2018.01.15 M 7.1 Peru Update #1
2017.06.30 M 6.0 Ecuador
2017.04.24 M 6.9 Chile
2017.04.23 M 5.9 Chile
2016.12.25 M 7.6 Chile
2016.11.24 M 7.0 El Salvador
2016.11.04 M 6.4 Maule, Chile
2016.04.16 M 7.8 Ecuador
2016.04.16 M 7.8 Ecuador Update #1
2015.11.29 M 5.9 Argentina
2015.11.11 M 6.9 Chile
2015.11.24 M 7.6 Peru
2015.11.26 M 7.6 Peru Update
2015.09.16 M 8.3 Chile
2014.04.01 M 8.2 Chile
2010.02.27 M 8.8 Chile
1960.05.22 M 9.5 Chile

Social Media

References:

Beck, S., Barrientos, S., Kausel, E., and Reyes, M., 1998. Source Characteristics of Historic Earthquakes along the Central Chile Subduction Zone in Journal of South American Earth Sciences, v. 11, no. 2, p. 115-129, https://doi.org/10.1016/S0895-9811(98)00005-4
Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
Gardi, A., A. Lemoine, R. Madariaga, and J. Campos (2006), Modeling of stress transfer in the Coquimbo region of central Chile, J. Geophys. Res., 111, B04307, https://doi.org/10.1029/2004JB003440
Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
Horton, B.K., 2018. Sedimentary record of Andean mountain building< in Earth-Science Reviews, v. 178, p. 279-309, https://doi.org/10.1016/j.earscirev.2017.11.025
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.
Leyton, F., Ruiz, J., Campos, J., and Kausel, E., 2009. Intraplate and interplate earthquakes in Chilean subduction zone:
A theoretical and observational comparison in Physics of the Earth and Planetary Interiors, v. 175, p. 37-46, https://doi.org/10.1016/j.pepi.2008.03.017
Marot, M., Monfret, T., Gerbault, M.,. Nolet, G., Ranalli, G., and Pardo, M., 2014. Flat versus normal subduction zones: a comparison based on 3-D regional traveltime tomography and petrological modelling of central Chile and western Argentina (29◦–35◦S) in GJI, v. 199, p. 1633-164, https://doi.org/10.1093/gji/ggu355
Métois, M., Vigny, C., and Socquet, A., 2016. Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38°–18°S) in Pure Applied Geophysics, https://doi.org/10.1007/s00024-016-1280-5
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: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
Rhea, S., Hayes, G., Villaseñor, A., Furlong, K.P., Tarr, A.C., and Benz, H.M., 2010. Seismicity of the earth 1900–2007, Nazca Plate and South America: U.S. Geological Survey Open-File Report 2010–1083-E, 1 sheet, scale 1:12,000,000.
Ruiz, S. and Madariaga, R., 2018. Historical and recent large megathrust earthquakes in Chile in Tectonophysics, v. 733, p. 37-56, https://doi.org/10.1016/j.tecto.2018.01.015
Saillard, M., L. Audin, B. Rousset, J.-P. Avouac, M. Chlieh, S. R. Hall, L. Husson, and D. L. Farber, 2017. From the seismic cycle to long-term deformation: linking seismic coupling and Quaternary coastal geomorphology along the Andean megathrust in Tectonics, 36, https://doi:10.1002/2016TC004156.

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Earthquake Report: San Clemente Island

Posted on June 6, 2019 by Jay Patton

Well, yesterday was the start of a sequence of earthquakes offshore of San Clemente Island, about 100 km west of San Diego, California. The primary tectonic player in southern CA is the Pacific – North America plate boundary fault, the San Andreas (SAF).

2019.06.05 10:47:18 (UTC)M 4.3
2019.06.05 14:32:09 (UTC)M 4.3
2019.06.05 14:37:35 (UTC)M 4.3
2019.06.05 16:13:43 (UTC)M 4.3
2019.06.05 22:33:25 (UTC)M 3.3
2019.06.06 01:44:33 (UTC)M 2.4
2019.06.06 02:21:17 (UTC)M 2.3
2019.06.06 11:18:09 (UTC)M 2.8
2019.06.06 11:25:36 (UTC)M 3.5
2019.06.06 17:19:10 (UTC)M 1.6

The region offshore where this ongoing sequence is called the California Continental Borderlands (CCB). There exists an excellent record of how the North America – Pacific plate margin boundary has evolved through time (remember, prior to about 29 million years ago, this plate boundary in southern CA was a subduction zone).
There was an earthquake offshore of Los Angeles last year. Check out my earthquake report and report update.
In places the SAF is a single thoroughgoing fault (e.g. in the southern San Joaquin Valley), in others it splays into multiple strands (in Orange County between the Santa Ana Mtns and Lake Elsinore), and in other places it bends to create regions of uplift (like in Ventura or the Santa Monica Mtns). The active faulting in the CCB is basically a series of right-lateral faults that step and bend to form uplifted islands and terraces, along with pull-apart sedimentary basins.
San Clemente Island is a region of uplifted non-marine Tertiary volcanic rocks (andesite and dacite) with ages ranging from 14.8 – 16.5 million years ago (Yeats, 1968; Merifield et al., 1971; Ward and Valenise, 1996). These rocks are overlain by Tertiary (Miocene) sediments (limestone, siltstone, shale, and diatomite; correlates to the Monterey Formation) and Plio-Pleistocene sediments (sandstones and conglomerates; correlates to the Fernando Formation found onshore; Stadum & Susuki, 1976; Ward and Valenise, 1996).
The bedrock is folded into a northwest trending anticline (rocks are folded upwards with the crest in the center of the island, forming a convex upward fold). Moore (1969) use regional compilations of seismic reflection data to show that this type of tectonic folding is ubiquitous throughout the CCB.
Ward and Velensise (1996) suggest that the San Clemente island formed via uplift during progressive slip on two, southeast striking, southwest dipping, blind thrust faults. These faults initiated movement between 3 and 5 Ma. There are a suite of Pleistocene marine terraces (2.56 Ma and younger) that provide evidence that uplift is continuing. Using fossil age determinations and correlation of marine terrace elevations with global eustatic sea level curves, the island is currently uplifting at rates between 0.2 and .5 mm/year. So, the underlying thrust faults are slipping at about 0.6-1.5 mm/yr (Ward and Velensise, 1996).
Muhs et al. (2014) used numerical ages (uranium-series analysis of corrals and amino acid geochronology of mollusks) to calculate marine terrace uplift rates in the CCB. When compared to uplift rates from different tectonic regimes, the terrace uplift rates in CCB is comparable to regions where strike-slip tectonics are dominant. These authors suggest that uplift like that found at the Big Bend (e.g. Ventura and Santa Monica Mtns) is not influencing terrace uplift rates in the CCB.
Along with this compression, there is a right-lateral (dextral) strike-slip fault on the east side of the island, the San Clemente fault, which has a slip rate of about 1 – 4 mm.yr (Ward and Valensise, 1996). The Southern California Earthquake Center suggests the slip rate is about 1.5 mm/yr for the SCF.
The ongoing sequence of earthquakes near the San Clemente Island are small in magnitude. If these were foreshocks to a larger earthquake, this would be felt across the southland, possibly cause damage on the island (where there is a U.S. Naval base), could possibly trigger submarine landslides or a small tsunami. Strike-slip earthquakes are not always considered a significant source for large tsunami, but there is abundant evidence that they do, though often much smaller than tsunami generated from thrust or subduction zone earthquakes. It is possible, if not probable, that this sequence will fizzle out.

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 ≥ 3.0 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.

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 lower right corner is a map from Wallace (1990) that shows the plate boundary and major faults overlain upon a topographic/bathymetric map. Check out the patterns made by the uplifted regions and the faulting (e.g. pull-apart basins). I placed a blue star in the general location of this ongoing sequence.
In the upper right corner there is a map that shows more detailed fault mapping and bathymetric contours (Chaytor et al., 2008).
In the upper left corner, there is a map from Legg et al. (2015) that shows how the strike-slip faults transect the CCB. Select earthquake mechanisms are shown (use legend at the top of the poster to help interpret these symbols) for some historic earthquakes. These authors collected and interpreted a number of seismic reflection profiles, including C-C.’
Below the Chaytor et al. (2008) map is seismic reflection profile C-C’ which shows how the basins are filled with sediment, the islands and terraces are also constructed of sedimentary rocks, and there are some steeply dipping faults. This profile is not travel time corrected, so depth is in two-way-travel-time (in seconds), not in depth. The faults probably dip more shallowly than is shown on the figure. The faults in this figure are aligned with the San Clemente fault system labeled on the map. Note that there are some faults that bound the Santa Nicolas Basin.
In the lower left corner is a figure that shows how a right-lateral strike-slip fault can create a geometry (e.g. in a step over) where there is extension that forms a pull-apart basin. This is one way to explain the formation of the Santa Cruz, Santa Nicolas, and Catalina basins shown on the maps.

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

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

Here is a map that shows detailed bathymetry data for the region (Dartnell et al., 2016, 2017) overlain on GEBCO coarser bathymetry data downloaded from GMRT. The land data are at 10 m resolution from The National Map (NED).
I plot USGS Quaternary Fault and Fold Database faults as faint white lines. Earthquakes include the past month for magnitudes M ≥ 0.5 and events since 1919 for M ≥ 4.0.
Look at the bathymetry surrounding the island. We can clearly see the SCF to the east of the island. There is evidence for a north-south striking fault to the west of the island. In the area just southeast of the earthquakes, there appears bedrock sticking up out of the continental shelf. This bedrock aligns with a ridge in the slop to the south of the island. This ridge may just be sediment, but it may also be tectonic in origin.

This map has the USGS MMI contours. The two M 4.3 temblors were felt across the southland.

Here is a larger scale map so that we can look at the bathymetry surrounding San Clemente Island in greater detail. I updated the USGS seismicity for 2019.06.06 at 20:00 Pacific time.

Other Report Pages

Some Relevant Discussion and Figures

Here is the figure showing the evolution of the SAF since its inception about 29 Ma. I include the USGS figure caption below as a blockquote.

EVOLUTION OF THE SAN ANDREAS FAULT.
This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendicino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendicino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendicino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system. (Image modified after original illustration by Irwin, 1990 and Stoffer, 2006.)

Here is a map that shows the tectonic provides in this region (Legg et al. (2015). While the region inherits topography and geologic structures from past tectonic regimes, the dominant tectonic control here is currently the North America – Pacific plate boundary.

Map of the California Continental Borderland showing major tectonic features and moderate earthquake locations (M >5.5). The dashed box shows area of this study. The large arrows show relative plate motions for the Pacific-North America transform fault boundary (~N40° ± 2°W; RM2 and PA-1 [Plattner et al., 2007]). BP = Banning Pass, CH = Chino Hills, CP = Cajon Pass, LA = Los Angeles, PS = Palm Springs, V = Ventura, ESC = Santa Cruz Basin, ESCBZ = East Santa Cruz Basin fault zone, SCI = Santa Catalina Island, SCL = San Clemente Island, SMB = Santa Monica Basin, and SNI = San Nicolas Island. Base map from GeoMapApp/Global Multi-Resolution Topography (GMRT) [Ryan et al., 2009].

This map (Legg et al., 2007) shows an interpretation of the tectonics in this area. Note the location of the seismic reflection profile 116. San Clemente Island is on the southern edge of this map.

Shaded relief map of Santa Catalina Island and vicinity, where several restraining-bend pop-ups and releasing-bend basins exist along major fault zones. Epicentres for two moderate earthquakes (1981 Santa Barbara Island, M 6.0; 1986 Oceanside, M 5.8) and aftershocks bound the Santa Catalina Island restraining bend (locations by Astiz & Shearer 2000; focal mechanism from Corbett 1984). Other restraining-bend pop-ups include the Palos Verdes Hills (PVH) and Lasuen knoll along the Palos Verdes fault zone, and Signal Hill (SH) and possibly the San Joaquin Hills (SJH) along the Newport–Inglewood fault zone. Small pop-ups and pull-apart basins in the vicinity of Crespi knoll are shown in Figure 14. Total relief across the Catalina Fault is almost 2000 m, from Catalina Basin to Mt Orizaba. From 60 to 72 km of right-slip on San Clemente Fault is inferred from offset of Emery Knoll crater rim (Legg et al. 2004b).

Here is the USGS seismic reflection profile 116 (Legg et al., 2007). The San Clemente fault zone and the Catalina fault are shown. Check out the pull-apart basin.

Seismic-reflection profile USGS-116 across the Catalina basin (see Fig. 12 for profile location). Note the thin sediment cover over an irregular basement surface. A pull-apart basin exists where the San Clemente Fault steps to the NE to eventually merge with the Catalina Fault. The major faults have subvertical dips, typical of strike-slip faults. Convergence across the Catalina Fault has elevated Santa Catalina Island, and uplift occurs on both sides of the PDZ. Seismic data from USGS (J. Childs 2005, pers. comm.) FK migration at 4800 fps velocity was applied to 22-fold USGS stacked data.

Here is the figure with more details about the tectonic interpretation of the area (Legg et al., 2015)

Map showing bathymetry, Quaternary faults, and recent seismicity in the Outer Borderland. Fault locations are based on the high-resolution bathymetry, available high-resolution seismic reflection profiles, and published fault maps [cf. California Geological Survey (CGS), 2010]. The red symbols show magnitude-scaled (M>4) epicenters for seismicity recorded for the period of 1932 to 2013. Seismicity data and focal mechanisms are derived from the Southern California Seismograph Network catalogs, National Earthquake Information Center [2012–2013], and Legg [1980]. Focal mechanism event numbers correspond to Table S2 in the supporting information. The black rectangle shows location of Figure 10. The light blue lines show tracklines of multichannel seismic profiles—the labeled white profiles are shown in Figures 12 (124) and 13 (108 and 126).

Here is the summary figure from Legg et al. (2015). This helps us put these faults systems into context. Seismic reflection profiles from their publication are shown here (profile C-C’ is located in the rectangle labeled Fig 6 and plotted below).

Map showing major active tectonic elements of the northern part of the California Continental Borderland. Major active (Quaternary) faults are shown in red (SAF = San Andreas fault, ABF = Agua Blanca fault, SCF = San Clemente fault, and SCCR = Santa Cruz-Catalina Ridge, Ferrelo). Major strike-slip offsets are shown by shaded areas with estimated displacement (EK = Emery Knoll crater; Tanner Basin near DB = Dall Bank; and SDT = San Diego Trough, small pull-apart near Catalina). Other symbols show oblique fault character including transpressional restraining bends (CAT = Santa Catalina Island, CB = Cortes Bank, and TB = Tanner Bank), uplifts (SRI = Santa Rosa Island, SCz = Santa Cruz Island, SNI = San Nicolas Island, CB = Cortes Bank, TB = Tanner Bank, and SBM = San Bernardino Mountains), and transtensional pull-apart basins (SD = San Diego, ENS = Ensenada, SCB = San Clemente Basin, and SIB = San Isidro Basin). The large arrows show Pacific-North America relative plate motions with the blue dashed line (PAC-NAM) along a small circle for the RM2 [Minster and Jordan, 1978] plate motions model through San Clemente Island (SCL). Boundary between the Inner and Outer Borderland follows the East Santa Cruz Basin fault zone (dotted line; modified from Schindler [2010] and De Hoogh [2012]). Holocene volcanoes exist along the coast (SQ= San Quintín) and within the Gulf of California Rift (CP = Cerro Prieto and Obsidian Buttes, Salton Trough). Dates show year of earthquakes with mapped focal mechanisms (see Table S2 in the supporting information). SB = Santa Barbara, LA = Los Angeles, and PS = Palm Springs.

Here is the seismic reflection profile C-C’ shown on the poster (Legg et al., 2015).

High-resolution 24-channel 4 kJ sparker seismic profiles along the Santa Cruz-Catalina Ridge (C. Goldfinger, personal communications, 2001). The profiles are arranged from north to south along the southern part of the ridge (see Figure 6a for profile locations). Profile OSU-128 located near the bifurcation of the Santa Cruz-Catalina Ridge east of Santa Barbara Island. The colored horizons are correlated to stratigraphy mapped by Sorlien et al. [2013] (see Figure 6 for profile location). The East San Clemente fault zone merges with the Catalina Ridge fault zone within the eastern valley (Figure 5a). Reverse slip is evident on several steep faults that bound blocks of sedimentary rocks that were squeezed up in this transpressional fault system.

Here is a map from Maier et al. (2018) that shows how the faults are configured, as well as the sedimentary distribution systems (the focus of their paper). I grew up on the [concrete] banks of the San Gabriel River and this is where the submarine canyon and channels send their sediment loads.

Color-contoured slope-shaded multibeam bathymetry gridded at 10 meters. A) The Catalina Basin and the San Gabriel Canyon–Channel depositional system. Dashed line in the Catalina Basin indicates approximate extent of channels resolved on the seafloor.

Below are seismic reflection profiles plotted on the above map (Maier et al., 2018)

Northwest channels and lobes. See Figure 1A for profile locations. Gray lines represent profile intersections. A) Chirp profile across the northwestern Catalina Basin shows the stacking of lobes that do not reach the Kimki Fault (KF). B) An obliquely oriented chirp profile shows that the lobe deposits originate from the northwest channels, end before reaching the San Clemente Fault (SCF), and do not overlap in extent with lobe b.

This shows the timeline of what has controlled the tectonics in this region (Legg et al., 2015).

Chronology of major Cenozoic events in the Southern California region (after Wright [1991] and Legg and Kamerling [2012]). Intensity of tectonic deformation is represented by the curve. Local (Los Angeles Basin) biostratigraphic zonation is shown. The slanted labels for Neogene stages represent the time-transgressive nature of these boundaries.

Pleistocene Marine Terraces

Schematic cartoon illustrating the cutting and abandonment of marine terraces in an actively uplifting landscape in relation to sea level fluctuation. (a) Marine terrace cut during a relative sea level high stand. (b) Sea level drops and the marine terrace is uplifted. (c) During the next relative sea level high stand a new marine terrace is cut into the landscape below the older terrace. Modified after Nalin et al., (2007).

Here is a figure that shows the geomorphic features of a marine terrace (Wikipedia).

Here is a beautiful low angle oblique photo of the marine terraces on San Clemente Island (Yatsko, 2000). These authors studied the archaeological deposits on this island.

Emergent Pleistocene marine terraces on the west side of the island between Norton and Box canyons.

Here are some views of the terraces on San Clemente Island as photographed by Daniel Muhs (USGS).

Here is a map I prepared using the 2016 USGS Topobathy data (LiDAR and historic bathymetry mosaic).
I present these data as a shaded relief (hillshade) beneath an elevation raster with color representing height or depth. I also use a slopeshade raster to help highlight the changes in slope.
The 100 meter topographic contours are labeled. The inset shows the location of the main map in relation to the CCB with a pink polygon.

UPDATE: 2019.06.07
I prepared a couple maps that show the entire island. These are below, with 2 different color ramps.

Below is a fantastic summary showing the uplift rates for Pleistocene marine terraces along the North America – Pacific plate boundary system(Legg et al., 2015). Note the high uplift rates at the Big Bend and the Mendocino triple junction (another plate where there is a major change in SAF tectonics).

Map showing the plate tectonic setting of western North America (simplified from Drummond (1981) and Simkin et al. (2006)). SAF, San Andreas Fault; MTJ, Mendocino Triple Junction; CSZ, Cascadia subduction zone. Also shown are marine terrace localities with reliably dated ~120 ka, ~80 ka, or ~49 ka corals, or amino acid ratios in mollusks that permit correlation to ~120 ka, ~80 ka, or ~49 ka terrace localities, and elevation data that allow calculations of late Quaternary uplift rates. Paleo-sea levels, relative to present, used for uplift rate calculations are þ6 m (~120 ka), 11 m (~80 ka), and 62 m (~49 ka), derived from data in Muhs et al. (2012). Abbreviations and sources of data, south to north: CP, Cabo Pulmo (Muhs et al., 2002a); LP, La Paz (Sirkin et al., 1990); BH, Bahía Magdalena (Omura et al., 1979); IC, Isla Coronados and PC, Punta Chivato (Johnson et al., 2007; see also Table 2); MU, Mulege (Ashby et al., 1987); BT, Bahía de Tortugas (Emerson et al., 1981); PB, Punta Banda (Rockwell et al., 1989; Muhs et al., 2002a); PL, Point Loma (Kern, 1977; Muhs et al., 2002a); SCI, San Clemente Island (Muhs et al., 2002a, 2014); NB, Newport Bay (Grant et al., 1999); SNI, San Nicolas Island (Muhs et al., 2012); PV, Palos Verdes Hills (Muhs et al., 2006); NCI, Northern Channel Islands (this study); V, Ventura (Lajoie et al., 1979; Kennedy et al., 1982;Wehmiller, 1982); IV, Isla Vista (Gurrola et al., 2014; see also Table 2); SB, Shell Beach (Stein et al., 1991; Hanson et al., 1994); PSL, Point San Luis (Hanson et al., 1994; Muhs et al., 1994); C, Cayucos (Stein et al., 1991; Muhs et al., 2002a); AN, A~no Nuevo (Muhs et al., 2006); PA, Point Arena (Muhs et al., 2006); PD, Point Delgada (McLaughlin et al., 1983a, 1983b; Merritts and Bull, 1989); CC, Crescent City (Kennedy et al., 1982; Polenz and Kelsey, 1999); CB, Cape Blanco (Kelsey, 1990; Muhs et al., 1990); B, Bandon (McInelly and Kelsey, 1990; Muhs et al., 1990, 2006); YB, Yaquina Bay (Kennedy et al., 1982; Kelsey et al., 1996).

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

San Andreas fault

General Overview

1906.04.18 M 7.9 San Francisco

Earthquake Reports

Northern CA

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

Central CA

2018.07.30 M 3.7 San Pablo Bay
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

2019.06.05 M 4.3 San Clemente Island
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

Social Media

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, https://dx.doi.org/10.1130/B26316.1
Dartnell, P., Driscoll, N.W., Brothers, D., Conrad, J.E., Kluesner, J., Kent, G., and Andrews, B., 2015, Colored shaded-relief bathymetry, acoustic backscatter, and selected perspective views of the inner continental borderland, Southern California, U.S. Geological Survey Scientific Investigations Map 3324, 3 sheets, https://dx.doi.org/10.3133/sim3324.
Dartnell, P., Roland, E.C., Raineault, N.A., Castillo, C.M., Conrad, J.E., Kane, R.R., Brothers, D.S., Kluesner, J.W., Walton, M.A.L., 2017, Multibeam bathymetry and acoustic-backscatter data collected in 2016 in Catalina Basin, southern California and merged multibeam bathymetry datasets of the northern portion of the Southern California Continental Borderland: U.S. Geological Survey data release, https://doi.org/10.5066/F7DV1H3W.
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
Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
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
Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
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Legg., <.R., Goldfinger, C., Kamerling, M.J., Chaytor, J.D., and Einstein, D.E., 2007. Morphology, structure and evolution of California Continental Borderland restraining bends in W. D. & Mann, P. (Eds) Tectonics of Strike-Slip Restraining And Releasing Bends. Geological Society, London, Special Publications, v. 290, p. 143–168
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.
Merifield, P.M., Lamar, D.L., and Stout, M.L., 1971. Geology of Central San Clemente Island, California in GSA Bulletin, v. 82, p. 1989-1994
Maier, K.L., Roland, E.C., Walton., A.L., Conrad,m J.E., Brothers, D.S., Bartnell, P., and Kleusner, J.W., 2018. The Tectonically Controlled San Gabriel Channel–Lobe Transition Zone, Catalina Basin, Southern California Borderland in Journal of Sedimentary Research, v. 88, p. 942-959, http://dx.doi.org/10.2110/jsr.2018.50
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
Muhs, Daniel R.; Simmons, Kathleen R.; Schumann, R. Randall; Groves, Lindsey T.; DeVogel, Stephen B.; Minor, Scott A.; and Laurel, DeAnna, “Coastal tectonics on the eastern margin of the Pacific Rim: late Quaternary sea-level history and uplift rates, Channel Islands National Park, California, USA” (2014). USGS Staff — Published Research. 932.
http://digitalcommons.unl.edu/usgsstaffpub/932
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
Nalin, R., Massari, F., and Zecchin, M., 2007, Superimposed Cycles of Composite Marine Terraces: The Example of Cutro Terrace (Calabria, Southern Italy): Journal of Sedimentary Research, v. 77, no. 4, p. 340-354.
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
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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].
Yeats, R. S., 1968. Southern California structure, sea-floor spreading, and history of the Pacific Basin in Geol. Soc. America Bull., v. 79, p. 1693-1702

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