Category Archives: asia

Earthquake Report for M 6.9 Earthquake in Taiwan

Posted on by
Reply

I don’t always have the time to write a proper Earthquake Report. However, I prepare interpretive posters for these events.

Because of this, I present Earthquake Report Lite. (but it is more than just water, like the adult beverage that claims otherwise). I will try to describe the figures included in the poster, but sometimes I will simply post the poster here.

There was a magnitude M 6.9 earthquake in Taiwan on 18 September 2022.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000i90q/executive

Taiwan is an interesting place, from a tectonic perspective. There is an intersection of several plate boundary fault systems here. Along the western boundary of Taiwan the Eurasia plate subducts (dives beneath) the Philippine Sea plate forming the Manila trench. This megathrust subduction zone fault system terminates somewhere in central-northern Taiwan.

Intersecting central Taiwan from the east is another subduction zone where the Philippine Sea plate subducts beneath the Eurasia plate, forming the Ryukyu trench.

There was an earthquake in Taiwan in 1999 that has been commemorated by creating a park and museum that preserves some of the evidence of the earthquake. This Chi-Chi earthquake cause lots of damage and, sadly, lots of suffering. In addition, because of the dominance of the computer chip manufacturing industry in Taiwan at the time, the price of computer chips was greatly inflated. The global economy suffered following this earthquake.

This 18 September 2022 M 6.9 earthquake occurred on a crustal fault that strikes (trends) parallel to the coast. Because of the mapped faults, I interpret this to have been a left-lateral strike slip earthquake.

There was a foreshock, a mag M 6.5 earthquake, nearby, the day before.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 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.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures.

  • In the upper left corner is a map that shows the plates, their boundaries, and a century of seismicity.
  • In the upper right are two maps that show models of how there may have been landslides or liquefaction because of the earthquake shaking and impacts. Read more about landslides and liquefaction here. I include both the USGS epicenter and the Central Weather Bureau Seismological Center epicenter (which is probably more accurate). However, these ground failure models are based on the USGS epicenter/location.
  • To the left of those two maps is a low angle oblique view of the tectonic plates and how they are oriented relative to each other.
  • Below that figure, in the center, is a map from Chen at al. (2020) that shows the earthquake fault mapping along eastern Taiwan. I place a yellow star in the location of the M 6.9 epicenter (the location of the earthquake on the ground surface).
  • In the lower right corner is a map that shows the ground shaking from the earthquake, with color representing intensity using the Modified Mercalli Intensity (MMI) scale. The closer to the earthquake, the stronger the ground shaking. The colors on the map represent the USGS model of ground shaking. The colored circles represent reports from people who posted information on the USGS Did You Feel It? part of the website for this earthquake. There are things that affect the strength of ground shaking other than distance, which is why the reported intensities are different from the modeled intensities.
  • To the left of the intensity map is a map that shows seismicity from the Central Weather Bureau Seismological Center. The locations of earthquakes from this center are better than those from the USGS since this organization runs a local seismic network (the USGS runs a global network). The local network uses more seismometers than the global network (so can detect more events, in this region).
  • To the left of this seismicity map is a plot that shows how the shaking intensity models and reports relate to each other. The horizontal axis is distance from the earthquake and the vertical axis is shaking intensity (using the MMI scale, just like in the map to the right: these are the same datasets).
  • In the upper left-center is a figure that shows the USGS earthquake slip model. This shows how much the fault slipped in different areas (based on their modeling, not observation). The model shows that there were places that may have slipped over 1.5 meters (5 feet).
  • Here is the map with 3 month’s seismicity plotted.


    Supportive Figures

  • I could not help myself. I am so excited to have this website back up and running, like a fully operational space station, that I include below some additional figures that help us understand the tectonic setting.
  • Here is the low angle oblique view of the plate configuration in Taiwan.


  • Here is the map from Chen at al. (2020) that shows the fault mapping in this area of eastern Taiwan.


    • Geologic map of the Coastal Range on shaded relief (after Wang and Chen, 1993). The Longitudinal Valley Fault (LVF) can be subdivided into the Linding and Juisui locked Fault and the Chihshang and Lichi creeping Fault. Vertical cross-sections of VS perturbation tomography along the AeA0 and BeB0 profiles denote the Central Range, the Coastal Range, and the LVF. EU: Eurasian Plate; PH: Philippine Sea Plate.

  • Here is an oblique view of the plate configuration in this region. This is from Chang (2001).


  • Here is a great interpretation showing how the Island of Taiwan is being uplifted and exhumed. This is from Lin (2002).


  • Needless to say, this is an excellent map showing the complicated faulting of this region. This is from Theunissen et al. (2012).


  • Here is another tectonic interpretation map from here.

  • Here is a great general overview of the tectonics of the region from Shyu et al. (2005). I include their figure caption below the image as a blockquote.


    • A neotectonic snapshot of Taiwan and adjacent regions. (a) Taiwan is currently experiencing a double suturing. In the south the Luzon volcanic arc is colliding with the Hengchun forearc ridge, which is, in turn, colliding with the Eurasian continental margin. In the north both sutures are unstitching. Their disengagement is forming both the Okinawa Trough and the forearc basins of the Ryukyu arc. Thus, in the course of passing through the island, the roles of the volcanic arc and forearc ridge flip along with the flipping of the polarity of subduction. The three gray strips represent the three lithospheric pieces of Taiwan’s tandem suturing and disarticulation: the Eurasian continental margin, the continental sliver, and the Luzon arc. Black arrows indicate the suturing and disarticulation. This concept is discussed in detail by Shyu et al. [2005]. Current velocity vector of the Philippine Sea plate relative to the Eurasian plate is adapted from Yu et al. [1997, 1999]. Current velocity vector of the Ryukyu arc is adapted from Lallemand and Liu [1998]. Black dashed lines are the northern and western limits of the Wadati-Benioff zone of the two subducting systems, taken from the seismicity database of the Central Weather Bureau, Taiwan. DF, deformation front; LCS, Lishan-Chaochou suture; LVS, Longitudinal Valley suture; WF, Western Foothills; CeR, Central Range; CoR, Coastal Range; HP, Hengchun Peninsula. (b) Major tectonic elements around Taiwan. Active structures identified in this study are shown in red. Major inactive faults that form the boundaries of tectonic elements are shown in black: 1, Chiuchih fault; 2, Lishan fault; 3, Laonung fault; 4, Chukou fault. Selected GPS vectors relative to the stable Eurasian continental shelf are adapted from Yu et al. [1997]. A,Western Foothills; B, Hsueshan Range; C, Central Range and Hengchun Peninsula; D, Coastal Range; E, westernmost Ryukyu arc; F, Yaeyama forearc ridge; G, northernmost Luzon arc; H, western Taiwan coastal plains; I, Lanyang Plain; J, Pingtung Plain; K, Longitudinal Valley; L, submarine Hengchun Ridge; M, Ryukyu forearc basins.

  • This figure from Shyu et al. (2005) shows their interpretation of the different tectonic domains in Taiwan. This is a complicated region that includes collision zones in different orientations as the Okinawa Trough, Ryukyu Trench, and Manila Trench (all subduction zones) each intersect beneath and adjacent to Taiwan. I include their figure caption below the image as a blockquote.


    • Map of major active faults and folds of Taiwan (in red) showing that the two sutures are producing separate western and eastern neotectonic belts. Each collision belt matures and then decays progressively from south to north. This occurs in discrete steps, manifested as seven distinct neotectonic domains along the western belt and four along the eastern. A distinctive assemblage of active structures defines each domain. For example, two principal structures dominate the Taichung Domain. Rupture in 1999 of one of these, the Chelungpu fault, caused the disastrous Chi-Chi earthquake. The Lishan fault (dashed black line) is the suture between forearc ridge and continental margin. Thick light green and pink lines are boundaries of domains.

  • This map from Shyu et al. (2005) shows the earthquake slip regions for proposed earthquake scenarios. I include their figure caption below the image as a blockquote.


    • Proposed major sources for future large earthquakes in and around Taiwan. Thick red lines are proposed future ruptures, and the white patches are rupture planes projected to the surface. Here we have selected only a few representative scenarios from Table 1. Earthquake magnitude of each scenario is predicted value from our calculation.

  • This map from here shows the basement geology of Taiwan. Note the accretionary belts, including the forearc basin. This is a compilation from Teng et al. (2001) and Hsiao et al. (1998) as presented in Ustaszewski et al. (2012).


Return to the Earthquake Reports page.

Earthquake Report: 2018 Summary

Posted on by
3

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


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

Below is my summary poster for this earthquake year

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


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

2018 Earthquake Report Pages

Other Annual Summaries

2018 Earthquake Reports

    General Overview of how to interact with these summaries

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

    Background on the Earthquake Report posters

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

    Magnetic Anomalies

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

2018.01.10 M 7.6 Cayman Trough

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

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


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


    •

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


    •

2018.01.14 M 7.1 Peru

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


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

2018.01.23 M 7.9 Gulf of Alaska

  • 2018.01.23 M 7.9 Gulf of Alaska UPDATE #1
  • 2018.01.24 M 7.9 Gulf of Alaska UPDATE #2

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

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


      •

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


      •

    • Large Scale Interpretive Map (from update report)


      •

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


    2018.02.16 M 7.2 Oaxaca, Mexico

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

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


      •

    2018.02.25 M 7.5 Papua New Guinea

  • 2018.02.26 M 7.5 Papua New Guinea Update #1

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

  • The same map without historic seismicity.



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

    • Here is the “update” map with aftershocks


      •

    2018.03.08 M 6.8 New Ireland

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

    Main Interpretive Poster with emag2


    Earthquakes M≥ 6.5 with emag2


    2018.03.26 M 6.6 New Britain

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

    2018.03.26 M 6.9 New Britain

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

    2018.04.02 M 6.8 Bolivia

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

    2018.05.04 M 6.9 Hawai’i

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

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

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

    • Hilo, Hawaii


    • Kawaihae, Hawaii


      •

    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.05.05 Pele, the Hawai’i Goddess of Fire, Lightning, Wind, and Volcanoes
    2018.05.06 Pele, la Diosa Hawaiana del Fuego, los Relámpagos, el Viento y los Volcanes de Hawái

    2018.08.05 M 6.9 Lombok, Indonesia

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

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


      •

    2018.08.15 M 6.6 Aleutians

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

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


      •

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


      •

    2018.08.18 M 8.2 Fiji

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

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


      •

    2018.08.19 M 6.9 Lombok, Indonesia

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

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


      •

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


      •

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


      •

    2018.08.21 M 7.3 Venezuela

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

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


      •

    2018.08.24 M 7.1 Peru

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

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


      •

    2018.09.05 M 6.6 Hokkaido, Japan

    Following the largest typhoon to strike Japan in a very long time, there was an earthquake on the island of Hokkaido, Japan today. There is lots on social media, including some spectacular views of disastrous and deadly landslides triggered by this earthquake (earthquakes are the number 1 source for triggering of landslides). These landslides may have been precipitated (sorry for the pun) by the saturation of hillslopes from the typhoon. Based upon the USGS PAGER estimate, this earthquake has the potential to cause significant economic damages, but hopefully a small number of casualties. As far as I know, this does not incorporate potential losses from earthquake triggered landslides [yet].
    This earthquake is in an interesting location. to the east of Hokkaido, there is a subduction zone trench formed by the subduction of the Pacific plate beneath the Okhotsk plate (on the north) and the Eurasia plate (to the south). This trench is called the Kuril Trench offshore and north of Hokkaido and the Japan Trench offshore of Honshu.
    One of the interesting things about this region is that there is a collision zone (a convergent plate boundary where two continental plates are colliding) that exists along the southern part of the island of Hokkaido. The Hidaka collision zone is oriented (strikes) in a northwest orientation as a result of northeast-southwest compression. Some suggest that this collision zone is no longer very active, however, there are an abundance of active crustal faults that are spatially coincident with the collision zone.
    Today’s M 6.6 earthquake is a thrust or reverse earthquake that responded to northeast-southwest compression, just like the Hidaka collision zone. However, the hypocentral (3-D) depth was about 33 km. This would place this earthquake deeper than what most of the active crustal faults might reach. The depth is also much shallower than where we think that the subduction zone megathrust fault is located at this location (the fault formed between the Pacific and the Okhotsk or Eurasia plates). Based upon the USGS Slab 1.0 model (Hayes et al., 2012), the slab (roughly the top of the Pacific plate) is between 80 and 100 km. So, the depth is too shallow for this hypothesis (Kuril Trench earthquake) and the orientation seems incorrect. Subduction zone earthquakes along the trench are oriented from northwest-southweast compression, a different orientation than today’s M 6.6.
    So today’s M 6.6 earthquake appears to have been on a fault deeper than the crustal faults, possibly along a deep fault associated with the collision zone. Though I am not really certain. This region is complicated (e.g. Kita et al., 2010), but there are some interpretations of the crust at this depth range (Iwasaki et al., 2004) shown in an interpreted cross section below.

    • Here is the map with a centuries seismicity plotted.


      •

    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.09.06 Violent shaking triggers massive landslides in Sapporo Japan earthquake

    2018.09.09 M 6.9 Kermadec

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

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


      •

    • Here is the map with a centuries seismicity plotted.


      •

    2018.09.28 M 7.5 Sulawesi

  • 2018.10.16 M 7.5 Sulawesi UPDATE #1

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

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




      •

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


      •

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


      •

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

    My 2018.10.01 BC Newshour Interview

    InSAR Analysis

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

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


    M 7.5 Landslide Model vs. Observation Comparison

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

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


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

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


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

    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.09.28 The Palu-Koro fault ruptures in a M=7.5 quake in Sulawesi, Indonesia, triggering a tsunami and likely more shocks
    2018.10.03 Tsunami in Sulawesi, Indonesia, triggered by earthquake, landslide, or both
    2018.10.16 Coseismic Landslides in Sulawesi, Indonesia

    2018.10.10 M 7.0 New Britain, PNG

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

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


      •

    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.10.10 M 7.5 Earthquake in New Britain, Papua New Guinea

    2018.10.22 M 6.8 Explorer plate

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

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


      •

    2018.10.25 M 6.8 Greece

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

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


      •

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


      •

    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.10.26 Greek earthquake in a region of high seismic hazard

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

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

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


      •

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


      •

    2018.11.30 M 7.0 Alaska

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

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


      •

    Temblor Reports:

    • Click on the graphic to see a pdf version of the article.
    • Click on the html link (date) to visit the Temblor site.
    2018.11.30 Exotic M=7.0 earthquake strikes beneath Anchorage, Alaska
    2018.12.11 What the Anchorage earthquake means for the Bay Area, Southern California, Seattle, and Salt Lake City

    2018.12.05 M 7.5 New Caledonia

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

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


      •

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


      •

    2018.12.20 M 7.4 Bering Kresla

  • 2018.12.20 M 7.3 Bering Kresla UPDATE #1

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

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


      •

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


      •

    UPDATE #1

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


      •

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


      •

    2018.12.29 M 7.0 Philippines

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

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


      •

    Geologic Fundamentals

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

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


      •

    • Here is another way to look at these beach balls.

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

      • Strike Slip:

      Compressional:

      Extensional:

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


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

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


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

    Return to the Earthquake Reports page.

    Earthquake Report: Bering Kresla / Pacific plate

    Posted on by
    Reply

    We just had a large earthquake in the region of the Bering Kresla fracture zone, a strike-slip fault system that coincides with the westernmost portion of the Aleutian trench (which is a subduction zone further to the east).
    At first, when I noticed the location, I hypothesized that this may be a strike-slip earthquake. womp womp. The earthquake mechanism from the USGS shows that this M = 7.4 earthquake was a normal fault earthquake (extension).
    This earthquake happened in an interesting region of the world where there is a junction between two plate boundaries, the Kamchatka subduction zone with the Aleutian subduction zone / Bering-Kresla Shear Zone. The Kamchatka Trench (KT) is formed by the subduction (a convergent plate boundary) beneath the Okhotsk plate (part of North America). The Aleutian Trench (AT) and Bering-Kresla Shear Zone (BKSZ) are formed by the oblique subduction of the Pacific plate beneath the Pacific plate. There is a deflection in the Kamchatka subduction zone north of the BKSZ, where the subduction trench is offset to the west. Some papers suggest the subduction zone to the north is a fossil (inactive) plate boundary fault system. There are also several strike-slip faults subparallel to the BKSZ to the north of the BKSZ.
    Today’s M = 7.4 earthquake shows northwest-southeast directed extension. This is consistent with slab tension in the direction of the Kurile subduction zone. It may also represent extension due to bending in the Pacific plate, but this seems less likely to me. Basically, the Pacific plate, as it subducts beneath the Okhotsk plate, the downgoing slab (the plate) exerts forces on the rest of the plate that pulls it down, into the subduction zone.
    A second cool thing about this earthquake is that this may be evidence that the Kuril subduction zone extends north of the intersection of the BKSZ with Kamchatka. I discussed this in my earthquake report from 2017 here.
    There are a couple analogy earthquakes, but one is the best. There were several strike-slip earthquakes nearby in 1982, 1987, and 1999. However, there was a M = 6.2 earthquake in almost the same location as the M = 7.4 from today. This M = 6.2 earthquake was slightly deeper (33 km) relative to the M = 7.4 (9.6 km).

    Check out my update here

  • 2018.12.20 M 7.4 Bering Kresla UPDATE #1

    • Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0 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.li>

      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.

      Age of Oceanic Lithosphere

    • In the map below, I include a transparent overlay of the age of the oceanic crust data from Agegrid V 3 (Müller et al., 2008).
    • Because oceanic crust is formed at oceanic spreading ridges, the age of oceanic crust is youngest at these spreading ridges. The youngest crust is red and older crust is yellow (see legend at the top of this poster).

      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 I include a map that shows the tectonic setting of this region, with the major plate boundary faults and volcanic arc designated by triangles (Bindeman et al., 2002). I placed a blue star in the general location of the M 7.4 earthquake. Note the complicated nature of the faulting in this region.
    • In the upper left corner I include a figure from Portnyagin and Manea (2008 ) that shows a low angle oblique view of the downgoing Pacific plate slab. I post this figure and their figure caption below.
    • In the upper right corner I include a map that shows more details about the faulting in the region.
    • Here is the map with a month’s seismicity plotted. The lower map shows the age of the crust.




      •

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




      •

    Other Report Pages

    Some Relevant Discussion and Figures

    • Here is the tectonic map from Bindeman et al., 2002. The original figure caption is below in blockquote.


      • Tectonic setting of the Sredinny and Ganal Massifs in Kamchatka. Kamchatka/Aleutian junction is modified after Gaedicke et al. (2000). Onland geology is after Bogdanov and Khain (2000). 1, Active volcanoes (a) and Holocene monogenic vents (b). 2, Trench (a) and pull-apart basin in the Aleutian transform zone (b). 3, Thrust (a) and normal (b) faults. 4, Strike-slip faults. 5–6, Sredinny Massif. 5, Amphibolite-grade felsic paragneisses of the Kolpakovskaya series. 6, Allochthonous metasedimentary and metavolcanic rocks of the Malkinskaya series. 7, The Kvakhona arc. 8, Amphibolites and gabbro (solid circle) of the Ganal Massif. Lower inset shows the global position of Kamchatka. Upper inset shows main Cretaceous-Eocene tectonic units (Bogdanov and Khain 2000): Western Kamchatka (WK) composite unit including the Sredinny Massif, the Kvakhona arc, and the thick pile of Upper Cretaceous marine clastic rocks; Eastern Kamchatka (EK) arc, and Eastern Peninsulas terranes (EPT). Eastern Kamchatka is also known as the Olyutorka-Kamchatka arc (Nokleberg et al. 1998) or the Achaivayam-Valaginskaya arc (Konstantinovskaya 2000), while Eastern Peninsulas terranes are also called Kronotskaya arc (Levashova et al. 2000).

    • This map shows the configuration of the subducting slab. The original figure caption is below in blockquote.


      • Kamchatka subduction zone. A: Major geologic structures at the Kamchatka–Aleutian Arc junction. Thin dashed lines show isodepths to subducting Pacific plate (Gorbatov et al., 1997). Inset illustrates major volcanic zones in Kamchatka: EVB—Eastern Volcanic Belt; CKD—Central
      Kamchatka Depression (rift-like tectonic structure, which accommodates the northern end of EVB); SR—Sredinny Range. Distribution of Quaternary volcanic rocks in EVB and SR is shown in orange and green, respectively. Small dots are active vol canoes. Large circles denote CKD volcanoes: T—Tolbachik; K l — K l y u c h e v s k o y ; Z—Zarechny; Kh—Kharchinsky; Sh—Shiveluch; Shs—Shisheisky Complex; N—Nachikinsky. Location of profiles shown in Figures 2 and 3 is indicated. B: Three dimensional visualization of the Kamchatka subduction zone from the north. Surface relief is shown as semi-transparent layer. Labeled dashed lines and color (blue to red) gradation of subducting plate denote depths to the plate from the earth surface (in km). Bold arrow shows direction of Pacific Plate movement.

    • Here is the more detailed tectonic map from Konstantinovskaia et al. (2001).




      •

    • This is the cross section associated with the above map.




      •

    • Here is the Rhea et al. (2010) poster.


      •

    • Finally, here is an earthquake report for an earthquake also north of today’s M 7.4 earthquake.


      •

    Geologic Fundamentals

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

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


      •

    • Here is another way to look at these beach balls.

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

      • Strike Slip:

      Compressional:

      Extensional:

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


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

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


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

    Return to the Earthquake Reports page.


    Earthquake Report: Iran

    Posted on by
    Reply

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

    Below is my interpretive poster for this earthquake

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

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

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

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


      •

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


      •

    Other Report Pages

    Some Relevant Discussion and Figures

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


      •

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


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

    • Here is the Scharf et al., 2015 map.


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

    • This is the Enami et al., 2010 figure.


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

    • This is the tectonic map from Hessami, 2002.


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

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


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

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


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

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


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

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


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

    Geologic Fundamentals

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

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


      •

    • Here is another way to look at these beach balls.

    • 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.

      References:

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

    Return to the Earthquake Reports page.


    Earthquake Report: Hokkaido, Japan

    Posted on by
    2

    Following the largest typhoon to strike Japan in a very long time, there was an earthquake on the island of Hokkaido, Japan today. There is lots on social media, including some spectacular views of disastrous and deadly landslides triggered by this earthquake (earthquakes are the number 1 source for triggering of landslides). These landslides may have been precipitated (sorry for the pun) by the saturation of hillslopes from the typhoon. Based upon the USGS PAGER estimate, this earthquake has the potential to cause significant economic damages, but hopefully a small number of casualties. As far as I know, this does not incorporate potential losses from earthquake triggered landslides [yet].

    This earthquake is in an interesting location. to the east of Hokkaido, there is a subduction zone trench formed by the subduction of the Pacific plate beneath the Okhotsk plate (on the north) and the Eurasia plate (to the south). This trench is called the Kuril Trench offshore and north of Hokkaido and the Japan Trench offshore of Honshu.

    The okhotsk plate is considered part of the North America plate on some maps. The location of the plate boundary of the Okhotsk plate are not well understood (e.g. using GPS plate motion velocities, it is difficult to find the northern boundary with the North America plate).

    Many of the earthquakes in this region are related to the subduction zone. Most notably is the 2011 Tohoku-oki M 9.1 tsunamigenic earthquake. More background information about the 2011 earthquake can be found here and information about the tsunami can be found here.

    The 2011 earthquake had lots of aftershocks and was quite complicated. One interesting thing that happened is that there was an extensional earthquake in the Pacific plate to the west of the Japan Trench. This M 7.7 earthquake happened along faults formed as the Pacific plate bends near where it meets the trench. Similar subduction zone / outer rise earthquake pairs are known, including some along the New Hebrides Trench in the western equatorial Pacific ocean, as well as further north along the Kuril subduction zone. I spend time discussing the 2006/2007 Kuril earthquake pair in this report.

    There was also a subduction zone earthquake in 2003, the Tokachi-oki earthquake, that triggered submarine landslides. These landslides transformed into turbidity currents and these were directly observed with offshore instrumentation.

    One of the interesting things about this region is that there is a collision zone (a convergent plate boundary where two continental plates are colliding) that exists along the southern part of the island of Hokkaido. The Hidaka collision zone is oriented (strikes) in a northwest orientation as a result of northeast-southwest compression. Some suggest that this collision zone is no longer very active, however, there are an abundance of active crustal faults that are spatially coincident with the collision zone.

    Today’s M 6.6 earthquake is a thrust or reverse earthquake that responded to northeast-southwest compression, just like the Hidaka collision zone. However, the hypocentral (3-D) depth was about 33 km. This would place this earthquake deeper than what most of the active crustal faults might reach. The depth is also much shallower than where we think that the subduction zone megathrust fault is located at this location (the fault formed between the Pacific and the Okhotsk or Eurasia plates). Based upon the USGS Slab 1.0 model (Hayes et al., 2012), the slab (roughly the top of the Pacific plate) is between 80 and 100 km. So, the depth is too shallow for this hypothesis (Kuril Trench earthquake) and the orientation seems incorrect. Subduction zone earthquakes along the trench are oriented from northwest-southweast compression, a different orientation than today’s M 6.6.

    So today’s M 6.6 earthquake appears to have been on a fault deeper than the crustal faults, possibly along a deep fault associated with the collision zone. Though I am not really certain. This region is complicated (e.g. Kita et al., 2010), but there are some interpretations of the crust at this depth range (Iwasaki et al., 2004) shown in an interpreted cross section below.

    I present more about the basics behind ground shaking, triggered landslides, and possible earthquake triggering on Temblor here:

    Below is my interpretive poster for this earthquake

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

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

      Magnetic Anomalies

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

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

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


      •

    • Here is the map with a centuries seismicity plotted.


      •

    Some Relevant Discussion and Figures

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


      •

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


      •

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


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

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


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

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


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

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


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

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


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

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


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

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


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


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

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


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

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

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


      •

    Earthquake Triggered Landslides

    • Here is the aerial video from NHK that shows some of the landslides triggered by this sequence of earthquakes today. This comes from a tweet below.
    • Well, here is a great figure from Keefer (1984) that shows that the larger the magnitude of an earthquake, the larger an area can be subject to triggering of landslides from the ground shaking from that earthquake.


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

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


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

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


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

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


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

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


    • Here is a view of the landslide from my truck.


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


    • Here is another cool view of the ghost forest.


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


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


      •

    Geologic Fundamentals

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

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


      •

    • Here is another way to look at these beach balls.

    • 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.

      References:

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

    Return to the Earthquake Reports page.


    °
    ≥
    ñ

    Earthquake Report: Burma!

    Posted on by
    4

    There was an earthquake in Burma today! The epicenter plotted very close to the Sagaing fault (SF), a major dextral (right-lateral) strike-slip fault system, part of the plate boundary between the India and Eurasia plates. This fault system accommodates much of the dextral relative movement between these two plates.
    I initially thought this would be a strike-slip earthquake. However, the USGS fault plane solution (moment tensor, read more about them below) shows that this was a thrust (compressional) earthquake. There is a region of uplift to the west of the SF, where there is a fold and thrust belt (the Bago-Yoma Range). This region may be experiencing compression due to the relative plate motion here and the orientation of the SF (strain partitioning). There is a GPS rate map below that shows geodetic motion oblique to the SF, showing compression.
    There were two M 7.2 and M 7.4 earthquakes just to the southeast in 1930 and an earthquake in 1994. The 1994 earthquake was a dextral strike-slip earthquake, but the 1930 earthquakes are too old to have this type of analytical results on the USGS website (see Sloan et al., 2017 figure below for the M 7.3 1930 earthquake, which shows a strike-slip mechanism).

    Below is my interpretive poster

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 (and down to M ≥ 4.5 in a second poster).
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.0 earthquake, 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 contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

      I include some inset figures.

      • In the upper left corner, is a map from Maurin and Rangin (2009) that shows the regional tectonics at a regional scale. The Sunda Trench is formed along the Sumatra-Andaman subduction zone, where the India plate subducts beneath the Eurasia, Burma, and Sunda plates. The Sagaing fault is the right-lateral strike-slip plate boundary fault between the Burma and Sunda plates. The black arrows show the relative plate motions between the India : Sunda and India : Burma plates. The Sagaing fault links with the Sumatra fault via the Andaman spreading ridge system. I place a blue star in the general location of today’s earthquake sequence.
      • To the right of the Maurin and Rangin (2009) map is a map from Wang et al (2014) that shows how the Sangaing fault can be broken up into segments. Warm colors are higher elevation than cooler colors. Other than national boundaries, red and black lines represent faults. I place a blue star in the general location of today’s earthquake sequence.
      • In the lower left corner is a figure from Sloan et al. (2017) that shows the fault systems here along with the GPS derived plate motions. On the left, we can see the triangle-barbed red lines, which are ~north-south striking thrust faults in the Indo-Burmese Wedge (“Ranges” on the map). I place a blue star in the general location of today’s earthquake sequence.
      • In the lower right corner is a large scale view of the earthquake faults and historic seismicity of this region (Wang et al., 2014). These authors also plotted some moment tensor data for historic earthquakes. I place a blue star in the general location of today’s earthquake sequence.
      • In the upper right corner is a map showing historic earthquakes on the Sagaing fault (Hurukawa and Maung, 2011). The right panel shows where the authors hypothesize that there is a seismic gap north of 20 degrees latitude, north of where this M 6.0 earthquake happened. I place a blue star in the general location of today’s earthquake sequence.


    • Here is the same map for USGS historic seismicity for earthquakes M ≥ 4.5. This map shows nicely how seismicity gets deeper to the east along the Sumatra-Andaman subduction zone (the Sunda Trench) along the southern part of the poster. This also shows how seismicity also deepens to the east along the Indo-Burmese we3dge (IBW), which is the convergent plate boundary system to the west of the SF.


    USGS Earthquake Pages

    Some Relevant Discussion and Figures

    • Here is a map from Maurin and Rangin (2009) that shows the regional tectonics at a larger scale. They show how the Burma and Sunda plates are configured, along with the major plate boundary faults and tectonic features (ninetyeast ridge). The plate motion vectors for India vs Sunda (I/S) and India vs Burma (I/B) are shown in the middle of the map. Note the Sunda trench is a subduction zone, and the IBW is also a zone of convergence. There is still some debate about the sense of motion of the plate boundary between these two systems. This map shows it as strike slip, though there is evidence that this region slipped as a subduction zone (not strike-slip) during the 2004 Sumatra-Andaman subduction zone earthquake. I include their figure caption as a blockquote below.


      • Structural fabric of the Bay of Bengal with its present kinematic setting. Shaded background is the gravity map from Sandwell and Smith [1997]. Fractures and magnetic anomalies in black color are from Desa et al.[2006]. Dashed black lines are inferred oceanic fracture zones which directions are deduced from Desa et al. in the Bay of Bengal and from the gravity map east of the 90E Ridge. We have flagged particularly the 90E and the 85E ridges (thick black lines). Gray arrow shows the Indo-Burmese Wedge (indicated as a white and blue hatched area) growth direction discussed in this paper. For kinematics, black arrows show the motion of the India Plate with respect to the Burma Plate and to the Sunda Plate (I/B and I/S, respectively). The Eurasia, Burma, and Sunda plates are represented in green, blue, and red, respectively.

    • Here is a different cross section that shows how Maurin and Rangin (2009) interpret this plate boundary to have an oblique sense of motion (it is a subduction zone with some strike slip motion). Typically, these different senses of motion would be partitioned into different fault systems (read about forearc sliver faults, like the Sumatra fault. I mention this in my report about the earthquakes in the Andaman Sea from 2015.07.02). This cross section is further to the south than the one on the interpretation map above. I include their figure caption as a blockquote below.


      • Present cross section based on industrial multichannel seismics and field observations. The seismicity from USGS catalog and Engdahl [2002] is represented as black dots. Focal mechanisms from Global CMT (http://www.globalcmt.org/CMTsearch.html) catalog are also represented.

    • This figure shows the interpretation from Maurin and Rangin (2009) about how the margin has evolved over the past 10 Ma.


      • Cartoon showing the tectonic evolution of the Indo-Burmese Wedge from late Miocene to present.

    • Wang et al. (2014) also have a very detailed map showing historic earthquakes along the major fault systems in this region. They also interpret the plate boundary into different sections, with different ratios of convergence:shear. I include their figure caption as a blockquote below.


      • Simplified neotectonic map of the Myanmar region. Black lines encompass the six neotectonic domains that we have defined. Green and Yellow dots show epicenters of the major twentieth century earthquakes (source: Engdahl and Villasenor [2002]). Green and yellow beach balls are focal mechanisms of significant modern earthquakes (source: GCMT database since 1976). Pink arrows show the relative plate motion between the Indian and Burma plates modified from several plate motion models [Kreemer et al., 2003a; Socquet et al., 2006; DeMets et al., 2010]. The major faults west of the eastern Himalayan syntax are adapted from Leloup et al. [1995] and Tapponnier et al. [2001]. Yellow triangle shows the uncertainty of Indian-Burma plate-motion direction.

    • Here is the map showing the SF fault segments (Wang et al., 2014).


      • Fault segments and historical earthquakes along the central and southern parts of the Sagaing fault. Green dots show relocated epicenters from Hurukawa and Phyo Maung Maung [2011]. Dashed and solid gray boxes surround segments of the fault that ruptured in historical events. NTf = Nanting fault; Lf = Lashio fault; KMf = Kyaukme fault; PYf = Pingdaya fault; TGf = Taunggyi fault.

    • Here is the Curray (2005) plate tectonic map.


      • Tectonic map of part of the northeastern Indian Ocean. Modified from Curray (1991).

    • Here is the Sloan et al. (2017) map showing the faults and GPS derived plate motion.


      • Seismotectonic map of Myanmar (Burma) and surroundings. Faults are from Taylor & Yin (2009) with minor additions and adjustments. GPS vectors show velocities relative to a fixed India from Vernant et al. (2014), Gahalaut et al. (2013), Maurin et al. (2010) and Gan et al. (2007). Coloured circles indicateMw > 5 earthquakes from the EHB catalogue. Grey events are listed for depths <50 km, yellow for depths of 50–100 km and red for depths >100 km. The band of yellow and red earthquakes beneath the Indo-Burman Ranges represents the Burma Seismic Zone. The dashed black line shows the line of the cross-section in Figure 2.13. ASRR, Ailao Shan–Red River Shear Zone.

    • Here is a Sloan et al. (2017) map that shows fault plane solutions (including the 1930 M 7.3 SF earthquake) for earthquakes in the region.


      • Seismotectonic map of Myanmar (Burma). Faults are from Taylor & Yin (2009) with minor additions and adjustments. GPS vectors show velocities relative to a fixed Eurasia from Maurin et al. (2010). Slip rate estimates on the Sagaing Fault are given in blue and are from a, Bertrand et al. (1998); b, Vigny et al. (2003); c, Maurin et al. (2010); and d, Wang et al. (2011). Major earthquakes (Ms ≥7) are shown by yellow stars for the period 1900–76 from International Seismological Centre (2011) and by red stars for the period 1836–1900 from Le Dain et al. (1984). The location and magnitude of theMb 7.5 1946 earthquake is taken from Hurukawa&Maung Maung (2011). Earthquake focal mechanisms are taken from the GCMT catalogue (Ekström et al. 2005) and show Mw ≥5.5 earthquakes, listed as being shallower than 30 km in the period 1976–2014. IR, Irrawaddy River; CR, Chindwin River; HV, Hukawng Valley; UKS, Upper Kachin State; SF, Sagaing Fault; KF, Koma Fault. The inset panel is an enlargement of the area within the dashed grey box. It shows the dense GPS network in this area.

    • This map shows that the region where today’s M 6.0 earthquake is located is in the region of uplifted regions along the SF.


      • Regional setting, and fault geometries and uplift distribution associated with the Sagaing Fault.

    • Here is a comprehensive map showing the complicated tectonics of this region (Sloan et al., 2017).


      • Regional tectonic setting of the Andaman Sea Region modified from Morley (2017). See text for explanation of labels A–E. The locations of Figures 2.15– 2.17 are indicated.

    • This map shows how Rangin (2017) hypothesizes about the platelets formed along the plate boundary.


      • Extension of the Burma–Andaman–Sumatra microplate (shown in green). The Burma Platelet is the northern part in Myanmar. Active faults are shown in red and inactive faults in purple. The post-Santonian magnetic anomalies and associated transform faults of the Indian and Australian plates are suggested in blue. Left-lateral red arrows along the 90° E Ridge illustrate left-lateral motion between the Indian and Australian plates. India/Eurasia relative motion is shown with a yellow arrow, India/Sunda motion with purple arrows and Australia/Sunda motion with black arrows (modified from Rangin 2016).

    • This is a great summary figure from Ranging (2017) showing how these plates and platelets interact in this region.


      • Structural map of the active buckling of the Burma Platelet considered not to be rigid. The curved Sagaing Fault, Lelong, Kaladan and coastal faults outline this arched platelet. WSW extrusion of the platelet is outlined by the NE–SW diffuse dextral shear south of the South Assam Shear Zone into the north and by the left lateral Pyay-Prome shear zone in the south. The western margin (CSM: collapsing Sunda margin) of this platelet is affected by dextral wrench and active collapse of the continental margin, but no sign of active subduction was found. This platelet is bracketed tectonically between the drifted 90° E Ridge and the accreted volcanic ridges into the south and the Eurasian Buttress (Himalayas and Shillong) into the north. The East Himalaya Crustal Flow (EHCF; large curved red arrow) imaged in the East Himalaya Syntaxis (EHS) is induced by the Tibet Plateau collapse and could be an important component of the tectonic force causing the platelet buckling. The Burma Platelet is jammed between the Accreted Volcanic Ridges in the south, and the Shillong Plateau crustal block in the north, participate to the buckling of the Myanmar Platelet. BBacc, Bay of Bengal attenuated continental crust (Rangin & Sibuet 2017); CMB, Central Myanmar Basins; CMF, Churachandpur-Mao Fault (Gahalaut et al. 2013).

      References:

    • Curray, J.R., 2005. Tectonics and history of the Andaman Sea Region in Journal of Asian Earth Sciences, v. 25, p. 187-232.
    • Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
    • Hurukawa, N. and Maung, P.M., 2011. Two seismic gaps on the Sagaing Fault, Myanmar, derived from relocation of historical earthquakes since 1918 in GRL, v. 38, L01310, doi:10.1029/2010GL046099
    • Maurin, T. and Rangin, C., 2009. Structure and kinematics of the Indo-Burmese Wedge: Recent and fast growth of the outer wedge in Tectonics, v. 28, TC2010, doi:10.1029/2008TC002276
    • Rangin, C., 2017. Active and recent tectonics of the Burma Platelet in Myanmar in BARBER, A. J., KHIN ZAW & CROW, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, v. 48, p. 53–64, https://doi.org/10.1144/M48.3
    • Sloan, R.A., Elliot, J.R., Searle, M.P., and Morley, C.K., 2017. Active tectonics of Myanmar and the Andaman Sea in BARBER, A. J., KHIN ZAW & CROW, M. J. (eds) 2017. Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs, v. 48, p. 19–52, https://doi.org/10.1144/M48.2
    • Wang, Y., K. Sieh, S. T. Tun, K.-Y. Lai, and T. Myint, 2014. Active tectonics and earthquake potential of the Myanmar region in J. Geophys. Res. Solid Earth, 119, 3767–3822, doi:10.1002/2013JB010762.

    Earthquake Report: Iran

    Posted on by
    Reply

    A couple weeks following the earthquake in eastern Iraq, there was a sequence of earthquakes in central eastern Iran. These earthquakes are too distant to be related. The Iranian sequence includes a M 6.1 foreshock on 2017.12.01 and two M 6.0 aftershocks on 2017.12.12. Here is my report for the M 7.3 earthquake.
    While putting together my annual summary for 2017, I wanted to include a poster that shows these two earthquakes as they relate to regional historic seismicity (with fault plane solutions).

    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 1917-2017 with magnitudes M > 6.5.
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.1 earthquake. I also include USGS fault plane solutions for most of the earthquakes in the region.

    • 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. Based upon the tectonics associated with the San Andreas and Maacama faults, I interpret this M 4.3 earthquake to be a right-lateral strike-slip fault.
    • 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 MMI contours for most of the earthquakes that have fault plane solutions plottted.

      • I include some inset figures.

      • In the upper right comer is a map from Peretti et al. (2011) that shows the plate boundaries in this region. I place blue stars in the general location of the M 7.3 and M 6.1 earthquakes. The M 6.1 earthquake happened in a region of north striking strike slip faults.
      • In the lower left corner is a map from Javadi et al. (2013) which shows the tectonic domains for this region. I place two blue stars in the general location of the M 7.3 and M 6.1 earthquakes. The M 6.1 earthquake sequence appears related to the Kubanan or Naiband faults.


      • Here is my poster for the M 7.3 earthquake. See the Earthquake Report page for more information about the tectonics in the region.


        •

      • Here is the tectonic map from Peretti et al. (2011).


        • Tectonic sketch map of the Persian Gulf and Arabian Peninsula, modified from Al-Husseini (2000), Ziegler (2001) and Pollastro (2003).

      • Here is the map from Javadi et al. (2013).


        • (Colour online) (a) Tectonic setting of Iran in the Middle East and presentation of major convergence vectors of the region. (b) Main sedimentary-structural zones of Iran (modified from Aghanabati, 2004). Major faults discussed in the text are shown. White and black arrows from Sella, Dixon & Mao (2002) and Vernant et al. (2004), respectively. DFS – Doruneh Fault System, MRZF – Main Zagros Reverse Fault, HZF – High Zagros Fault, MFF – Mountain Frontal Fault, ZFF – Zagros Foredeep Fault.

      • Here is a great fault map from Walker and Jackson (2004). The M 6.1 earthquake sewuence was located in the region of Fig. 8 a (shown below as a Landsat map).


        • GTOPO30 image of central and eastern Iran showing the major fault zones and geographical regions. Black and gray arrows represent Arabia-Eurasia plate motions. Rates are in millimeters per year. Black arrows are GPS estimates from Sella et al. [2002] and gray arrows represent 3 Ma magnetic anomaly plate motions which are a combination of the Africa-Eurasia plate motion from Chu and Gordon [1998] and the Africa-Arabia plate motion of DeMets et al. [1994] (see Jackson et al. [1995] for method). Arabia-Eurasia convergence occurs in the Zagros, the Alborz, and Kopeh Dagh, and possibly in central Iran by the rotation of strike-slip faults (see later discussion). Right-lateral shear between central Iran and Afghanistan is taken up on N–S right-lateral faults of the Gowk-Nayband and Sistan suture zone systems, which surround the Dasht-e-Lut. North of 34N, the right-lateral shear is taken up on left-lateral faults that rotate clockwise.

      • Here is a map showing the location of the Gowk fault, also with the geomorphology (shown on the LANDSAT map) associated with this fault system. This is the map labeled as Fig 8.


        • (a) GTOPO30 topography of the Kerman region centered on the Gowk fault (see Figure 1 for location). Fault plane solutions of shallow (<35 km) earthquakes are shown. Black solutions are events modeled using body waveforms (listed by Jackson [2001], Walker [2003], and Talebian and Jackson [2004]); dark gray represents events from the Harvard CMT catalogue with >70% double-couple component; light gray represents first-motion solutions [from McKenzie, 1972]. Zones of shortening and thrust faulting are seen both to the north of Kerman, where the Gowk fault splits into the Kuh-Banan, Lakar-Kuh, and Nayband faults, and south of Mahan, where NW–SE trending thrust faults occupy the region between the Sabzevaran and Gowk faults. These zones of intense deformation may be partly caused by rotation of crustal blocks, as marked by black arrows (see section 5.3). The box marks the location of Figure 8b. (b) Landsat TM image of the central part of the Gowk fault. Restoration of drainage and structural features indicate between 12 and 15 km of cumulative right-lateral displacement [Walker and Jackson, 2002]. Restoration of 15 km of right-lateral slip aligns dark-colored lithologies (marked X), although it is not certain that the dark-colored rocks at either side of the fault are from a single displaced unit.

      • Here is the aerial image map of this region (Walker and Jackson, 2002). The M 6.1 sequence occurred to the northeast of Fandogo.


        • LANDSAT TM image and location map of the Gowk fault region.

      • This figure shows the Walker and Jackson (2002) interpretation for the structures in the region of the Gowk fault. The M 6.1 earthquake is most likely related to the Shahad thrust fault system (also noted on the above map).


      • This is a plot from the International Seismological Center (ISC) that shows seismicity in plan view (the map) and cross sectional view.


        • Historical seismicity map based in ISC Bulletin data for yesterdays Mw 7.3 on Iran-Iraq border. Mostly shallow thrust events in a complex tectonic setting.

      • UPDATE: After chatting with Dr. Eric Fielding on twitter, I discovered a paper that he wrote discussing the faults in the region of the M 6.1 sequence. Perfect! They relate fault growth in a fold and thrust belt (Shahad thrust faults) to aseismic slip, based upon modeling (constrained by InSAR data) of the 1998.03.14 Fandoqa M 6.6 earthquake. However, given the M 6.1 sequence, we now know that all the growth is probably not aseismic.


        • A: Shaded relief topographic map of Shahdad area with active faults (medium black lines) (Walker and Jackson, 2002), XX9 profile location (thick black line), moderate earthquakes (black filled circles), four large earthquakes since 1981 (white filled circles), and fault-plane solution (upper right) for Fandoqa earthquake (Berberian et al., 2001). Rectangles with thin black lines are Fandoqa rupture (F) and Shahdad basalthrust (S) dislocations shown in other figures. Thick dashed white line—Gowk fault zone; P—central Iranian plateau; L—Lut block. B: Topographic profile and depth cross section of Fandoqa main shock, Shahdad basal thrust, and splay slip planes. Solid lines show positions of fault planes from inversion after adjustment for topography; dashed lines are unadjusted. Gray fill shows Shahdad thrust wedge.


        A: Average of two interferograms, converted to radar range change (motion in radar line of sight) in millimeters. Faults (black lines) and profile location (white line) as in Figure 1A. Rectangles (thin lines) show surface locations of Fandoqa and Shahdad basalthrust dislocation models. B: Surface deformation from Fandoqa main-shock elastic model, shown as radar range change. Large rectangle outlines area shown in C and D. C: Residual interferogram after subtracting Fandoqa main shock model shown in B. Note that color scale and area are different from A and B. Green labels are Universal Transverse Mercator zone 40 coordinates and tics are every 10 km. Thin red lines show updip projections of Fandoqa and Shahdad basal thrust to surface. Larger rectangle shows extended Shahdad basal thrust used in distributed slip inversion (Fig. 3) and Poly3D (Fig. 4). D: Surface deformation predicted by slip model of Shahdad basal thrust and splays shown in Figure 4, projected into radar line of sight. Same area and colors as C.

    Here are the USGS pages for the main earthquake in this sequence.

      References

    • Allen, M.B., Saville, C., Blac, E.K-P., Talebian, M., and Nissen, E., 2013. Orogenic plateau growth: Expansion of the Turkish-Iranian Plateau across the Zagros fold-and-thrust belt in Tectonics, v. 32, p. 171-190, doi:10.1002/tect.20025
    • Fielding, E.J., Wright, T.J., Muller, J., Parsons, B.E., and Walker, R., 2004. Aseismic deformation of a fold-and-thrust belt imaged by synthetic aperture radar interferometry near Shahdad, southeast Iran in Geology, v. 32, no. 7, p. 577-580, doi: 10.1130/G20452.1
    • Giardini, D., Grunthal, G., Shedlock, K., Zhang. P., and Global Seismic Hazards Program, 1999. Global seismic hazards map: Accessed on Jan. 9, 2007 at http://www.seismo.ethz.ch/GSHAP.
    • Javadi, H. R., M. Esterabi Ashtiani, B. Guest, A. Yassaghi, M. R. Ghassemi, M. Shahpasandzadeh, and A. Naeimi (2015), Tectonic reversal of the western Doruneh Fault System: Implications for Central Asian tectonics, Tectonics, 34, 2034–2051, doi:10.1002/ 2015TC003931.
    • Jenkins, Jennifer, Turner, Bethan, Turner, Rebecca, Hayes, G.P., Sinclair, Alison, Davies, Sian, Parker, A.L., Dart, R.L., Tarr, A.C., Villaseñor, Antonio, and Benz, H.M., compilers, 2013. Seismicity of the Earth 1900–2010 Middle East and vicinity (ver 1.1, Jan. 28, 2014): U.S. Geological Survey Open-File Report 2010–1083-K, scale 1:7,000,000, https://pubs.usgs.gov/of/2010/1083/k/.
    • Perotti, C.R., S. Carruba, M. Rinaldi, G. Bertozzi, L. Feltre and M. Rahimi, 2011. The Qatar–South Fars Arch Development (Arabian Platform, Persian Gulf): Insights from Seismic Interpretation and Analogue Modelling in Earth and Planetary Sciences » Geology and Geophysics » “New Frontiers in Tectonic Research – At the Midst of Plate Convergence”, book edited by Uri Schattner, ISBN 978-953-307-594
    • Stern, R.J. and Johnson, P., 2010. Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis in Earth-Science Reviews, v. 101, p. 29-67.
    • Taymaz, T., Yilmaz, Y., and Dilek, Y., 2007. The geodynamics of the Aegean and Anatolia: introduction in Geological Society, London, Special Publications, v. 291; p. 1-16, doi:10.1144/SP291.1
    • Verges, J., Saura, E., Casciello, E., Fernandez, M., Villasenor, A., Jimenez-Munt, I., and Garcia-Castellanos, D., 2011. Crustal-scale cross-sections across the NW Zagros belt: implications for the Arabian margin reconstruction in Geol. Mag., v. 148, no. 5-6, p. 739-761
    • Walker, R. and Jackson, J., 2002. Offset and evolution of the Gowk fault, S.E. Iran: a major Intra-continental Strike-Slip System in Journal of Structural Geology, v. 24, p. 1677-1698.
    • Walker, R. and Jackson, J., 2004. Active tectonics and late Cenozoic strain distribution in central and eastern Iran in Tectonics, v. 23, doi:10.1029/2003TC001529
    • Woudloper, 2009. Tectonic map of southern Europe and the Middle East, showing tectonic structures of the western Alpide mountain belt.

    Earthquake Report: Iraq

    Posted on by
    Reply

    A month and a half ago, I was attending the PATA conference and an earthquake hit Iran and Iraq the night before our first field trip. Thus, I did not have the time to address this earthquake at the time. I am preparing this report in support of my annual summary.
    This was a damaging earthquake and is the most deadly for 2017. Over 500 people were killed and thousands were injured.
    I post lots of material below that was developed in the 6 weeks following the earthquake.
    There is a page here with some photos of the damage: Earthquake-Report.com.

    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 1917-2017 with magnitudes M > 6.5.
    I plot the USGS fault plane solutions (moment tensors in blue) for the M 7.3 earthquake.

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. Based upon the tectonics associated with the San Andreas and Maacama faults, I interpret this M 4.3 earthquake to be a right-lateral strike-slip fault.
    • 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.

    Here are the USGS pages for the main earthquake in this sequence.

    I include some inset figures.

    • In the lower right corner I include a map that shows the major plate boundary and major crustal faults in the region, as well as relative plate motions plotted as arrows (Taymaz et al.,
      2007). I place a green star in the general location of the M 7.3 earthquake. Note that this M 7.3 earthquake happened along the Bitis-Zagros Fold Belt.
    • In the upper right corner is a map that shows the results of interfereometric RADAR analyses as prepared by GSI in Japan. This map shows a region of subsidence to the southwest of the M 7.3 epicenter (the largest orange circle) and a region of uplift to the northeast of this M 7.3 earthquake. More about this map below.
    • To the left of this interferogram, I include a basic tectonic map of this region (Woudloper, 2009). Maps with local (larger) scale have much more detailed views of the faulting. I place a green star in the general location of this M 7.3 earthquake.
    • In the upper left corner are two maps that show how Earth’s surface moved during the earthquake (and shortly afterwards). The left panel shows east-west motion and the right panel shows up-down motion (this looks similar to the figure in the upper right corner.
    • In the lower left corner I place a map that shows the large scale details of the crustal faults in the Bitis-Zagros Fold Belt (Allen et al., 2013). I place a green star in the general location of this M 7.3 earthquake.
    • To the right of this fault map is a cross section A-A.’ The location of this cross section is designated by a blue line on the map in the lower left corner, as well as on the main interpretive poster map.


    • Here is a comparison between the “Did You Feel It?” map and the Shakemap. Both maps represent shaking intensity with the same scale, the MMI scale (described above). The DYFI map on the left is based on peoples’ observations as they report using the USGS DYFI website. The map on the right is the result of numerical simulations of shaking intensity. Below each map are regressions of those data.


      •

    • This map shows the plate boundary and intraplate faults of the region. Also shown are the relative plate motions as black arrows. Note how the Bitis-Zagros Fold Belt (BZFB) is a dextral oblique (right-lateral thrust) fault system. This fault system is part of the Alpide belt, which is oriented parallel to the Arabia-Anatolia relative plate motion (ergo the strike-slip motion).


      • (a) Seismicity of the Eastern Mediterranean region and surroundings reported by USGS–NEIC during 1973–2007 with magnitudes for M . 3 superimposed on a shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b). (b) Summary sketch map of the faulting and bathymetry in the Eastern Mediterranean region, compiled from our observations and those of Le Pichon & Angelier (1981), Taymaz (1990), Taymaz et al. (1990, 1991a, b); S¸arogˇlu et al. (1992), Papazachos et al. (1998), McClusky et al. (2000) and Tan & Taymaz (2006). Large black arrows show relative motions of plates with respect to Eurasia (McClusky et al. 2003). Bathymetry data are derived from GEBCO/97–BODC, provided by GEBCO (1997) and Smith & Sandwell (1997a, b). Shaded relief map derived from the GTOPO-30 Global Topography Data taken after USGS. NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; PTF, Paphos Transform Fault; CTF, Cephalonia Transform Fault; PSF, Pampak–Sevan Fault; AS, Apsheron Sill; GF, Garni Fault; OF, Ovacık Fault; MT, Mus¸ Thrust Zone; TuF, Tutak Fault; TF, Tebriz Fault; KBF, Kavakbas¸ı Fault; MRF, Main Recent Fault; KF, Kagˇızman Fault; IF, Igˇdır Fault; BF, Bozova Fault; EF, Elbistan Fault; SaF, Salmas Fault; SuF, Su¨rgu¨ Fault; G, Go¨kova; BMG, Bu¨yu¨k Menderes Graben; Ge, Gediz Graben; Si, Simav Graben; BuF, Burdur Fault; BGF, Beys¸ehir Go¨lu¨ Fault; TF, Tatarlı Fault; SuF, Sultandagˇ Fault; TGF, Tuz Go¨lu¨ Fault; EcF, Ecemis¸ Fau; ErF, Erciyes Fault; DF, Deliler Fault; MF, Malatya Fault; KFZ, Karatas¸–Osmaniye Fault Zone.

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


      •

    • This is a great map showing some details of the tectonics associated with the Arabia plate (Stern and Johnson, 2010).


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

    • This map (Allen et al., 2013) shows focal mechanisms (fault plane solutions) for earthquakes associated with the BZFB. GPS velocities are also plotted in blue (rates of motion at points on the earth, measured in mm per year), relative to Iran.


      • (a) Regional topography and seismicity of the Arabia-Eurasia collision. Large dots are epicenters of earthquakes of M >6 from 1900 to 2000 [Jackson, 2001], small dots are epicenters from the EHB catalogue 1964–1999, M >5. Red arrows show GPS-derived velocity with respect to Asia from Sella et al. [2002]. A= Alborz; TIP = Turkish-Iranian plateau; Z = Zagros. (b) Seismicity of the Zagros: focal mechanisms reported in Nissen et al. [2011] and references therein. Note the scarcity of thrusts above the smoothed 1250m regional elevation contour (derived using a Gaussian filter with a radius of 50 km). Earthquake epicenters are accurate to within 20 km [Nissen et al., 2011]. GPS vectors are from Walpersdorf et al. [2006]. MZRF =Main Zagros Reverse Fault (Zagros suture).

    • This map shows a detailed view of faults and folds in the BZFB (Allen et al., 2013).


      • (a) Location map and major structures of the Zagros Simply Folded Belt, Iran. Derived from NIOC [1975, 1977], Berberian [1995], Hessami et al. [2001], Blanc et al. [2003], Agard et al. [2005], and Babaie et al. [2006]. Key to fault abbreviations: B = Borazjan; Iz = Izeh; K= Kazerun; KB= Kareh Bas; Kh = Khanaqin; S = Sarvestan; SP = Sabz-Pushan; BL = Balarud Line; A= Kuh-e Asmari. b) Earthquake epicentres across the Zagros, from Nissen et al. [2011] and references therein, divided by fault type. MZRF =Main Zagros Reverse Fault.


    • This is cross section A-A’ from the map above (also on poster). Note the thrust faults and the strike-slip faults represented in this section (Allen et al., 2013). While this section is to the south of the M 7.3 earthquake, it still represents the generalized tectonics in the region (dextral oblique plate boundary).


      • (a) Cross-section through the Dezful Embayment and the Bakhtyari Culmination.

    • The Geospatial Information Authority of Japan (GSI) conducted some analyses using Synthetic Aperture Radar (SAR). “Two or more line-of-sight displacements with different observing directions can be decomposed to quasi east-west and up-down components.” They describe their interpretation below.




      • Large displacement (~90 cm upward and ~50 cm westward) has been detected around 20 km NNW of Sarpol-e Zahab. Around the epicenter, ~30 cm downward and ~35 cm westward displacement has been detected.

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


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

    Other Social Media Posts

    • Here is a plot showing historic seismicity from Dr. Jascha Polet (Cal Poly Pomona Seismologist).

      References

    • Allen, M.B., Saville, C., Blac, E.K-P., Talebian, M., and Nissen, E., 2013. Orogenic plateau growth: Expansion of the Turkish-Iranian Plateau across the Zagros fold-and-thrust belt in Tectonics, v. 32, p. 171-190, doi:10.1002/tect.20025
    • Giardini, D., Grunthal, G., Shedlock, K., Zhang. P., and Global Seismic Hazards Program, 1999. Global seismic hazards map: Accessed on Jan. 9, 2007 at http://www.seismo.ethz.ch/GSHAP.
    • Jenkins, Jennifer, Turner, Bethan, Turner, Rebecca, Hayes, G.P., Sinclair, Alison, Davies, Sian, Parker, A.L., Dart, R.L., Tarr, A.C., Villaseñor, Antonio, and Benz, H.M., compilers, 2013, Seismicity of the Earth 1900–2010 Middle East and vicinity (ver 1.1, Jan. 28, 2014): U.S. Geological Survey Open-File Report 2010–1083-K, scale 1:7,000,000, https://pubs.usgs.gov/of/2010/1083/k/.
    • Stern, R.J. and Johnson, P., 2010. Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis in Earth-Science Reviews, v. 101, p. 29-67.
    • Taymaz, T., Yilmaz, Y., and Dilek, Y., 2007. The geodynamics of the Aegean and Anatolia: introduction in Geological Society, London, Special Publications, v. 291; p. 1-16, doi:10.1144/SP291.1
    • Woudloper, 2009. Tectonic map of southern Europe and the Middle East, showing tectonic structures of the western Alpide mountain belt.

    Earthquake Report: Bengkulu (Sumatra)!

    Posted on by
    Reply

    Last night (my time) while I was tending to other business, there was an earthquake along the Sunda Megathrust. Here is the USGS website for this M 6.4 earthquake.
    This M 6.4 earthquake happened down-dip (“deeper than”) along the megathrust from the 2007.09.12 M 8.4 megathrust earthquake. Here is the USGS website for the M 8.4 earthquake. This M 6.4 earthquake occurred in a region of low seismogenic coupling (as inferred by Chlieh at al., 2008), albeit with sparse GPS data in this region. Chlieh et al. (2008) used coral geodetic and paleogeodetic data, along with Global Positioning System (GPS) observations, to constrain their model. Because there are no forearc islands in this part of the subduction zone, there are no GPS nor coral data with which to constrain their model (so it may underestimate the coupling %, i.e. coupling ratio).
    Based upon the USGS fault plane slip model, this M 6.4 earthquake actually happened in a region of higher slip from the M 8.4 earthquake. We may consider this M 6.4 earthquake to be an aftershock of the M 8.4 earthquake.
    Here is a report from earthquake-report.com.

    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 also include USGS epicenters from 1917-2017 for magnitudes M ≥ 7.
    I also include the USGS moment tensor for today’s earthquake, as well as for the 2007 M 8.4 earthquake. I label the other epicenters with large magnitudes (2004, 2005, and 2012). Find more details about these earthquakes in my reports listed at the bottom of this page, above the references.

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

      I include some inset figures in the poster.

    • In the upper right corner, I include a map from Hayes et al. (2013) that shows the epicenters of earthquakes from the past century or so. There is also a cross section that is in the region of the 2007 and 2017 M 8.4 and M 6.4 earthquakes. I also placed a C-C’ green line on the main map to show where this cross section is compared to the other features on my map. I placed a blue star in the general location of the M 6.4 earthquake.
    • In the lower left corner, I include a map I made that delineates the spatial extent for historic earthquakes along the Sunda Megathrust. This came from a paper that I had submitted to Marine Geology. I include this figure below with attributions to the publications that I used as references for this map. I outlined the slip patch for the M 8.4 earthquake in transparent orange.
    • In the upper left corner, I present a figure from Chlieh et al. (2008 ). These authors use GPS and coral geodetic and paleogeodetic data to constrain the proportion of the plate motion rates that are accumulated as tectonic strain along the megathrust fault. Basically, this means how much % that the fault is storing energy to be released in subduction zone earthquakes. This is just a model and is limited by the temporal and spatial extent of their observations which form the basis for their model. However, this is a well respected approach to estimate the potential for future earthquake (given the assumptions that I here mention).


    • I prepared this figure to show the difference in MMI Intensity for these two closely spaced earthquakes. The data here come from the USGS websites listed above.


      •

    • Here is my map. I include the references below in blockquote.


      • Sumatra core location and plate setting map with sedimentary and erosive systems figure. A. India-Australia plate subducts northeastwardly beneath the Sunda plate (part of Eurasia) at modern rates (GPS velocities are based on regional modeling of Bock et al, 2003 as plotted in Subarya et al., 2006). Historic earthquake ruptures (Bilham, 2005; Malik et al., 2011) are plotted in orange. 2004 earthquake and 2005 earthquake 5 meter slip contours are plotted in orange and green respectively (Chlieh et al., 2007, 2008). Bengal and Nicobar fans cover structures of the India-Australia plate in the northern part of the map. RR0705 cores are plotted as light blue. SRTM bathymetry and topography is in shaded relief and colored vs. depth/elevation (Smith and Sandwell, 1997). B. Schematic illustration of geomorphic elements of subduction zone trench and slope sedimentary settings. Submarine channels, submarine canyons, dune fields and sediment waves, abyssal plain, trench axis, plunge pool, apron fans, and apron fan channels are labeled here. Modified from Patton et al. (2013 a).

    • This is the main figure from Hayes et al. (2013) from the Seismicity of the Earth series. There is a map with the slab contours and seismicity both colored vs. depth. There are also some cross sections of seismicity plotted, with locations shown on the map.


      •

    • Here is a great figure from Philobosian et al. (2014) that shows the slip patches from the subduction zone earthquakes in this region.


      • Map of Southeast Asia showing recent and selected historical ruptures of the Sunda megathrust. Black lines with sense of motion are major plate-bounding faults, and gray lines are seafloor fracture zones. Motions of Australian and Indian plates relative to Sunda plate are from the MORVEL-1 global model [DeMets et al., 2010]. The fore-arc sliver between the Sunda megathrust and the strike-slip Sumatran Fault becomes the Burma microplate farther north, but this long, thin strip of crust does not necessarily all behave as a rigid block. Sim = Simeulue, Ni = Nias, Bt = Batu Islands, and Eng = Enggano. Brown rectangle centered at 2°S, 99°E delineates the area of Figure 3, highlighting the Mentawai Islands. Figure adapted from Meltzner et al. [2012] with rupture areas and magnitudes from Briggs et al. [2006], Konca et al. [2008], Meltzner et al. [2010], Hill et al. [2012], and references therein.

    • This is a figure from Philobosian et al. (2012) that shows a larger scale view for the slip patches in this region.


      • Recent and ancient ruptures along the Mentawai section of the Sunda megathrust. Colored patches are surface projections of 1-m slip contours of the deep megathrust ruptures on 12–13 September 2007 (pink to red) and the shallow rupture on 25 October 2010 (green). Dashed rectangles indicate roughly the sections that ruptured in 1797 and 1833. Ancient ruptures are adapted from Natawidjaja et al. [2006] and recent ones come from Konca et al. [2008] and Hill et al. (submitted manuscript, 2012). Labeled points indicate coral study sites Sikici (SKC), Pasapuat (PSP), Simanganya (SMY), Pulau Pasir (PSR), and Bulasat (BLS).

    • Here are a series of figures from Chlieh et al. (2008 ) that show their data sources and their modeling results. I include their figure captions below in blockquote.
    • This figure shows the coupling model (on the left) and the source data for their inversions (on the right). Their source data are vertical deformation rates as measured along coral microattols. These are from data prior to the 2004 SASZ earthquake.


      • Distribution of coupling on the Sumatra megathrust derived from the formal inversion of the coral and of the GPS data (Tables 2, 3, and 4) prior to the 2004 Sumatra-Andaman earthquake (model I-a in Table 7). (a) Distribution of coupling on the megathrust. Fully coupled areas are red, and fully creeping areas are white. Three strongly coupled patches are revealed beneath Nias island, Siberut island, and Pagai island. The annual moment deficit rate corresponding to that model is 4.0 X 10^20 N m/a. (b) Observed (black vectors) and predicted (red vectors) horizontal velocities appear. Observed and predicted vertical displacements are shown by color-coded large and small circles, respectively. The Xr^2 of this model is 3.9 (Table 7).

    • This is a similar figure, but based upon observations between June 2005 and October 2006.


      • Distribution of coupling on the Sumatra megathrust derived from the formal inversion of the horizontal velocities and uplift rates derived from the CGPS measurements at the SuGAr stations (processed at SOPAC). To reduce the influence of postseismic deformation caused by the March 2005 Nias-Simeulue rupture, velocities were determined for the period between June 2005 and October 2006. (a) Distribution of coupling on the megathrust. Fully coupled areas are red and fully creeping areas are white. This model reveals strong coupling beneath the Mentawai Islands (Siberut, Sipora, and Pagai islands), offshore Padang city, and suggests that the megathrust south of Bengkulu city is creeping at the plate velocity. (b) Comparison of observed (green) and predicted (red) velocities. The Xr^2 associated to that model is 24.5 (Table 8).

    • This is a similar figure, but based on all the data.


      • Distribution of coupling on the Sumatra megathrust derived from the formal inversion of all the data (model J-a, Table 8). (a) Distribution of coupling on the megathrust. Fully coupled areas are red, and fully creeping areas are white. This model shows strong coupling beneath Nias island and beneath the Mentawai (Siberut, Sipora and Pagai) islands. The rate of accumulation of moment deficit is 4.5 X 10^20 N m/a. (b) Comparison of observed (black arrows for pre-2004 Sumatra-Andaman earthquake and green arrows for post-2005 Nias earthquake) and predicted velocities (in red). Observed and predicted vertical displacements are shown by color-coded large and small circles (for the corals) and large and small diamonds (for the CGPS), respectively. The Xr^2 of this model is 12.8.

    • Here is the figure I included in the poster above.


      • Comparison of interseismic coupling along the megathrust with the rupture areas of the great 1797, 1833, and 2005 earthquakes. The southernmost rupture area of the 2004 Sumatra-Andaman earthquake lies north of our study area and is shown only for reference. Epicenters of the 2007 Mw 8.4 and Mw 7.9 earthquakes are also shown for reference. (a) Geometry of the locked fault zone corresponding to forward model F-f (Figure 6c). Below the Batu Islands, where coupling occurs in a narrow band, the largest earthquake for the past 260 years has been a Mw 7.7 in 1935 [Natawidjaja et al., 2004; Rivera et al., 2002]. The wide zones of coupling, beneath Nias, Siberut, and Pagai islands, coincide well with the source of great earthquakes (Mw > 8.5) in 2005 from Konca et al. [2007] and in 1797 and 1833 from Natawidjaja et al. [2006]. The narrow locked patch beneath the Batu islands lies above the subducting fossil Investigator Fracture Zone. (b) Distribution of interseismic coupling corresponding to inverse model J-a (Figure 10). The coincidence of the high coupling area (orange-red dots) with the region of high coseismic slip during the 2005 Nias-Simeulue earthquake suggests that strongly coupled patches during interseismic correspond to seismic asperities during megathrust ruptures. The source regions of the 1797 and 1833 ruptures also correlate well with patches that are highly coupled beneath Siberut, Sipora, and Pagai islands.

    • This figure shows the authors’ estimate for the moment deficit in this region of the subduction zone. This is an estimate of how much the plate convergence rate, that is estimated to accumulate as tectonic strain, will need to be released during subduction zone earthquakes.


      • Latitudinal distributions of seismic moment released by great historical earthquakes and of accumulated deficit of moment due to interseismic locking of the plate interface. Values represent integrals over half a degree of latitude. Accumulated interseismic deficits since 1797, 1833, and 1861 are based on (a) model F-f and (b) model J-a. Seismic moments for the 1797 and 1833 Mentawai earthquakes are estimated based on the work by Natawidjaja et al. [2006], the 2005 Nias-Simeulue earthquake is taken from Konca et al. [2007], and the 2004 Sumatra-Andaman earthquake is taken from Chlieh et al. [2007]. Postseismic moments released in the month that follows the 2004 earthquake and in the 11 months that follows the Nias-Simeulue 2005 earthquake are shown in red and green, respectively, based on the work by Chlieh et al. [2007] and Hsu et al. [2006].

    • For a review of the 2004 and 2005 Sumatra Andaman subduction zone (SASZ) earthquakes, please check out my Earthquake Report here. Below is the poster from that report. On that report page, I also include some information about the 2012 M 8.6 and M 8.2 Wharton Basin earthquakes.
      • I include some inset figures in the poster.
      • In the upper left corner, I include a map that shows the extent of historic earthquakes along the SASZ offshore of Sumatra. This map is a culmination of a variety of papers (summarized and presented in Patton et al., 2015).
      • In the upper right corner I include a figure that is presented by Chlieh et al. (2007). These figures show model results from several models. Each model is represented by a map showing the amount that the fault slipped in particular regions. I present this figure below.
      • In the lower right corner I present a figure from Prawirodirdjo et al. (2010). This figure shows the coseismic vertical and horizontal motions from the 2004 and 2005 earthquakes as measured at GPS sites.
      • In the lower left corner are the MMI intensity maps for the two SASZ earthquakes. Note these are at different map scales. I also include the MMI attenuation curves for these earthquakes below the maps. These plots show the reported MMI intensity data as they relate to two plots of modeled estimates (the orange and green lines). These green dots are from the USGS “Did You Feel It?” reports compared to the estimates of ground shaking from Ground Motion Prediction Equation (GMPE) estimates. GMPE are empirical relations between earthquakes and recorded seismologic observations from those earthquakes, largely controlled by distance to the fault, ray path (direction and material properties), and site effects (the local geology). When seismic waves propagate through sediment, the magnitude of the ground motions increases in comparison to when seismic waves propagate through bedrock. The orange line is a regression of data for the central and eastern US and the green line is a regression through data from the western US.

    • The 2004/2005 SASZ earthquakes also tended to load strain in the crust in different locations. On 2012.04.11 there was a series of strike-slip earthquakes in the India plate crust to the west of the 2004/2005 earthquakes. The two largest magnitudes for these earthquakes were M 8.6 and M 8.2. The M 8.6 is the largest strike-slip earthquake ever recorded.
    • On 2016.03.22 there was another large strike-slip earthquake in the India-Australia plate. This is probably related to this entire suite of subduction zone and intraplate earthquakes. I presented an interpretive poster about this M 7.8 earthquake here. Below is my interpretive poster for the M 7.8 earthquake. Here is the USGS website for this earthquake.
    • I include a map in the upper right corner that shows the historic earthquake rupture areas.


    • Here is a poster that shows some earthquakes in the Andaman Sea. This is from my earthquake report from 2015.11.08.


    • This map shows the fracture zones in the India-Australia plate.


      •

    References:

    • Abercrombie, R.E., Antolik, M., Ekstrom, G., 2003. The June 2000 Mw 7.9 earthquakes south of Sumatra: Deformation in the India–Australia Plate. Journal of Geophysical Research 108, 16.
    • Bassin, C., Laske, G. and Masters, G., The Current Limits of Resolution for Surface Wave Tomography in North America, EOS Trans AGU, 81, F897, 2000.
    • Bock, Y., Prawirodirdjo, L., Genrich, J.F., Stevens, C.W., McCaffrey, R., Subarya, C., Puntodewo, S.S.O., Calais, E., 2003. Crustal motion in Indonesia from Global Positioning System measurements: Journal of Geophysical Research, v. 108, no. B8, 2367, doi: 10.1029/2001JB000324.
    • Bothara, J., Beetham, R.D., Brunston, D., Stannard, M., Brown, R., Hyland, C., Lewis, W., Miller, S., Sanders, R., Sulistio, Y., 2010. General observations of effects of the 30th September 2009 Padang earthquake, Indonesia. Bulletin of the New Zealand Society for Earthquake Engineering 43, 143-173.
    • Chlieh, M., Avouac, J.-P., Hjorleifsdottir, V., Song, T.-R.A., Ji, C., Sieh, K., Sladen, A., Hebert, H., Prawirodirdjo, L., Bock, Y., Galetzka, J., 2007. Coseismic Slip and Afterslip of the Great (Mw 9.15) Sumatra-Andaman Earthquake of 2004. Bulletin of the Seismological Society of America 97, S152-S173.
    • Chlieh, M., Avouac, J.P., Sieh, K., Natawidjaja, D.H., Galetzka, J., 2008. Heterogeneous coupling of the Sumatran megathrust constrained by geodetic and paleogeodetic measurements: Journal of Geophysical Research, v. 113, B05305, doi: 10.1029/2007JB004981.
    • DEPLUS, C. et al., 1998 – Direct evidence of active derormation in the eastern Indian oceanic plate, Geology.
    • DYMENT, J., CANDE, S.C. & SINGH, S., 2007 – Oceanic lithosphere subducting beneath the Sunda Trench: the Wharton Basin revisited. European Geosciences Union General Assembly, Vienna, 15-20/05.
    • Hayes, G. P., Wald, D. J., and Johnson, R. L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
    • Hayes, G.P., Bernardino, Melissa, Dannemann, Fransiska, Smoczyk, Gregory, Briggs, Richard, Benz, H.M., Furlong, K.P., and Villaseñor, Antonio, 2013. Seismicity of the Earth 1900–2012 Sumatra and vicinity: U.S. Geological Survey Open-File Report 2010–1083-L, scale 1:6,000,000, https://pubs.usgs.gov/of/2010/1083/l/.
    • JACOB, J., DYMENT, J., YATHEESH, V. & BHATTACHARYA, G.C., 2009 – Marine magnetic anomalies in the NE Indian Ocean: the Wharton and Central Indian basins revisited. European Geosciences Union General Assembly, Vienna, 19-24/04.
    • Ji, C., D.J. Wald, and D.V. Helmberger, Source description of the 1999 Hector Mine, California earthquake; Part I: Wavelet domain inversion theory and resolution analysis, Bull. Seism. Soc. Am., Vol 92, No. 4. pp. 1192-1207, 2002.
    • Ishii, M., Shearer, P.M., Houston, H., Vidale, J.E., 2005. Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array. Nature 435, 933.
    • Kanamori, H., Rivera, L., Lee, W.H.K., 2010. Historical seismograms for unravelling a mysterious earthquake: The 1907 Sumatra Earthquake. Geophysical Journal International 183, 358-374.
    • Konca, A.O., Avouac, J., Sladen, A., Meltzner, A.J., Sieh, K., Fang, P., Li, Z., Galetzka, J., Genrich, J., Chlieh, M., Natawidjaja, D.H., Bock, Y., Fielding, E.J., Ji, C., Helmberger, D., 2008. Partial Rupture of a Locked Patch of the Sumatra Megathrust During the 2007 Earthquake Sequence. Nature 456, 631-635.
    • Maus, S., et al., 2009. EMAG2: A 2–arc min resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne, and marine magnetic measurements, Geochem. Geophys. Geosyst., 10, Q08005, doi:10.1029/2009GC002471.
    • Malik, J.N., Shishikura, M., Echigo, T., Ikeda, Y., Satake, K., Kayanne, H., Sawai, Y., Murty, C.V.R., Dikshit, D., 2011. Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India: Geology, v. 39, p. 559-562.
    • Meltzner, A.J., Sieh, K., Chiang, H., Shen, C., Suwargadi, B.W., Natawidjaja, D.H., Philobosian, B., Briggs, R.W., Galetzka, J., 2010. Coral evidence for earthquake recurrence and an A.D. 1390–1455 cluster at the south end of the 2004 Aceh–Andaman rupture. Journal of Geophysical Research 115, 1-46.
    • Meng, L., Ampuero, J.-P., Stock, J., Duputel, Z., Luo, Y., and Tsai, V.C., 2012. Earthquake in a Maze: Compressional Rupture Branching During the 2012 Mw 8.6 Sumatra Earthquake in Science, v. 337, p. 724-726.
    • Natawidjaja, D.H., Sieh, K., Chlieh, M., Galetzka, J., Suwargadi, B., Cheng, H., Edwards, R.L., Avouac, J., Ward, S.N., 2006. Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls. Journal of Geophysical Research 111, 37.
    • Newcomb, K.R., McCann, W.R., 1987. Seismic History and Seismotectonics of the Sunda Arc. Journal of Geophysical Research 92, 421-439.
    • Philibosian, B., Sieh, K., Natawidjaja, D.H., Chiang, H., Shen, C., Suwargadi, B., Hill, E.M., Edwards, R.L., 2012. An ancient shallow slip event on the Mentawai segment of the Sunda megathrust, Sumatra. Journal of Geophysical Research 117, 12.
    • Prawirodirdjo, P., McCaffrey,R., Chadwell, D., Bock, Y, and Subarya, C., 2010. Geodetic observations of an earthquake cycle at the Sumatra subduction zone: Role of interseismic strain segmentation, JOURNAL OF GEOPHYSICAL RESEARCH, v. 115, B03414, doi:10.1029/2008JB006139
    • Rivera, L., Sieh, K., Helmberger, D., Natawidjaja, D.H., 2002. A Comparative Study of the Sumatran Subduction-Zone Earthquakes of 1935 and 1984. BSSA 92, 1721-1736.
    • Shearer, P., and Burgmann, R., 2010. Lessons Learned from the 2004 Sumatra-Andaman Megathrust Rupture, Annu. Rev. Earth Planet. Sci. v. 38, pp. 103–31
    • SATISH C. S, CARTON H, CHAUHAN A.S., et al., 2011 – Extremely thin crust in the Indian Ocean possibly resulting from Plume-Ridge Interaction, Geophysical Journal International.
    • Sieh, K., Natawidjaja, D.H., Meltzner, A.J., Shen, C., Cheng, H., Li, K., Suwargadi, B.W., Galetzka, J., Philobosian, B., Edwards, R.L., 2008. Earthquake Supercycles Inferred from Sea-Level Changes Recorded in the Corals of West Sumatra. Science 322, 1674-1678.
    • Singh, S.C., Carton, H.L., Tapponnier, P, Hananto, N.D., Chauhan, A.P.S., Hartoyo, D., Bayly, M., Moeljopranoto, S., Bunting, T., Christie, P., Lubis, H., and Martin, J., 2008. Seismic evidence for broken oceanic crust in the 2004 Sumatra earthquake epicentral region, Nature Geoscience, v. 1, pp. 5.
    • Smith, W.H.F., Sandwell, D.T., 1997. Global seafloor topography from satellite altimetry and ship depth soundings: Science, v. 277, p. 1,957-1,962.
    • Sorensen, M.B., Atakan, K., Pulido, N., 2007. Simulated Strong Ground Motions for the Great M 9.3 Sumatra–Andaman Earthquake of 26 December 2004. BSSA 97, S139-S151.
    • Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., McCaffrey, R., 2006. Plate-boundary deformation associated with the great Sumatra–Andaman earthquake: Nature, v. 440, p. 46-51.
    • Tolstoy, M., Bohnenstiehl, D.R., 2006. Hydroacoustic contributions to understanding the December 26th 2004 great Sumatra–Andaman Earthquake. Survey of Geophysics 27, 633-646.
    • Zhu, Lupei, and Donald V. Helmberger. “Advancement in source estimation techniques using broadband regional seismograms.” Bulletin of the Seismological Society of America 86.5 (1996): 1634-1641.

    Earthquake Report: Philippines!

    Posted on by
    Reply

    There was a deep focus earthquake in the Philippines today. This shaker was located near the city of Manila (I live in Manila. Manila, California). The hypocenter was quite deep (168 km) so (a) had lesser shaking due to the distance to the earthquake from Earth’s surface and (b) was not related to the subduction zone fault. Seismicity associated with the megathrust fault is typically less than 40 km or so. As the oceanic lithosphere dives into the upper mantle, there are lots of processes that can lead to earthquakes (e.g. internal deformation due to bending of the slab). Many are familiar with extensional earthquakes in this region (e.g. the 2001 Nisqually Earthquake in Washington, USA). But, today’s earthquake is compressional.
    Here is the USGS website for today’s M 6.2 earthquake.
    There was a series of earthquakes earlier this year and here is my earthquake report for those earthquakes.
    Here is a report from earthquake report dot com.

    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 also include USGS epicenters from 1917-2017 for magnitudes M ≥ 7.
    I also include the USGS moment tensor for today’s earthquake, as well as for the other historic earthquake in the region that had a similarly deep focus (the 1985.04.23 M 7.0 earthquake). The 1985 earthquake was an extensional earthquake and slightly deeper (188 km).

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

      I include some inset figures in the poster.

    • On the left side of the poster I include a small scale (upper panel) and a large scale (bottom panel) view of the regional tectonics (Zahirovic et al., 2014). Plate boundary fault symbology (and other features, like fracture zones) is shown in the legend. I place a blue star on the map in the general location of this earthquake epicenter.
    • In the upper right corner I include a map showing the seismicity and tectonic plate boundary faults for this region (Smoczyk et al., 2013). Earthquakes are plotted with color representing depth and diameter representing magnitude (see legend). To the left of the map I include the cross section C-C’ that shows the earthquake hypocenters.
    • In the lower right corner is another tectonic map (Fan et al., 2015) that shows a seismic tomographic cross section. Seismic Tomography is a way of using seismic waves (like X-Rays in a CT scan) to peer into Earth’s interior. Based upon how seismic waves travel at different velocities depending upon the type of material (e.g. oceanic crust or the asthenospheric mantle), we can deduce the location of subducted slabs inside the Earth. To the left of the map is a tomographic cross section with the subducting Sunda plate colored blue as it subducts to the east, forming the Manila Trench. This cross section F-F’ is 2 degrees north of today’s M 6.2 earthquake. Below the cross section is an illustration showing how the authors interpret the subducting slab to be configured beneath the Philippines.


    • Here is the poster from the earthquakes that happened earlier this year.


      •

    • Below is a series of figures from Wu et al., 2016). These figures reveal information about the subduction zone forming the Manila Trench west of the northern Philippines. I will include their original figure caption in blockquote beneath the figures.
    • Here is a map showing the regional topography/bathymetry, along with the relative plate motions (in the inset map, note the vector for scale).


      • (a) Present-day Philippine Sea and East Asian tectonic setting. Plate motion azimuths in Figure 1a are shown relative to stable Eurasia from the MORVEL model [deMets et al., 2010]. Paleomagnetic sample locations in Figure 3 are shown by the colored symbols. (b) Present-day differential velocities across the Philippine Sea plate boundaries calculated from MORVEL. The southeast Philippine Sea plate is strongly edge coupled to the Pacific plate through the Caroline Sea, which has Pacific-like velocities. HB, Huatung Basin; AP, Amami Plateau; DR, Daito Ridge; ODR, Oki-Daito Ridge; BR, Benham Rise; KPR, Kyushu-Palau ridge; MS, Molucca Sea minor plate; BH, Bird’s Head minor plate; Hal, Halmahera.

    • This map shows the magnetic anomalies for the crust in the western Pacific. These magnetic anomalies help us interpret the age of the crust and the history of plate motion. As new oceanic crust is formed at mid ocean ridges, when Earth’s magnetic polarity flips, anomalies from parallel to the oceanic ridges. There is an inset map showing the plate age in colors. (Wu et al., 2016)


      • (a) EMAG2 gridded magnetic anomalies for the Philippine Sea and East Asia [Maus et al., 2009]. Plate motion azimuths as in Figure 1. (b) Philippine Sea gridded seafloor spreading model used in this study (modified from Müller et al. [2008] and Seton et al. [2012]). WPB, West Philippine Basin; SB, Shikoku Basin; PVB, Parece Vela Basin; MT, Mariana Trough; DRP, Daito ridges province; PB, Palau Basin; L, Luzon; KPR, Kyushu-Palau ridge; HB, Huatung Basin.

    • Here is a tomographic cross section in the region just north of the 2017.08.11 M 6.2 earthquake. Note the downgoing “Eurasia slab” highlighted showing the subducting Sunda plate. The Sunda plate is part of Eurasia, but is oceanic crust. There is also a larger scale cross section for the Mariana Trench. (Wu et al., 2016)


      • (a) MITP08 tomographic cross section oriented along the mean 0 to 50Ma Pacific convergence direction showing the subvertical Pacific “slab wall” under the central Marianas. (b) Maximum E-W width of the Pacific slab wall anomaly under the northern and central Marianas calculated from a 0% dVp cutoff along three transects. (c) Pacific slab wall from Figure 15a shown undistorted within spherical Earth model. Three possible Pacific slab areas A to C (dashed colored lines) were picked from the tomographic section that were guided by dVp cutoffs of 0.2% to 0%, respectively. We measured unfolded slab lengths between 3041 km and 4447 km for areas A to C using cross-sectional area unfolding (for method, see Figure 8). Our unfolded slab lengths were corrected for PREM density-depth changes [Dziewonski and Anderson, 1981] and assumed an incoming 100 km thick Pacific slab. (d) Total Pacific slab subduction times for slab areas A to C was 48 ± 10 Ma, based on a comparison of unfolded Pacific slab lengths to the Pacific convergence rate at the central Marianas from Seton et al. [2012].

    • Here are some more tomographic cross sections in the region. Cross section E-E’ is south of the 2017.08.11 M 6.2 earthquake and section D-D’ is just to the north of this earthquake. Section D-D’ is probably most representative of the subduction zone near this M 6.2 earthquake. (Wu et al., 2016)


      • (a to f) Interpreted slabs and mantle structure under East Asia from MITP08 tomography vertical cross sections. The study area is dominated by subhorizontal, relatively lower amplitude detached slabs at 500 to 1100 km depths that we call the East Asian Sea slabs. Inset map shows section locations. Seismicity shown by red dots. AUS, Australian craton; Ayu, deep Ayu Trough slab; BMS, Bismarck Sea; MS, Molucca Sea slab; NH, New Hebrides slab; OR, Ordos block; Pac, Pacific slabs; PP, proto-Pacific slabs; PSCS, proto-South China Sea slabs; PSP, Philippine Sea plate; PT, Philippine Trench slab; Ryu, Philippine Sea Ryukyu slab; SCS, South China Sea and Eurasian slabs; Shk, Philippine Sea Shikoku slab; SMar, southern Marianas detached slab; SolE, Solomon east slab; SolW, Solomon west slab; SolS, Solomon south slab; SS, Solomon Sea plate; Sulu, Sulu Sea slab; Su, Sunda slabs.

    • This figure shows details about the megathrust that forms the Philippine Trench to the southeast of the 2017.08.11 M 6.2 earthquake. I include this figure to show that the Philippine Trench subduction zone is unlikely to be responsible for today’s M 6.2 earthquake. (Wu et al., 2016)


      • Slab constraints for the SW Philippine Sea and surrounding areas. (a) Philippine Trench slab, Molucca Sea slabs, and detached “deep Ayu Trough” midslab maps. (b) Three-dimensional oblique view from west showing projected seismicity within 50 km of the Philippine Trench and Molucca Sea west midslab surfaces. (c) Unfolded Philippine Trench and Molucca Sea slabs colored by their dVp midslab seismic velocities. (d and e) MITP08 vertical tomographic cross sections and Benioff zone seismicity (red spheres) showing the interpreted fast-slab anomalies. Section locations are shown in Figure 21a. Note that the unfolded Molucca Sea slab in Figure 21c was the minimumlength model. A longer unfolded Molucca Sea slab is possible based on possible deeper (>900 km) anomalies in Figure 21b and the slab buckling in Figure 21e. PSP, Philippine Sea plate; Phil, Philippines; MS, Molucca Sea.

    • Here is the money shot, showing the subducting Sunda plate (labeled “Eurasia slab”) beneath the northern Philippines. h/t to JD Dianala for pointing out this figure in a Twitversation online. This is a fantastic paper that discusses the tectonics of the Philippine Sea plate and surrounding regions. (Wu et al., 2016)


      • Eurasian slab constraints shown by (a) map of Benioff zone seismicity within 50 km of the midslab surface, (b) Eurasia midslab depth map. (c) Unfolded Eurasian slab colored by its intraslab seismic velocity dVp. The unfolded Eurasian slab has a 400 to 500 km E-W width and has an eastern edge that terminates near Ishigaki at the Ryukyu Islands. Similarly, the unfolded northern proto-South China Sea detached slabs shown by the purple dashed line also have a similar eastern limit. This suggests a linked origin, namely, that the South China Sea opened as a back-arc basin through subduction of the proto-South China Sea. (d) Three dimensional visualization of the subvertical Eurasian midslab surface between Taiwan and Palawan. Coastlines in white.

    • This is a compilation of their (Wu et al., 2016) plate tectonic reconstruction. This is not important to interpret today’s earthquake, but it is interesting to know an alternate view of how the modern plate boundary configuration formed. (Wu et al., 2016)


      • Preferred Philippine Sea plate reconstruction Model 1 showing its origin near the Manus mantle plume (yellow dot). Stippled areas show the unfolded slab constraints from this study. Purple polygons show oceanic plateaus from Ishizuka et al. [2013] and the UTIG University of Texas LIPs database [Coffin, 2011]. HB, Huatung Basin.

    • Here is another view of the megathrust at the Manila Trench. This figure shows how the slab is more shallowly dipping in the north and more steeply dipping in the south (where the 2017.08.11 M 6.2 earthquake happened).


    • Here is a figure from Wang and Bilek (2014) that shows what proportion of the fault is creeping vs. being seismogenically coupled. Typically, people present figures like this and display the coupling ratio (what % of the plate motion is contributing to elastic strain), but this paper is about creeping faults, so they plot the opposite. One may look at it like this: a 20% creeping ratio = a 80% coupling ratio.


      • Creeping state and seismicity of the Manila Trench. Megathrust creeping ratio was determined by Hsu et al. (2012) by inverting GPS data from shown sites (triangles); Galgana et al. (2007) reported almost 100% creeping based on similar data.

    • This is an interesting figure to compare to the last one. Particularly in panel a, which shows the convergence rate as constrained by Global Positioning Systems (GPS) observations (Hsu et al., 2012). Note how the southern margin has a lower convergence rate (where the megathrust is creeping more).


      • Plate boundary deformation and trench parallel gravity anomaly along the Manila subduction zone. (a) Black vectors are GPS station velocities in the Sunda fixed reference frame. Error ellipses indicate 95% confidence intervals of GPS velocities. Blue vectors are velocities corrected for fault locking effect on the Philippine Fault (PHF in (b)). The yellow–red color scale indicates plate convergence rate. Bathymetry is shown in grey scale. The inset shows the regional geography with a red box indicating the study area (b) The seismicity is in the time period between 1973 and 2010 from NEIC (the US Geological Survey National Earthquake Information Center). The moment magnitude is in the range between 4.6 and 7.7. Color scale indicates focal depth. (c) The shaded relief topography and estimated free-air TPGA on the Manila subduction zone (Sandwell and Smith, 2009). The color bar indicates the amplitude of TPGA values. The black barbed and dashed lines denote the Manila Trench and 50 km slab iso-depth, respectively. (d) The Bouguer TPGA at the Manila subduction zone.

    References:

    • Bock et al., 2003. Crustal motion in Indonesia from Global Positioning System measurements in JGR, v./ 108, no. B8, 2367, doi:10.1029/2001JB000324
    • Hall, R., 2011. Australia–SE Asia collision: plate tectonics and crustal flow in 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, 75–109.
    • Fan, J-k., Wu, S-g., Spence, G., 2015. Tomographic evidence for a slab tear induced by fossil ridge subduction at Manila Trench, South China Sea in International Geology Review, v. 57, p. 998-1013, DOI: 10.1080/00206814.2014.929054
    • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
    • Hsu, Y-J., Yu, S-B., Song, T-R.A., and Bacolcol, T., 2012. Plate coupling along the Manila subduction zone between Taiwan and northern Luzo in Journal of Asian Earth Sciences, v. 51, p. 98-108
    • McCaffrey, R., Silver, E.A., and Raitt, R.W., 1980. Crustal Structure of the Molucca Sea Collision Zone, Indonesia in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands-Geophysical Monograph 23, p. 161-177.
    • Noda, A., 2013. Strike-Slip Basin – Its Configuration and Sedimentary Facies in Mechanism of Sedimentary Basin Formation – Multidisciplinary Approach on Active Plate Margins http://www.intechopen.com/books/mechanism-of-sedimentarybasin-formation-multidisciplinary-approach-on-active-plate-margins http://dx.doi.org/10.5772/56593
    • Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013. Seismicity of the Earth 1900–2012 Philippine Sea plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, 1 sheet, scale 1:10,000,000.
    • Waltham et al., 2008. Basin formation by volcanic arc loading in GSA Special Papers 2008, v. 436, p. 11-26.
    • Wang, K., and Bilek, S.L., 2014. Invited review paper: Fault creep caused by subduction of rough seafloor relief in Tectonophysics, v. 610, p./ 1-24.
    • Wu, J., J. Suppe, R. Lu, and R. Kanda, 2016. Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods, J. Geophys. Res. Solid Earth, 121, doi:10.1002/2016JB012923.
    • Zahirovic et al., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014.