Earthquake Report: M 6.9 Sumatra

Posted on November 20, 2022 by Jay Patton

While I was travelling back from a USGS Powell Center Workshop on the recurrence of earthquakes along the Cascadia subduction zone, there was an earthquake (gempa) offshore of Sumatra.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000iqpn/executive

There was actually a foreshock (more than one): https://earthquake.usgs.gov/earthquakes/eventpage/us7000iq2d/executive
I need to run to catch the sunset and will complete the intro later tonight.

OK, sunset led to nap, led to bed.

The plate boundary offshore of Sumatra, Indonesia, is a convergent (moving together) plate boundary. Here, the Australia plate subducts northwards beneath the Sunda plate (part of the Eurasia plate) along a megathrust subduction zone fault. This subduction forms a deep sea trench, the Sunda trench.

This was a shallow event near the trench formed by the subduction here. The magnitude was a little small for generating a large tsunami. However, it was shallow, so the deformation reached the sea floor and generated tsunami recorded on several tide gages in the region.

These gages are operated by the Indonesian Geospatial Reference System, though there are some gages that are posted on the European Union World Sea Levels website.

The water surface elevation data was a little noisy on these tide gage plots, but two of them had sufficient signal to justify my interpretation that these are tsunami. My interpretations could be incorrect and I include two plots below.

Here are the tide gage data. I label the locations for these two gage sites on the interpretive poster.

Many are familiar with the Boxing Day Earthquake and Tsunami from December 2004. This is one of the most deadly events in modern history, almost a quarter million people perished (mostly from the tsunami).

These lives lost did lead to changes in how tsunami risk is managed worldwide. So, these lives lost were not lost in vain (though it would be better if they were not lost, we can all agree to that).

The southern Sumatra subduction zone has an excellent record of prehistoric and historic earthquakes. For example, there is a couplet where earthquake slips overlapped slightly, the 1797 and 1833 earthquakes.

Many think that this area is the next place a large tsunamigenic earthquake may occur. Below we can see the analysis from Chlieh et al. (2008) where they suggest that there is considerable tectonic strain accumulated since these 1797 and 1833 earthquakes. There have been several large earthquakes in this area but they may not have released this strain.

If we look at the Chlieh et al. (2008) study, we will notice that this M 6.9 earthquake happened in an area thought to be in an area that is not accumulating much tectonic strain. I post a figure showing this later in the report.

There are millions of people who live in the coastal lowlands of Padang who may have difficulty evacuating in time should an earthquake like the 2004 Sumatra-Andaman subduction zone earthquake were to occur in this area.

For those that live along the coast here, the ground shaking from the earthquake is their natural notification to evacuate to high ground. For those that live across the ocean, they will get warning notifications to help them learn to evacuate since they won’t have the ground shaking as a warning. This is what happened to many people in December 2004 along the east coast of India and along the coast of Sri Lanka.

Here are some of the larger historic earthquakes in this area, ordered by magnitude:

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 1922-2022 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.
A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

In the upper right corner is a map showing historic seismicity, fault lines, and the global strain rate map (red shows area of higher tectonic strain).
To the left of the strain map is a figure that shows historic earthquake rupture areas and a representation of how strongly the megathrust subduction fault is (Chlieh et al., 2008).
In the upper left corner are maps that show the seismic hazard and seismic risk for Indonesia. I spend more time explaining this below.
In the center top-left is a map that shows earthquake intensity using the Modified Mercalli Intensity (MMI) Scale.
In the lower left center is a low angle oblique view of a cut away of the Earth along the subduction zone in Sumatra, Indonesia from EOS.
Above the oblique view is a plot of the tide gage from Cocos, Island.
In the right center is a great figure from Philobosian et al. (2014) that shows the slip patches from the subduction zone earthquakes in this region.

Here is the map with 3 month’s seismicity plotted.

Other Report Pages

Some Relevant Discussion and Figures

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. Note that today’s earthquake happened at the edge of the 7.9 earthquake slip patch.

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.

Here is the Chlieh et al. (2008) figure with the 18 November 2022 M 6.9 earthquake plotted as a blue star.
Note how the M 6.9 happened in a region of low seismogenic coupling. Beware that this is also in an area without any geodetic (GPS/GNSS) nor paleogeodetic (coral microattol) observations (the sources of data for the coupling model).

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.

Here is a map from Jacob et a. (2014) that shows the structure of the eastern Indian Ocean. Figure text below.

Free-air gravity anomaly map derived from satellite altimetry [Sandwell and Smith, 2009] over the Wharton Basin area.

Here is the map from Jacobs et a. (2014). Figure text below.

Structure and age of the Wharton Basin deduced from free-air gravity anomaly [Sandwell and Smith, 2009; background colors] for the fracture zones (thin black longitudinal lines), and marine magnetic anomaly profiles (not shown) for the isochrons (thin black latitudinal lines). The plain colors represent the oceanic lithosphere created during normal geomagnetic polarity intervals (see legend for the ages of Chrons 20 to 34 according to the time scale of Gradstein et al. [2004]). Compartments separated by major fracture zones are labeled A to H. Grey areas: oceanic plateaus, thick black line: Sunda Trench subduction zone.

This is a fascinating figure from Jacob et al. (2014). This shows a reconstruction of the magntic anomalies for the oceanic crust as they are subducted beneath Eurasia.

Reconstitution of the subducted magnetic isochrons and fracture zones of the northern Wharton Basin using the finite rotation parameters deduced from our two- and three-plate reconstructions. (a) First the geometry is restored on the Earth surface, then (b) it is draped on the top of the subducting plate as derived from seismic tomography [Pesicek et al., 2010] shown by the thin dotted lines at intervals of 100 km (b). Colored dots: identified magnetic anomalies; colored triangles: rotated magnetic anomalies, solid lines; observed fracture zones and isochrons, dashed lines: uncertain or reconstructed fracture zones, dotted lines: reconstructed isochrons from rotated magnetic anomalies (two-plate and three-plate reconstructions), colored area: oceanic lithosphere created during normal geomagnetic polarity intervals (see legend for the ages; the colored areas without solid or dotted lines have been interpolated), grey areas: oceanic plateaus, thick line: Sunda Trench subduction zone.

Finally, these authors present what their reconstruction implicates about this plate boundary system.

The deviation of the Sunda Trench from a regular arc shape (dotted lines) off Sumatra is explained by the presence of the younger, hotter and therefore lighter lithosphere in compartments C–F, which resists subduction and form an indentor (solid line). The very young compartment G was probably part of this indentor before oceanic crust formed at slow spreading rate near the Wharton fossil spreading center approached subduction: The weaker rheology of outcropping or shallow serpentinite may have favored the restoration of the accretionary prism in this area. Further south, the deviation off Java is explained by the resistance of the thicker Roo Rise, an oceanic plateau entering the subduction.

Seismic Hazard and Seismic Risk

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

The GEM Seismic Hazard Map:

The Global Earthquake Model (GEM) Global Seismic Hazard Map (version 2018.1) depicts the geographic distribution of the Peak Ground Acceleration (PGA) with a 10% probability of being exceeded in 50 years, computed for reference rock conditions (shear wave velocity, VS30, of 760-800 m/s). The map was created by collating maps computed using national and regional probabilistic seismic hazard models developed by various institutions and projects, and by GEM Foundation scientists. The OpenQuake engine, an open-source seismic hazard and risk calculation software developed principally by the GEM Foundation, was used to calculate the hazard values. A smoothing methodology was applied to homogenise hazard values along the model borders. The map is based on a database of hazard models described using the OpenQuake engine data format (NRML). Due to possible model limitations, regions portrayed with low hazard may still experience potentially damaging earthquakes.
Here is a view of the GEM seismic hazard map for Indonesia.

The GEM Seismic Risk Map:

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

Here is a view of the GEM seismic risk map for Indonesia.

Tsunami Hazard

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

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

Indonesia | Sumatra

General Overview

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

Earthquake Reports

2022.11.18 M 6.9 Sumatra
2022.02.25 M 6.2 Sumatra
2020.05.06 M 6.8 Banda Sea
2019.08.02 M 6.9 Indonesia
2019.06.23 M 7.3 Banda Sea
2019.04.12 M 6.8 Sulawesi, Indonesia
2018.09.28 M 7.5 Sulawesi
2018.10.16 M 7.5 Sulawesi UPDATE #1
2018.08.19 M 6.9 Lombok, Indonesia
2018.08.05 M 6.9 Lombok, Indonesia
2018.07.28 M 6.4 Lombok, Indonesia
2017.12.15 M 6.5 Java
2017.08.31 M 6.3 Mentawai, Sumatra
2017.08.13 M 6.4 Bengkulu, Sumatra, Indonesia
2017.05.29 M 6.8 Sulawesi, Indonesia
2017.03.14 M 6.0 Sumatra
2017.03.01 M 5.5 Banda Sea
2016.10.19 M 6.6 Java
2016.03.02 M 7.8 Sumatra/Indian Ocean
2015.07.22 M 5.8 Andaman Sea
2015.11.08 M 6.4 Nicobar Isles
2012.04.11 M 8.6 Sumatra outer rise
2004.12.26 M 9.2 Andaman-Sumatra subduction zone

Social Media

EarthquakeReport for M6.9 #Gempa #Earthquake offshore of #Sumatra #Indonesia

Appears to be on the megathrust subduction zone fault

Read more about the regional tectonics herehttps://t.co/sjXP2RmtVu https://t.co/bglPVLQUDt pic.twitter.com/KSdUDVh9HD

— Jason "Jay" R. Patton (@patton_cascadia) November 18, 2022

#EarthquakeReport for M 6.9 #Gempa #Earthquake offshore of #Sumatra #Indonesia

appears to be a megathrust subduction zone fault earthquake

generated a small tsunami recorded on tide gages

read more here:https://t.co/KKizpqJuSa pic.twitter.com/W4gCCJ9bKY

— Jason "Jay" R. Patton (@patton_cascadia) November 20, 2022

#EarthquakeReport for M 6.9 #Gempa #Earthquake offshore of #Sumatra #Indonesia

probably slip along megathrust subduction zone where Chlieh modeled low seismogenic coupling https://t.co/nuGY5m9iGD

*in area absent of GPS/microatoll data

read more here:https://t.co/KKizpqsrQa pic.twitter.com/oZP5u7JgiK

— Jason "Jay" R. Patton (@patton_cascadia) November 20, 2022

#EarthquakeReport #TsunamiReport for M 6.9 #Gempa #Earthquake offshore of #Sumatra #Indonesia

Cocos Isle gage updated due to twitter peer review from @Harold_Tobin thanks!

also added Bintuhan record

interp poster and plots updated in report herehttps://t.co/KKizpqsrQa pic.twitter.com/TGgCZeA1MV

— Jason "Jay" R. Patton (@patton_cascadia) November 20, 2022

Effects of the magnitude 6.9 #earthquake off #Sumatra #Indonesia was felt in my apartment over 776 km away in #Singapore. Managed to record this lamp swaying. #gempa #seismology #sismo #terremoto #geology pic.twitter.com/b1CdcxrCLm

— GeoGeorge (@GeoGeorgeology) November 18, 2022

Preliminary M6.9 #Earthquake
ID: #rs2022wsherl
Southwest of Sumatra, Indonesia
2022-11-18 13:37 UTC@raspishake #QuakeView

– Learn more about us at https://t.co/ojzht2DDAL

– EVENT: https://t.co/WbAhjnStUl pic.twitter.com/W5SOXtMdjn

— Raspberry Shake Earthquake Channel (@raspishakEQ) November 18, 2022

I love this figure by Kyle Bradley – really highlights the Mentawai Seismic Gap, a region at high risk of a large tsunamigenic earthquake offshore Sumatra. https://t.co/DJ1MSuKGVa pic.twitter.com/j8A0LjReNO

— Dr. Judith Hubbard (@JudithGeology) March 18, 2022

No #tsunami threat to Australia from magnitude 6.8 #earthquake near Southwest of Sumatra, Indonesia. Latest advice at https://t.co/Tynv3Zygqi. pic.twitter.com/ISugUTXpVm

— Bureau of Meteorology, Australia (@BOM_au) November 18, 2022

#White #Bulletin #2

NO TSUNAMI THREAT!

An earthquake occurred in South Sumatra region with following preliminary parameters 👇🏼

There is no tsunami threat to SL at present & coastal areas of SL are declared safe.#Tsunami #NoThreat #SriLanka #LKA pic.twitter.com/CPQMp12dqD

— Department of Meteorology Sri Lanka (@SLMetDept) November 18, 2022

Earthquakes commonly occur near Sumatra, as the Indo-Australian Plate subducts under the Sunda Plate. The M6.9 earthquake occurred at a depth of 25 km, likely on the subduction interface.https://t.co/OM7bvJsmTO pic.twitter.com/NS3rGVd8hR

— EarthScope Consortium (@EarthScope_sci) November 18, 2022

Surface waves from a M6.9 earthquake near Bengkulu, Indonesia at 18/11/2022 13:37:09UTC, received in the UK approximately an hour after the earthquake on @BGS and @raspishake devices. The frequency of these waves is shown on a plot from the BGS Elmsett seismometer. @rdlarter pic.twitter.com/OQkXOm5K1o

— Mark Vanstone (@wmvanstone) November 19, 2022

Waves from the M6.9 earthquake southwest of Sumatra shown on a nearby station using Station Monitor. https://t.co/Tir0KZmCJF pic.twitter.com/H5AjBzaKRH

— EarthScope Consortium (@EarthScope_sci) November 18, 2022

Location and First-motion mechanism: Mwp6.8 #earthquake Southwest of Sumatra, Indonesia https://t.co/kCIw9Vypa6 https://t.co/xebYrDiQ5S pic.twitter.com/May21uExD6

— Anthony Lomax 😷🇪🇺🌍🇺🇦 (@ALomaxNet) November 18, 2022

Watch the waves from the M6.9 earthquake in Sumatra, Indonesia roll across seismic stations in North America. (THREAD 🧵) pic.twitter.com/JMolvkAy4b

— EarthScope Consortium (@EarthScope_sci) November 18, 2022

Global surface and body wave sections from the M6.9 earthquake southwest of Sumatra, Indonesiahttps://t.co/a0ciLbpC9x pic.twitter.com/T4HDlrwvjy

— EarthScope Consortium (@EarthScope_sci) November 18, 2022

Back projection for the M6.9 earthquake southwest of Sumatra, Indonesiahttps://t.co/SLKRaU3oUA pic.twitter.com/glLYRXLj7X

— EarthScope Consortium (@EarthScope_sci) November 18, 2022

Earthquakes commonly occur near Sumatra, as the Indo-Australian Plate subducts under the Sunda Plate. The M6.9 earthquake occurred at a depth of 25 km, likely on the subduction interface.https://t.co/OM7bvJsmTO pic.twitter.com/NS3rGVd8hR

— EarthScope Consortium (@EarthScope_sci) November 18, 2022

Mw=6.9, SOUTHWEST OF SUMATRA, INDONESIA (Depth: 19 km), 2022/11/18 13:37:06 UTC – Full details here: https://t.co/uLuj3Ztf0a pic.twitter.com/3V9bk1wFaq

— Earthquakes (@geoscope_ipgp) November 18, 2022

Preliminary M6.9 #Earthquake
ID: #rs2022wsherl
Southwest of Sumatra, Indonesia
2022-11-18 13:37 UTC@raspishake #QuakeView

– Learn more about us at https://t.co/ojzht2DDAL

– EVENT: https://t.co/WbAhjnStUl pic.twitter.com/W5SOXtMdjn

— Raspberry Shake Earthquake Channel (@raspishakEQ) November 18, 2022

References:

Basic & General References

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

Specific References

Andrade, V. and Rajendran, K., 2014. The April 2012 Indian Ocean earthquakes: Seismotectonic context and implications for their mechanisms in Tectonophysics, v. 617, p. 126-139, http://dx.doi.org/10.1016/j.tecto.2014.01.024
Heidarzadeh, M., Harada, T., Satake, K., Ishibe, T., Takagawa, T., 2017. Tsunamis from strike-slip earthquakes in the Wharton Basin, northeast Indian Ocean: March 2016 Mw7.8 event and its relationship with the April 2012 Mw 8.6 event in GJI, v. 2110, p. 1601-1612, doi: 10.1093/gji/ggx395
Jacob, J., J. Dyment, and V. Yatheesh, 2014. Revisiting the structure, age, and evolution of the Wharton Basin to better understand subduction under Indonesia, J. Geophys. Res. Solid Earth, 119, 169–190, doi:10.1002/2013JB010285.
Yadav, R.K., Kundu, B., Gahalaut, K., Catherine, J., Gahalaut, V.K., Ambikapathy, A., and Naidu, MZ.S., 2013. Coseismic offsets due to the 11 April 2012 Indian Ocean earthquakes (Mw 8.6 and 8.2) derived from GPS measurements in Geophysical Research Letters, v. 40, p. 3389-3393, doi:10.1002/grl.50601
Wiseman, K. and Bürgmann, R., 2012. Stress triggering of the great Indian Ocean strike-slip earthquakes in a diffuse plate boundary zone in Geophysical research Letters, v. 39, L22304, doi:10.1029/2012GL053954
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.

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Earthquake Report: M 7.6 Earthquake in Mexico

Posted on November 9, 2022 by Jay Patton

There was a magnitude M 7.6 earthquake in Mexico on 1 September 2022.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000i9bw/executive

I am catching up on some Earthquake Reports that I did not yet post since my website was being migrated to a more secure and reliable server (and more expensive).

The tectonics of coastal southwestern Mexico is dominated by the convergent plate boundary between the Cocos plate (to the southwest) and the North America plate (to the northeast). Here, the Cocos plate subducts below (goes underneath) the North America plate.

The fault between these plates is called a megathrust subduction zone fault and the plate boundary forms the Middle America trench.

This M 7.6 earthquake mechanism (the “moment tensor”) shows that this event was a compressional earthquake (reverse or thrust).

Based on it’s location, the event probably happened along the megathrust fault.

This earthquake even generated a tsunami recorded on tide gages in the region!

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 the USGS epicenter as a red circle. However, these ground failure models are based on the USGS epicenter/location.
On the center right is a map that shows the historic subduction zone earthquake history for the subduction zone offshore of Mexico (National Seismological Service of Mexico).
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 (Manea et al., 2013).
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.
On the left, below the tectonic setting 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 1.25 meters (~4 feet).
In the lower left is a series of plots from the tide gages in the region. The location of these gages are shown on the main map. The tsunami wave height (vertical distance between the peak of the wave and the trough of the wave) ranged from 0.6m to 1.7m.

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 this tectonic region (Manea et al., 2013).

Development of the Tepic–Zacoalco (TZ), Colima, and Chapala rifts. The TZ rift is formed by the Rivera slab rollback, enhanced by the toroidal flow around the slab edges. The Colima rift is probably related with the oblique convergence between Rivera and NAM plates at ~5 Ma.

Here are the plots from the tide gages operated in the region.
These tide gages are organized north to south, top to bottom.
The dark blue line is the tidal forecast. The medium blue line is the tide gage record. The light blue line is the tide gage record minus the tidal forecast (basically the tsunami plus any other influence (like atmospheric pressure influences, or storm surge, etc.).
The locations for these gages are labeled on the interpretive poster above.
The earthquake origin time is labeled in orange.
Time is presented in UTC.

Here is tectonic map from Franco et al. (2012).

Tectonic setting of the Caribbean Plate. Grey rectangle shows study area of Fig. 2. Faults are mostly from Feuillet et al. (2002). PMF, Polochic–Motagua faults; EF, Enriquillo Fault; TD, Trinidad Fault; GB, Guatemala Basin. Topography and bathymetry are from Shuttle Radar Topography Mission (Farr&Kobrick 2000) and Smith & Sandwell (1997), respectively. Plate velocities relative to Caribbean Plate are from Nuvel1 (DeMets et al. 1990) for Cocos Plate, DeMets et al. (2000) for North America Plate and Weber et al. (2001) for South America Plate.

These figures are from the USGS publication (Benz et al., 2011) that presents an educational poster about the historic seismicity and seismic hazard along the Middle America Trench.
First is a map showing earthquake depth as color (green depth > red). Seismicity cross section B-B’ is shown on the map. Today’s M=6.6 quake is nearest this section.

Here is a map from Benz et al. (2011) that shows the seismic hazard for this region.

Here are some figures from Manea et al. (2013). First are the map and low angle oblique view of the Cocos plate.

A. Geodynamic and tectonic setting alongMiddle America Subduction Zone. JB: Jalisco Block; Ch. Rift—Chapala rift; Co. rift—Colima rift; EGG—El Gordo Graben; EPR: East Pacific Rise; MCVA: Modern Chiapanecan Volcanic Arc; PMFS: Polochic–Motagua Fault System; CR—Cocos Ridge. Themain Quaternary volcanic centers of the TransMexican Volcanic Belt (TMVB) and the Central American Volcanic Arc (CAVA) are shown as blue and red dots, respectively. B. 3-D view of the Pacific, Rivera and Cocos plates’ bathymetrywith geometry of the subducted slab and contours of the depth to theWadati–Benioff zone (every 20 km). Grey arrows are vectors of the present plate convergence along theMAT. The red layer beneath the subducting plate represents the sub-slab asthenosphere.

Here is a map showing the spreading ridge features, along with the plate boundary faults (Mann, 2007). This is similar to the inset map in the interpretive poster.

Marine magnetic anomalies and fracture zones that constrain tectonic reconstructions such as those shown in Figure 4 (ages of anomalies are keyed to colors as explained in the legend; all anomalies shown are from University of Texas Institute for Geophysics PLATES [2000] database): (1) Boxed area in solid blue line is area of anomaly and fracture zone picks by Leroy et al. (2000) and Rosencrantz (1994); (2) boxed area in dashed purple line shows anomalies and fracture zones of Barckhausen et al. (2001) for the Cocos plate; (3) boxed area in dashed green line shows anomalies and fracture zones from Wilson and Hey (1995); and (4) boxed area in red shows anomalies and fracture zones from Wilson (1996). Onland outcrops in green are either the obducted Cretaceous Caribbean large igneous province, including the Siuna belt, or obducted ophiolites unrelated to the large igneous province (Motagua ophiolites). The magnetic anomalies and fracture zones record the Cenozoic relative motions of all divergent plate pairs infl uencing the Central American subduction zone (Caribbean, Nazca, Cocos, North America, and South America). When incorporated into a plate model, these anomalies and fracture zones provide important constraints on the age and thickness of subducted crust, incidence angle of subduction, and rate of subduction for the Central American region. MCSC—Mid-Cayman Spreading Center.

Here is the McCann et al. (1979) summary figure, showing the earthquake history of the region.

Rupture zones (ellipses) and epicenters (triangles and circles) of large shallow earthquakes (after KELLEHER et al., 1973) and bathymetry (CHASE et al., 1970) along the Middle America arc. Note that six gaps which have earthquake histories have not ruptured for 40 years or more. In contrast, the gap near the intersection of the Tehuantepec ridge has no known history of large shocks. Contours are in fathoms.

This is a more updated figure from Franco et al. (2005) showing the seismic gap.
Here is a map from Franco et al. (2015) that shows the rupture patches for historic earthquakes in this region.

The study area encompasses Guerrero and Oaxaca states of Mexico. Shaded ellipse-like areas annotated with the years are rupture areas of the most recent major thrust earthquakes (M≥6.5) in the Mexican subduction zone. Triangles show locations of permanent GPS stations. Small hexagons indicate campaign GPS sites. Arrows are the Cocos-North America convergence vectors from NUVEL-1A model (DeMets et al., 1994). Double head arrow shows the extent of the Guerrero seismic gap. Solid and dashed curves annotated with negative numbers show the depth in km down to the surface of subducting Cocos plate (modified from Pardo and Su´arez, 1995, using the plate interface configuration model for the Central Oaxaca from this study, the model for Guerrero from Kostoglodov et al. (1996), and the last seismological estimates in Chiapas by Bravo et al. (2004). MAT, Middle America trench.

Earthquake Triggered Landslides

There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:
FOS = Resisting Force / Driving Force
When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.

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

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

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

If we look at the map at the top of this report, we might imagine that because the areas close to the fault shake more strongly, there may be more landslides in those areas. This is probably true at first order, but the variation in material properties and water content also control where landslides might occur.
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.

Social Media:

#EarthquakeReport for the M 7.6 (likely) subduction zone #Earthquake in #Mexico on 19 Sept 2022

catching up on reports that happened after my website went down

generated 0.6-1.7m wave height #Tsunami
probably triggered landslides/induced liquefactionhttps://t.co/9zpN2ZlAhw pic.twitter.com/tMDb4mSwCw

— Jason "Jay" R. Patton (@patton_cascadia) November 9, 2022