Earthquake Report: M 7.0 Vanuatu

Early this morning (my time) I got a notification from the Pacific Tsunami Warning Center that there was no tsunami threat from an M 7.2 earthquake in the Vanuatu Islands.

Later, as I woke up I checked the USGS website to see that there was an M7.0 earthquake offshore of the Vanuatu Islands.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000j2yw/executive

Based on the depth of the hypocenter (the 3-D location of the earthquake) it appears that this M 7.0 ruptured a thrust fault within the Australia plate. Given the uncertainty of the location of the megathrust fault, it is possible that this actually was on the megathrust subduction zone fault (so is what we call an “interface” event). I don’t think that the USGS finite fault model is correct (it seems unlikely that this earthquake ruptured a fault within the Australia plate and slipped up into the upper plate). But I could be wrong (which is quite common).

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. (UPDATE: I could not resist spending a little time looking at updated papers from this region, so have included some figures below.)

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.

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

Some Supporting Information

  • Here is the USGS poster showing the seismicity for this region from 1900-2010 (Benz et al., 2011). Below I include the legend (not the correct scale; click on this link for the entire poster (65 MB pdf)). Note the cross section F-F’ which I plot on the poster above.


  • Here is the cross section F-F’ again, with the legend below.


  • Here are some figures from Bergeot et al., 2009.
  • This first figure shows the tectonic setting and the plate convergence rates (the rates, in mm/year, that the Australia plate is converging relative to the North Fiji Basin.

  • (a) Geodynamic setting of the VSZ, with block motions relative to the North Fiji Basin [from Calmant et al., 2003]. The Vanuatu arc is split into three blocks, with anticlockwise rotation (north), convergence (center), and clockwise rotation (south). Dashed line is the BATB; solid lines are the spreading ridge; bold line is the VSZ. Bathymetry data are from Calmant et al. [2002]. The black rectangle is the central part of the Vanuatu arc. White arrows are velocities (millimeters per year) with respect to the Australian plate (AP); black arrows are block motion with respect to the North Fiji Basin. Dotted line is the cross section of Figure 2b. (b) Schematic of the central part of the VSZ [from Lagabrielle et al., 2003]. The direction of this cross section is west to east, and it intersects the Santo and Maewo Islands (dotted line in Figure 2a). Abbreviations are as follows: IAB, Aoba Intra-arc Basin; BATB, back-arc thrust belt; NFB, North Fiji Basin.

  • This figure shows the horizontal motion rates (in mm/year) for the GPS sites in the region.

  • Horizontal interseismic GPS velocities for the VSZ in an Australia-fixed reference frame. The Australian motion is estimated as a rigid rotation from our GPS results with a least squares inversion. Abbreviations are as follows: WTP, West Torres Plateau; DER, D’Entrecasteaux Ridge. Lines are (1) BATB, (2) spreading ridge, (3) VSZ, (4) discontinuity supposed between TGOA and Epi island, and (5) transition zone.

  • This figure shows a map where they plot, in the next figure, a comparison between their modeled vertical velocities with the observed vertical velocities.

  • Transects and GPS stations used to assess the locked zone parameters in this study. Shaded triangles represent the A-A0 (TNMR, LVMP, LMBU, WLRN, SWBY, VMVS, NSUP, RNSR, and AMBR) transect GPS stations, and solid triangles represent the B-B0 transect GPS stations (LISB, TASM, AVNA, RATA, RATU, SANC, AOBA, PNCT, and MAWO). The bold lines represent the A-A0 and B-B0 transects. The white arrows show the convergence direction. Abbreviations are as follows: DER, D’Entrecasteaux Ridge; WTP, Wet Torres Plateau. The stars indicate the edge of the locked zone as deduced from the GPS velocity interpretation (Figure 12). Lines are (1) BATB and (2) VSZ.

  • This figure shows the comparison between their modeled velocities and the observed velocities.

  • (top) Vertical and (bottom) horizontal (perpendicular to the trench) velocity profiles for the GPS stations of the A-A0 (open circle) and B-B0 (filled circle) transects. Distances are given with respect to the trench. The bold curves represent the best fit of the locked zone and long-term convergence rate model (dip, 20; width, 50 km; slip, 54 mm a1) estimated from observed velocities. Lines 2 and 3 represent the effect of the width variation in the model (45 and 60 km, respectively). See Figure 11 for the transect location.

  • Here are the two figures from Cleveland et al. (2014).
  • Figure 1. I include the figure caption below as a blockquote.

  • (left) Seismicity of the northern Vanuatu subduction zone, displaying all USGS-NEIC earthquake hypocenters since 1973. The Australian plate subducts beneath the Pacific in nearly trench-orthogonal convergence along the Vanuatu subduction zone. The largest events are displayed with dotted outlines of the magnitude-scaled circle. Convergence rates are calculated using the MORVEL model for Australia Plate relative to Pacific Plate [DeMets et al., 2010]. (right) All GCMT moment tensor solutions and centroids for Mw ≥ 5 since 1976, scaled with moment. This region experiences abundant moderate and large earthquakes but lacks any events with Mw >8 since at least 1900.

  • Figure 17. I include the figure caption below as a blockquote.

  • One hundred day aftershock distributions of all earthquakes listed in the ISC catalog for the 1966 sequence and in the USGS-NEIC catalog for the 1980, 1997, 2009, and 2013 sequences in northern Vanuatu. The 1966 main shocks are plotted at locations listed by Tajima et al. [1990]. Events of the 1997 and 2009 sequences were relocated using the double difference method [Waldhauser and Ellsworth, 2000] for P wave first arrivals based on EDR picks. The event symbol areas are scaled relative to the earthquake magnitudes based on a method developed by Utsu and Seki [1954]. Hypocenters of most aftershock events occurred at <50 km depth.

  • Figure 17. I include the figure caption below as a blockquote.

  • (right) Space-time plot of shallow (≤ 70 km) seismicity M ≥ 5.0 in northern Vanuatu recorded in the NEIC catalog as a function of distance south of ~10°N, 165.25°E. (left) The location of the seismicity on a map rotated to orient the trench vertically.

  • Craig et al. (2014) evaluated the historic record of seismicity for subduction zones globally. In particular, the evaluated the relations between upper and lower plate stresses and earthquake types (cogent for the southern New Hebrides trench). Below is a figure from their paper for this part of the world. I include their figure caption below in blockquote.

  • Outer-rise seismicity along the New Hebrides arc. (a) Seismicity and focal mechanisms. Seismicity at the southern end of the arc is dominated by two major outer-rise normal faulting events, and MW 7.6 on 1995 May 16 and an MW 7.1 on 2004 January 3. Earthquakes are included from Chapple & Forsyth (1979); Chinn & Isacks (1983); Liu & McNally (1993). (b) Time versus latitude plot.

  • Here is a summary figure from Craig et al. (2014) that shows different stress configurations possibly existing along subduction zones.

  • Schematic diagram for the factors influencing the depth of the transition from horizontal extension to horizontal compression beneath the outer rise. Slab pull, the interaction of the descending slab with the 660 km discontinuity (or increasing drag from the surround mantle), and variations in the interface stress influence both the bending moment and the in-plane stress. Increases in the angle of slab dip increases the dominance of the bending moment relative to the in-plane stress, and hence moves the depth of transition towards the middle of the mechanical plate from either an shallower or a deeper position. A decrease in slab dip enhances the influence of the in-plane stress, and hence moves the transition further from the middle of the mechanical plate, either deeper for an extensional in-plane stress, or shallower for a compressional in-plane stress. Increased plate age of the incoming plate leads to increases in the magnitude of ridge push and intraplate thermal contraction, increasing the in-plane compressional stress in the plate prior to bending. Dynamic topography of the oceanic plate seawards of the trench can result in either in-plane extension or compression prior to the application of the bending stresses.

  • Here are the figures from Richards et al. (2011) with their figure captions below in blockquote.
  • Here is the map showing the current configuration of the slabs in the region.

  • Map showing distribution of slab segments beneath the Tonga-Vanuatu region. West-dipping Pacifi c slab is shown in gray; northeast-dipping Australian slab is shown in red. Three detached segments of Australian slab lie below the North Fiji Basin (NFB). HFZ—Hunter Fracture Zone. Contour interval is 100 km. Detached segments of Australian plate form sub-horizontal sheets located at ~600 km depth. White dashed line shows outline of the subducted slab fragments when reconstructed from 660 km depth to the surface. When all subducted components are brought to the surface, the geometry closely approximates that of the North Fiji Basin.

  • This is the cross section showing the megathrust fault configuration based on seismic tomography and seismicity.

  • Previous interpretation of combined P-wave tomography and seismicity from van der Hilst (1995). Earthquake hypocenters are shown in blue. The previous interpretation of slab structure is contained within the black dashed lines. Solid red lines mark the surface of the Pacifi c slab (1), the still attached subducting Australian slab (2a), and the detached segment of the Australian plate (2b). UM—upper mantle;
    TZ—transition zone; LM—lower mantle.

  • Here is their time step interpretation of the slabs that resulted in the second figure above.

  • Simplifi ed plate tectonic reconstruction showing the progressive geometric evolution of the Vanuatu and Tonga subduction systems in plan view and in cross section. Initiation of the Vanuatu subduction system begins by 10 Ma. Initial detachment of the basal part of the Australian slab begins at ca. 5–4 Ma and then sinking and collision between the detached segment and the Pacifi c slab occur by 3–4 Ma. Initial opening of the Lau backarc also occurred at this time. Between 3 Ma and the present, both slabs have been sinking progressively to their current position. VT—Vitiaz trench; dER—d’Entrecasteaux Ridge.

  • Here are two great figures from Deng et al. (2022). This article focuses on the influence of the D’Entrecasteaux Ridge on subduction here in Vanuatu. They focus on geochemical data from magmatic rocks in the area.
  • This one shows a variety of processes going on in this area.

  • Geological and geophysical constraints regarding the evolution of the Vanuatu arc. (a) Bathymetric map showing the locations of islands for which samples were included in our geochemical compilation. Slab dip contours below the Vanuatu arc are displayed every 20° (from Hayes et al., 2018). (b) Bathymetric map of the Vanuatu arc and an inset showing depth-to-slab versus distance-from-trench for each of the sample localities included in our compilation (Table S1 in Supporting Information S1). Slab depth contours beneath the Vanuatu arc are displayed every 20 km (from Hayes et al., 2018). The orange lines show the chosen cross sections (i.e., Sections A, B, C) across the different blocks of the Vanuatu arc, which were used to estimate slab dips. Orange dots denote the location of Deep Sea Drilling Project Site 286 and Ocean Drilling Program Legs 134 Sites 828 and 831. (c) Interpreted geodynamic setting of the Vanuatu arc based on modern global positioning system velocity measurements (observed, black arrows; modeled, white arrows; from Bergeot et al., 2009). The Vanuatu arc can be divided into three tectonic blocks that are separated by two strike-slip faults (magenta dashed lines; Calmant et al., 2003; Taylor et al., 1995), which are the counterclockwise rotated Northern Block, the eastward migrated Central Block and the clockwise rotated Southern Block. Orange arrows indicate plate convergence velocities (in mm/year) with respect to the Australian plate (Bergeot et al., 2009). (d) Intermediate-depth seismicity distribution (50–170 km) since 1972 with magnitudes in the range of 4–7, from USGS Earthquake Catalog (https://earthquake.usgs.gov/earthquakes/search/). The seismic gap is highlighted by a solid polygon. The wide red arrow depicts the influx of hot sub-slab mantle to the forearc mantle wedge through a slab tear.


  • Schematic of the Vanuatu subduction zone to illustrate the model proposed by this study. The conceptual model highlights the role that the subducting buoyant D’Entrecasteaux ridge plays in the dynamic evolution of the Vanuatu arc. The introduction of D’Entrecasteauz Ridge causes shallow subduction and the development of a slab tear south of the ridge and the segmentation of the Vanuatu arc into the Northern Block, Central Block and Southern Block. Shallow slab subduction beneath the Central Block results in (a) squeezing out of the asthenospheric mantle; (b) scraping off the bottom of the ancient continental lithospheric mantle beneath the forearc, which then migrates ahead of the advancing slab and forms a bulldozed keel underneath the main-arc and (c) transmitting compressional stresses in the over-riding plate, which inhibits the formation of backarc spreading and instead produces a backarc thrust belt. Additionally, the ingression of hot subslab mantle causes partial melting of the cold forearc mantle and produces magmatism anomalously close to the trench (i.e., the Efate, Nguna, and Pele volcanoes that are situated in the forearc).

  • Here is a video that shows a simulation from Deng et al. (2022).
    • Baillard et al. (2015) provide some insight into the geometry of the Australia plate slab. I am adjusting my hypothesis to be that this M 7.0 probably was on the megathrust, based on their work.
    • This first figure shows a map and some cross sections.
    • Note how cross section 4 is really close to the M 7.0. This geometry places the top of the slab below the M 7.0 hypocenter.

    • Geometry of the subduction interface and updip/downdip extents of the seismogenic zone. (a) Map view. The green contour is at 27 km depth and marks the intersection with the fore-arc Moho. The dashed contours present the updip and downdip extents of the seismogenic zone. The numbered lines showthe location of cross sections plotted to the right. NDR: north d’Entrecasteaux ridge; BS: Bougainville seamount. (b) Geometric cross sections of the subduction interface (depth as a function of distance from the subduction front). (c) Dip cross sections (dip angle as a function of distance from the subduction front).

    • This figure shows how they interpret the seismicity in this area.

    • Cross section of seismic activity through the center of our total catalog (only events with residuals <0.2 s are plotted). Three clusters of activity are observed: (1) around the subduction interface (green), (2) within the subducting plate beneath the subduction interface (red), and (3) at intermediate depths (blue). The dotted line is our interpretation of the subduction interface.

    • This figure shows some specific earthquakes they used in their analyses. Earthquake 1 (pink) is really close to the M 7.0. They interpret that event to be in the upper part of the Australia plate. At this point, I suggest that it is equivocal, about whether or not the M 7.0 was an interface event or a slab or crustal event.

    • Clusters and focal mechanisms in the local catalog. Simple focal mechanisms are illustrated in black, composite focal mechanisms in colors corresponding to the cluster events (circles). P axes indicated in red. (a) Map view. The boxes indicate the orientation and dimensions of the cross sections. (b) Cross section beneath Santo Island. (c) Cross section between Santo and Malekula Islands. The cross sections also show the picked subduction interface (thick black curve), the Australian Plate Moho (dotted line, assuming a 8 km thick crust), and the North Fiji Basin Moho (dotted line, assuming a 27 km thick fore-arc crust).

    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

  • Baillard, C., W. C. Crawford, V. Ballu, M. Régnier, B. Pelletier, and E. Garaebiti (2015), Seismicity and shallow slab geometry in the central Vanuatu subduction zone, J. Geophys. Res. Solid Earth,120,5606–5623, https://doi.org/10.1002/2014JB011853
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
  • Bergeot, N., M. N. Bouin, M. Diament, B. Pelletier, M. Re´gnier, S. Calmant, and V. Ballu (2009), Horizontal and vertical interseismic velocity fields in the Vanuatu subduction zone from GPS measurements: Evidence for a central Vanuatu locked zone, J. Geophys. Res., 114, B06405, https://doi.org/10.1029/2007JB005249
  • Cleveland, K.M., Ammon, C.J., and Lay, T., 2014. Large earthquake processes in the northern Vanuatu subduction zone in Journal of Geophysical Research: Solid Earth, v. 119, p. 8866-8883, doi:10.1002/2014JB011289.
  • Deng, C., Jenner, F. E., Wan, B., & Li, J.-L. (2022). The influence of ridge subduction on the geochemistry of Vanuatu arc magmas. Journal of Geophysical Research: Solid Earth, 127, e2021JB022833. https://doi.org/10.1029/2021JB022833
  • 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.
  • Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
  • Oceanic-Oceanic Subduction Zone Figure
    Music Reference (in 1900-2016 seismicity video)

  • Bumba Crossing Kevin MacLeod (incompetech.com) | Licensed under Creative Commons: By Attribution 3.0 License | http://creativecommons.org/licenses/by/3.0/

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Earthquake Report: M 6.4 Gorda plate

Initial Narrative

Well, it has been a very busy week. I had gotten back from the American Geophysical Union Fall Meeting in Chicago late Saturday night. I had one day to hang out with my cats before I was to head down to Santa Cruz to meet with the city there to discuss installing a tide gage. Santa Cruz lacks a gage yet receives large tsunami inundations.

So, I drove down and got there about 10pm Monday evening. I was up for an hour or two and went to sleep.

At shortly after 2:30am I got a text message about a M 6.4 earthquake near Ferndale. I immediately got up and texted my colleague Cynthia Pridmore. We are tasked to prepare Earthquake Quick Reports that we (California Geological Survey, CGS) provide to the California Governor’s Office of Emergency Services (Cal OES). These reports provide technical information that helps them provide resources to local first responders during times following natural hazards impacts.

https://earthquake.usgs.gov/earthquakes/eventpage/nc73821036/executive

These reports are reviewed by the head of the Seismic Hazards Program (Tim Dawson) and by the State Geologist prior to being provided to the leadership in our organization and parent organizations. Reports for larger earthquakes and tsunami sometimes end up on the Governor’s desk.

We got our report submitted within about 45 minutes and we prepared for a long couple of days. We at CGS met at 8am to discuss our field response activities.

CGS and the U.S. Geological Survey (USGS) work closely together to document field evidence from earthquakes and tsunami. Kate Thomas (CGS) and Luke Blair (USGS) have a database ready to go within about 15 minutes after an earthquake. This database is used on mobile devices to collect observational information that include photos and other information. We use the ESRI Field Maps app for this purpose.

We decided to send CGS staff from the Eureka office out to collect information. I was to drive back to Humboldt and then join the field teams the following day.

Something that also happens following significant or damaging earthquakes is the activation of the California Earthquake Clearinghouse. Pridmore (CGS) is the chair of the EQCH and works with our partners (USGS, EERI, etc.) to decide when to activate the EQCH.

Data from these CGS/USGS field observations, along with data from other field teams, are posted onto the EQCH page for this event. Here is where those data are made available for this M 6.4 Ferndale Earthquake. The dataset of field observations are posted on that page are found by clicking on the “Resources” tab, also linked here.

When I returned to my home, the power was still out. We (CGS) had a scheduled meeting at 6pm and the EQCH meeting at 7pm. So, I went to the Eureka National Weather Service (NWS) Office on Woodley Island. They have electric power backup and satellite internet access. I work closely with the NWS and Cal OES and have been granted access to set up my workstation there during natural hazard emergencies like earthquake and tsunami. This was we can all better coordinate our actions without the burden of having power or internet outages at our residences. We are thankful for these relationships between CGS, the NWS (Ryan Aylward, Troy Nicolini) and Cal OES Eureka (Todd Becker).

So, I got up very early to work with my co-workers to continue the field investigations. There was little geological evidence from the earthquake. We identified some landslides and cracks in road fill. We did not locate any evidence for liquefaction, even though the USGS liquefaction susceptibility data suggested a high chance for that phenomena.

The Earthquake Report

This earthquake is in a tectonically complicated region of the western United States, the Mendocino triple junction. Here, three plate boundary fault systems meet (the definition of a triple junction): the San Andreas fault from the south, the Cascadia subduction zone from the north, and the Mendocino fault from the west. These plate boundary fault systems all overlap like fingers do when we fold our hands together.

The Cascadia subduction zone is a convergent (moving together) plate boundary where the Gorda and Juan de Fuca plates dive into the Earth beneath the North America plate. The fault formed here is called the megathrust subduction zone fault. Earthquakes on subduction zone faults generate the largest magnitude earthquakes of all fault types and also generate tsunami that can impact the local area and also travel across the ocean to impact places elsewhere. The most recent known Cascadia megathrust subduction zone fault earthquake was in January 1700.

The San Andreas and Mendocino fault systems are strike-slip (plates move side by side) fault systems. Many are familiar with the 1906 San Francisco Earthquake.

While the largest source of annual seismicity are intraplate Gorda plate earthquakes, the two largest contributors to seismic hazards in California are the Cascadia subduction zone (CSZ) and the San Andreas fault (SAF) systems. These sources overlap in the region of the Mendocino triple junction (MTJ) and may interact in ways we are only beginning to understand as evidenced by the 2016 M7.8 Kaikōura earthquake in New Zealand (Clark et al., 2017 Litchfield et al., 2018), which occurred along a similar subduction/transform boundary, and included co-seismic rupture of more than 20 faults.

The M 6.4 earthquake was a strike-slip earthquake within the downgoing Gorda plate (an intra plate earthquake). The earthquake started offshore and then the fault slipped to the east.

There is modest evidence that this earthquake generated focused seismic waves in the direction of fault slip (this is called directivity). In addition, the area of the lower Eel River Valley is a sedimentary basin. Sedimentary basins are known for amplifying ground shaking and trapping seismic waves, further increasing the ground shaking. The lower Eel River Valley is formed by tectonic folding caused by the northward migration of the Mendocino triple junction (read my contributions in the 2022 Pacific Cell Friends of the Pleistocene guidebook for more information about the structure of the Eel River and Van Duzen River valleys and surrounding regions.

So the seismic waves could have been trapped in the sedimentary basin formed within the Eel River Valley. However, there is an even older sedimentary basin here in which the Eel/Van Duzen river sediments are deposited within. These older sedimentary rocks have different seismic velocity properties that could also affect how seismic waves are transmitted here. There is a terrane bounding fault that separates these older rocks (Cretaceous Franciscan Formation) to the south from the younger rocks (Quaternary-Tertiary Wildcat Group) to the north.

Also, any of the large crustal fault systems (e.g., the Russ fault, the Little Salmon or Table Bluff faults, etc.) could guide seismic waves (a.k.a. act as wave guides), directing them in orientations relative to the fault systems.

My leading hypothesis is that the younger (latest Pleistocene to Holocene) river sediments that form the younger sedimentary basin and the crustal faults are both responsible for modifying the seismic wave transmission from this earthquake.

One thing people alkmost always ask si about whether or not ther eis a higher chance that there wil eb a Cascadia subduciton zone earthquake. This is currently impossible to tell. However, we can make some estimates of how forces within the Earth might have changed after a given earthquake. There was a Gorda plate earthquake sequence in 2018 that allowed us to consider these changes in the crust to see if the megathrust was brought more close to rupture. Here is the report from that Gorda plate earthquake sequence.

I will update this report further in the future, as we collect additional information.

One last thing for now. Bob McPherson formed a research group that we call Team Gorda. Team Gorda, supported by Connie Stewart at Cal Poly Humboldt, is using recently constructed fiber cables as a seismic instrument (called distributed acoustic seismic, DAS) to learn more about the underlying tectonic structures in the region. This fiber cable acts as thousands of little seismometers. Jeff McGuire and his team just installed the interrogator in our office at the Arcata City Hall. Horst from the Berkeley Seismic Lab is also working with Bob to install seismometers along the fiber cable so that we can calibrate the DAS observations.

We ran our first DAS experiment earlier this year and plan on doing more experiments far into the future, including fiber cables that are installed from here into the Pacific Ocean (on their way to Asia).

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 ≥ 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. 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 western US and a century of seismicity.
  • In the upper right corner is a map that displays a variety of earthquake intensity information. I plot the USGS modeled intensity, the USGS Did You Feel It? observations of intensity, and the shaking magnitude using the Peak Ground Acceleration scale in units of g (gravitational acceleration). I describe this map later in the report.
  • To the left of the intensity map are two maps that show the probability (the chance of) earthquake triggered landslides and the susceptibility (the chance of) earthquake induced liquefaction. I will discuss these ground failure models later in the report.
  • In the lower right corner I include a plot of aftershocks from a three day period.
  • Here is the map with 3 month’s seismicity plotted.

  • Here is an updated interpretive poster with 3 day’s seismicity plotted. I describe how this poster is different
  • In the lower right corner is a map from the USGS. This map shows where they interpret the location for the causative fault for this earthquake. There are also arrows (vectors) that show how Global Navigation Satellite System (GNSS, used to be called GPS) sites moved during the earthquake and how the moved using a computer simulation of the Earth that incorporate a fault that slipped like shown on the map. These arrows show the direction of motion and the amount of motion.
  • To the left of this map is the USGS finite fault model for this earthquake. The colors represent the amount that the fault slipped during the earthquake. This is the fault model that they used to estimate how the GNSS sites moved in the map to the right.
  • In the upper right corner is a map that shows the seismicty from the past week (in orange) and seismicity associated with the earthquake sequence from exactly one year before (in blue).
  • In the main part of the map I plot the earthquake mechanisms from the past century.

    Seismicity Profile

  • I felt the M 4.1 earthquake this morning (24 Dec 2022). It was an extensional earthquake in the eastern part of the aftershock region.
  • Today I plotted the seismicity along an east-west profile.
  • I traced the Gorda plate geometry from Guo et al. (2018). This is from their profile B-B’ which is just about at 41 degrees north.
  • We can see that the mainshock (the M 6.4) and most of the aftershocks are within the Gorda plate.

  • Here is an updated plot that includes the USGS Finite Fault Model as a transparent overlay.
  • Note how most of the slip is in the North America plate.

  • Here is an updated plot that displays M 6.4 in blue and M 5.4 in green.

  • And if someone wants to learn more about what a hypocenter is, here you go >>>

    Aftershock Patterns

  • Yesterday I got to feel one of the aftershocks, an M 4.2 to the southeast of the main sequence.
  • Today I plotted all the aftershocks to date as of this morning. It appears that there were two main faults involved. One about 45 km long and another one about 25 km long.
  • I include earthquake mechanisms for all events that I could download today. I placed some mechanisms that may not be related to these 2 faults at 50% transparency.
  • This poster below includes a map (lower right corner) of the Cascadia subduction zone and the cross section showing how the crust deforms between (interseismic) and during (coseismic) earthquakes.
  • I also include a schematic showing where earthquakes might happen (upper left center). Earthquakes along the megathrust subduction zone fault are called interplate earthquakes (like the interstate highways connect between states).
  • Earthquakes within the Gorda or North America plates are called intraplate earthquakes. The M 6.4 was an intraplate earthquake within the Gorda plate. I don’t really have a good way to show intraplate strike-slip faults in this diagram (room for future work!).
  • In the upper right corner is the seismicity profile that I also show above in the report. When comparing the seismicity with the Guo et al. (2021) slab model, it appears that most of the earthquakes are within the Gorda crust. There are some above, possibly in the North America crust.

  • Here is another updated map, updated on 2 January 2023 to include the M 5.4 related earthquakes.
  • Now it appears that there are three main faults involved, at least.

  • Yesterday I was chatting with Bob McPherson as we were looking at the USGS finite fault slip model. Bob suggested that this model shows that the earthquake slipped in the Gorda and North America plates. If the slip model is correct, then Bob is correct. This is quite interesting if true.
  • UPDATE comment (4 jan ’23): I have seen other slip models that do not place M 6.4 slip above the Gorda plate. We must remember that these slip models are non unique solutions and that there is quite a bit of wiggle room for their solutions. Basically, there are knobs to turn on these models (allowing one to change parameters, such as the material properties of the Earth (e.g., the “rheology” of the crust or mantle)) and changing these parameters can change the results while still keeping a good fit to the observational data. It is not uncommon that the slip models for both nodal planes (the two possible fault planes shown on earthquake mechanisms (focal mechanisms or moment tensors)) each fit the data equally well. I have seen the fault model that was fit to the incorrect (incorrect relative to aftershocks) fault plane being chosen as the preferred slip model. So, we must remember this when we are interpreting model results like these fault slip models.
  • First lets just look at the finite fault slip model. Below we see a plot with color representing how much the fault slipped. The white star is the M 6.4 hypocenter (the 3-D location of the M 6.4). East is to the left and west is to the left (pretend you are looking at the diagram from the north side of the fault).
  • There are gray lines that represent times (10 seconds and 20 seconds after the M 6.4 mainshock) where the rupture propagation front was. So, the fault started slipping at the white star. Then, the fault moved and the outer limit of this motion radiated outwards and was at the first gray line in 10 seconds and at the second gray line at 20 seconds.
  • There are small gray arrows that show the direction and magnitude of slip motion along the fault. If we combine this plot with our knowledge that this was a left-lateral strike-slip earthquake, and we are looking to the south at the fault, we can surmise that these vectors are on the north side of the fault. Also, that the fault slipped from east of the hypocenter towards the hypocenter, and updip (shallower).

  • Yes, this would be quite interesting, if the fault broke both Gorda and North America crust. This would make our interpretation of the Mendocino triple junction even more complicated. There are not currently any faults mapped in the North America plate that align with this M 6.4 sequence.
  • It is possible that there are faults there, or that they may be blind (not reach the ground surface). If these faults are young, they may not have sufficient offset to produce deformation at the Earth’s surface.
  • We do have examples of this elsewhere, where there are crustal faults in the downgoing plate that are also in the same location but in the upper plate.
  • For example, Goldfinger et al. (1997) mapped a series of faults that cut across strike to the Cascadia subduction zone fault. Two, the Daisy Bank and Wecoma faults, are shown in their figure below. Note how these faults are mapped in the Juan de Fuca plate and propagate upwards into the accretionary prism (let’s call this the upper plate).

  • Block rotation model for the central Cascadia forearc. SeaBeam bathymetry shaded from the north. The Wecoma and Daisy Bank faults are show, with the Daisy Bank fault exposed in the foreground. Well-mapped fault traces are in solid; discontinuous traces are dashed. The arc-parallel component of oblique subduction creates a dextral share couple, which is accommodated by WNW trending left-lateral strike-slip faults. We propose that shearing of the slab due to oblique subduction is responsible for the fault involving oceanic crust. WF, Wecoma fault; DBF, Daisy bank fault; FF, Fulmar fault, “pr,” pressure ridge; “DB,” Daisy Bank; “OT?,” possible old left-lateral fault strand. Arrow heads and tails show strike-slip motion. White arrows at western end of Wecoma fault show eastward increasing slip calculated from isopach offsets.

  • Another place where I have seen this is offshore of Sumatra. When we were coring there for my Ph.D. research, we identified a strike-slip fault in the India Australia plate that propagated upwards into the accretionary prism (the “upper plate”).
  • One thing That this almost certainly requires is that the megathrust fault be seismogenically coupled in this area.
  • Basically, we need a mechanism by which, when the lower plate fault slips, that the forces are exerted to the upper plate to move in the same direction and manner as that observed in the lower plate. Having a coupled megathrust fault is one way to do this
  • And we have several examples of this in the southern CSZ. There are a number of strike-slip fault earthquakes within the Gorda plate (or along the Mendocino fault) offshore of the megathrust that generated differential motion for geodetic sites (like GNSS or GPS stations) during the earthquake.
  • Further down in the report I present the map from Dengler et al. (1994) that shows how geodetic sites in North America plate move in response to the 1994 Cape Mendocino fault right-lateral strike-slip earthquake.
  • The USGS pages for the GNSS network provide static offsets for the GNSS stations as observed for these Gorda plate earthquakes. Williams and McPherson (2006) present another example of this. Below we can see the coseismic displacements from the 2005 northeast striking left-lateral strike-slip fault earthquake.

  • Coseismic displacements from the 15-Jun-2005 M7.2 Gorda plate earthquake located (off the map) 156 km (97 miles) W (280°) from Trinidad, CA and 157 km (98 miles) WSW (251°) from Crescent City, CA. Note the similarity to the deformation pattern of the 1994 event. Continuously operating GPS stations shown here are operated and maintained through the Plate Boundary Observatory component (pboweb.unavco. org) of the National Science Foundation’s EarthScope project
    (www.earthscope.org).

  • Regardless of whether or not there is a throughgoing fault, it is clear that the megathrust fault is locked here. (either from the presence of a throughoing fault or from the static offsets at these GNSS sites.
  • Below is the USGS finite fault slip model and a comparison between the observed GNSS offsets and the offsets modeled by placing slip on the finite fault model in an elastic half space.
  • Once we have better INSAR data (presuming these data will exist), this slip model may improve.

  • Mapped Geology

  • Here is a map that shows the mapped geologic units. Some of the map is from McLaughlin et al. (2000) and some is from the California Division of Mines and Geology (CDMG, 1999) which is now the California Geological Survey.
  • There are about 30 units in each dataset, so I chose to simply use their labels from the respective databases. The CDMG labels are basically the same as the geologic unit (e.g., Franciscan is something like TKJf) while McLaughlin mapped units relative to their geomorphic expression, so units have strange labels (e.g., Franciscan is something like co1 or cm1).
  • Note that the seismicity trend from the M 6.4 does not align with the faults nor the geologic units in the North America plate. This makes the linkage between rupturing in the Gorda and the North America plates more tenuous (though still possible).

    Earlier Report Interpretive Posters

  • Here is the poster from last year’s earthquake sequence.

  • Here are two relevant interpretive posters from the 1992 Cape Mendocino Earthquake.
  • This one is an overview of the earthquake.

  • This one helps us compare the mainshock and two main triggered earthquakes.

  • Here is a poster that shows a comparison between the 1991 Honeydew and 1992 Cape Mendocino earthquakes..

    Some Relevant Discussion and Figures

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

    • Here is a version of the CSZ cross section alone (Plafker, 1972). This shows two parts of the earthquake cycle: the interseismic part (between earthquakes) and the coseismic part (during earthquakes). Regions that experience uplift during the interseismic period tend to experience subsidence during the coseismic period.

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

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

    The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. In my mind, these two aftershocks aligned on what may be the eastern extension of the Mendocino fault. However, looking at their locations, my mind was incorrect. These two earthquakes were not aftershocks, but were either left-lateral or right-lateral strike-slip Gorda plate earthquakes triggered by the M 7.1 thrust event.

    These two quakes appear to be aligned with the two northwest trends in seismicity and the 18 March 2020 M 5.2. The orientation of the mechanisms are not as perfectly well aligned, but there are lots of reasons for this (perhaps the faults were formed in a slightly different orientation, but have rotated slightly).

    There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.

    • This is the map used in the animation below. Earthquake epicenters are plotted (some with USGS moment tensors) for this region from 1917-2017 with M ≥ 6.5. I labeled the plates and shaded their general location in different colors.
    • I include some inset maps.
      • In the upper right corner is a map of the Cascadia subduction zone (Chaytor et al., 2004; Nelson et al., 2004).
      • In the upper left corner is a map from Rollins and Stein (2010). They plot epicenters and fault lines involved in earthquakes between 1976 and 2010.


    • Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).

    • Tectonic configuration of the Gorda deformation zone and locations and source models for 1976–2010 M ≥ 5.9 earthquakes. Letters designate chronological order of earthquakes (Table 1 and Appendix A). Plate motion vectors relative to the Pacific Plate (gray arrows in main diagram) are from Wilson [1989], with Cande and Kent’s [1995] timescale correction.

    • Here is a large scale map of the 1994 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles.

    • Here is a plot of focal mechanisms from the Dengler et al. (1995) paper in California Geology.

  • In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.

  • Here is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. The moment tensor (at the moment i write this) shows a north-south striking fault with a reverse or thrust faulting mechanism. While this region of faulting is dominated by strike slip faults (and most all prior earthquake moment tensors showed strike slip earthquakes), when strike slip faults bend, they can create compression (transpression) and extension (transtension). This transpressive or transtentional deformation may produce thrust/reverse earthquakes or normal fault earthquakes, respectively. The transverse ranges north of Los Angeles are an example of uplift/transpression due to the bend in the San Andreas fault in that region.

  • A: Mapped faults and fault-related ridges within Gorda plate based on basement structure and surface morphology, overlain on bathymetric contours (gray lines—250 m interval). Approximate boundaries of three structural segments are also shown. Black arrows indicated approximate location of possible northwest- trending large-scale folds. B, C: uninterpreted and interpreted enlargements of center of plate showing location of interpreted second-generation strike-slip faults and features that they appear to offset. OSC—overlapping spreading center.

  • These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.
  • Mw = 5 Trinidad Chaytor

    Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.

Mendocino triple junction video

Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
  • In the upper panel is the USGS Did You Feel It reports map, showing reports as colored dots using the MMI color scale. Underlain on this map are colored areas showing the USGS modeled estimate for shaking intensity (MMI scale).
  • I also plot, in colored squares, the ground motions recorded on seismometers operated by the CGS Strong Motion Instrument Program (SMIP), run by Hamid Haddadi. Units are relative to gravitation acceleration where 1 = 1g. g is defined as the acceleration at the Earth’s surface (9.8 m/s^2). Here is the data page for this M 6.4 earthquake. The largest acceleration (1.36g) is from a seismometer attached to a bridge and seismologists think that this large acceleration is due to the bridge in some way. Here is the SMIP data page for the M 5.4 earthquake.
  • Below the upper map plot is the USGS MMI Intensity scale, which lists the level of damage for each level of intensity, along with approximate measures of how strongly the ground shakes at these intensities, showing levels in acceleration (Peak Ground Acceleration, PGA) and velocity (Peak Ground Velocity, PGV).
  • In the lower panel is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orange and purple dots. Note how well (or poorly) the reports fit the brown line (the model that represents how MMI works based on quakes in California). The increased intensity on the left of the plot (which are closer to the earthquake) are the records that show intensities higher than expected from the modeling.

  • Here is an animation from the USGS and Cal Tech that shows a simulation of seismic waves from this M 6.4 earthquake.
  • There is a link to this video from the earthquake page.

Shaking Intensity and Potential for Ground Failure

  • Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.
  • There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

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


    Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.

    Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.

    An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.

    Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


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


  • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a low probability for landslides. However, we have already seen photographic evidence for landslides and the lower limit for earthquake triggered landslides is magnitude M 5.5 (from Keefer 1984)
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.

Seismic Hazard and Seismic Risk

  • These are two maps from the Global Earthquake Model (GEM) project, 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 the USA.

  • 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 the USA. Note how the seismic risk is higher in places of larger population (like Los Angeles and San Francisco).

Stress Triggering

  • When an earthquake fault slips, the crust surrounding the fault squishes and expands, deforming elastically (like in one’s underwear). These changes in shape of the crust cause earthquake fault stresses to change. These changes in stress can either increase or decrease the chance of another earthquake.
  • I wrote more about this type of earthquake triggering for Temblor here. Head over there to learn more about “static coulomb stress triggering.”
  • Rollins and Stein (2010) conducted this type of analysis for the 2010 M 6.5 Gorda Earthquake. They found that some of the faults in the region experienced an increase in fault stress (the red areas on the figure below). These changes in stress are very small, so require a fault to be at the “tipping point” for these changes in stress to cause an earthquake.
  • There was a triggered earthquake in this sequence. There was a M 5.9 event about 25 days after the mainshock, this earthquake happened in a region that saw increased stress after the M 6.5. The M 5.9 appears to have been on the same fault as the M 6.5
  • First, here is the fault model that Rollins and Stein used in their analysis of stress changes from the 2010 earthquake.

  • Source models for earthquakes S and T, 10 January 2010, M = 6.5, and 4 February 2010, Mw = 5.9.

  • Let’s take a look at some examples of analogic earthquakes to the 2010 temblor. First, here is a plot showing changes in stress following the 1980 Trinidad Earthquake (a very damaging earthquake in the region). This is the largest historic earthquake in the region at magnitude M 7.3 (other than the 1906 San Francisco Earthquake).

  • Coulomb stress changes imparted by the 1980 Mw = 7.3 earthquake (B) to a matrix of faults representing the Mendocino Fault Zone, the Cascadia subduction zone, and NE striking left‐lateral faults in the Gorda zone. The Mendocino Fault Zone is represented by right‐lateral faults whose strike rotates from 285° in the east to 270° in the west; Cascadia is represented by reverse faults striking 350° and dipping 9°; faults in the Gorda zone are represented by vertical left‐lateral faults striking 45°. The boundary between the left‐lateral “zone” and the reverse “zone” in the fault matrix is placed at the 6 km depth contour on Cascadia, approximated by extending the top edge of the Oppenheimer et al.
    [1993] model for the 1992 Cape Mendocino earthquake (J). Calculation depth is 5 km. The numbered brackets are groups of aftershocks from Hill et al. [1990].

  • Next let’s look at the stress changes following the 2005 M 7.2 earthquake.

  • Coulomb stress changes imparted by the Shao and Ji (2005) variable slip model for the 15 June 2005 Mw = 7.2 earthquake (P) to the epicenter of the 17 June 2005 Mw = 6.6 earthquake (Q). Calculation depth is 10 km.

  • Here is the figure we have all been waiting for (actually, the next one is cool too). This figure shows the changes in stress associated with the 2010 M 6.5 earthquake. Remember, these are just models.

  • Coulomb stress changes imparted by the D. Dreger (unpublished report, 2010, [no longer] available at http://seismo.berkeley.edu/∼dreger/jan10210_ff_summary.pdf) model for the January 2010 M = 6.5 shock (S) to nearby faults. East of the dashed line, stress changes are resolved on the Cascadia subduction zone, represented by a northward extension of the Oppenheimer et al. [1993] rupture plane for the 1992 Mw = 6.9 Cape Mendocino earthquake. West of the dashed line, stress changes are resolved on the NW striking nodal plane for the February 2010 Mw = 5.9 earthquake (T) at a depth of 23.6 km.

  • This is the main take-away figure from Rollins and Stein (2010). For each map, there is a source fault (in black) and receiver faults (red or blue, depending on the change in stress).
  • For example, in a, the source is a gorda plate left-lateral strike-slip fault. Parts of the Cascadia megathrust are represented on the right (triangles, labeled thrust). They also model changes in stress on the Mendocino fault (the red and blue lines at the bottom of “a”).

  • And, you thought it couldn’t get any better. Here is yet another fantastic figure showing the stress change on the Cascadia megathrust fault and on the Mendocino fault following the 2010 M 6.5 earthquake.

    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

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Guo, H., McGuire, J., and Zhang, H., 2021. Correlation of porosity variations and rheological transitions on the southern Cascadia megathrust in Nature Geoscience, https://doi.org/10.1038/s41561-021-00740-1
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
  • McLaughlin, R.J., Ellen, S.D., Blake, M.C. Jr., Jayko, A.S., Irwin, W.P., Aalto, F.R., Carver, G.A., and Clarke, S.H. Jr., 2000. Geology of the Cape Mendocino, Eureka, Garberville, and Southwestern Part of the Hayfork 30 x 60 Minute Quadrangles and Adjacent Offshore Area, Northern California, USGS Miscellaneous Field Studies Map MF-2336, http://pubs.usgs.gov/mf/2000/2336/
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].
  • Wells, D.L., and Coopersmith, K.J., 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement in BSSA, v. 84, no. 4, p. 974-1002
  • Wells, R.E., Blakely, R.J., Wech, A.G., McCrory, P.A., Michael, A., 2017. Cascadia subduction tremor muted by crustal faults in Geology, v. 45, no. 6, p. 515–518, https://doi.org/10.1130/G38835.1
  • Williams, T.B. and McPherson, R.C., (2006). Gorda Plate Deformation Contributes to Shortening Between the Klamath Block and the On-land Portion of the Accretionary Prism to the S. Cascadia Subduction Zone. In Hemphill-Haley, M., McPherson, R., Patton, J. R., Stallman, J., Leroy, T.H., Sutherland, D., and Williams, T.B., eds. (2006) Pacific Cell Friends of the Pleistocene Field Trip Guidebook, The Triangle of Doom: Signatures of Quaternary Crustal Deformation in the Mendocino Deformation Zone (MDZ) Arcata, CA.

Return to the Earthquake Reports page.

Earthquake Report: M 6.9 Sumatra

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.

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).

    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 7.3 Tonga trench outer rise

Early this morning I received some notifications of earthquakes along the Tonga trench (southwestern central Pacific Ocean). It was about 2am my local time.

I work on the tsunami program for the California state tsunami program (CTP) and we respond to tsunami to (1) help local communities do their first response activities so that they can help reduce suffering and to (2) document the impact of these tsunami.

Because of this work, our team is “at the ready” 24 hours a day, 7 days a week, to respond to these events. Luckily, this event was unlikely to generate a tsunami that would impact California. I went back to sleep.

This morning I put together a report and checked to see if there was a tsunami generated. Here is one place that I check for tsunami records as observed on tide gages http://www.ioc-sealevelmonitoring.org/map.php. I did not see anything convincing.

This earthquake, from last night my time, has a magnitude of M 7.3.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000ip0l/executive

This area of the Earth has a plate boundary fault system called a subduction zone. A subduction zone is a convergent plate boundary, which means that the plates on either side of the boundary move towards each other.

Here, the Pacific plate dives westwards beneath the Australia plate, forming the Tonga trench. Below is a schematic illustration showing what these plates may look like if we cut into the Earth and viewed this subduction zone from the side. Note the Pacific plate on the right and the Australia plate on the left, with the megathrust subduction zone fault where they meet.

This illustration shows where earthquakes may happen along this plate boundary. There could be interface earthquakes along the megathrust fault (megathrust earthquakes). These are what most people are familiar with when they are thinking about tsunami (e.g., the 2011 Great East Japan Earthquake and Tsunami).

In the upper plate (the Australia plate), there can be crustal fault earthquakes. In the lower plate (the Pacific plate) there can be slab earthquakes (events within the crust, aka the slab), and there can be outer rise earthquakes).

The outer rise is a part of the plate that is warping up and down because of the forces adjacent to the subduction zone. This warping can cause extension in the upper part, and compression in the lower part, of this plate.

This 11 Nov 2022 M 7.3 earthquake was a compressional (reverse) earthquake in the outer rise region of this plate boundary. It was pretty deep (for oceanic crust) so fits nicely in the correct place in this illustration:


But megathrust earthquakes are not the only type of earthquake that can cause a tsunami. The 2009 magnitude M 8.1 extensional (normal) fault earthquake near Samoa and American Samoa caused a tsunami that inundated the nearby islands (causing lots of damage and human suffering). This tsunami also travelled across the Pacific Ocean to impact California! (This is why the California Tsunami Program monitors tsunami across the Pacific Basin, so that we can help reduce suffering through the evacuation of coastal areas. Remember, the entire coast of California is a Tsunami Hazard Area.)

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 ≥ 7.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

  • In the upper left corner is a map that shows the plates, their boundaries, and a century of seismicity.
  • 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 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).
  • Further to the left is a diagram that shows the different types of earthquakes that can occur along a subduction zone.
  • In the upper right corner is a map that shows some of the historic earthquakes in the region, with the earthquake mechanisms. I labeled these events for the type of event that I interpret them to be.
  • In the upper left center is a map from Richards et al. (2011) that shows earthquake locations (epicenters) with color representing depth. I place a yellow star in the general location of today’s M 7.3 earthquake. These colors help us visualize how the Pacific plate dips deeper towards the left (yellow are shallow events and purple are deep events). The 2000.01.08 M 7.2 earthquake is an intermediate depth earthquake (see the map in the upper right corner).
  • In the right center is a map from Timm et al (2013) that also shows the depth to the slab (the downgoing Pacific plate). I place a yellow star in the general location of today’s M 7.3 earthquake.
  • Here is the map with 3 month’s seismicity plotted.

  • Well, I just looked at Pago Pago and the record is clear. I am kinda surprised since this gage is on the nodal plane for this event. I will plot these data up.

  • Here is a screenshot:

  • Here are the two plots for the gages listed above. The Pago pago record is quite clear. However, the Nukualofa gage is pretty noisy. I don’t have much confidence in the measurements of the wave size.
  • Data from both gages show a background wave sequence that makes it difficult to know when the tsunami ends. Someone who can filter out that wave series could probably do a better job at locating when the tsunami ends, at least for the Pago Pago data.
  • Pago Pago (American Samoa) https://webcritech.jrc.ec.europa.eu/SeaLevelsDb/Device/959

  • Nukualofa (Tonga Island) https://webcritech.jrc.ec.europa.eu/SeaLevelsDb/Device/950

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the map from Timm et al., 2013.

  • Bathymetric map of the Tonga–Kermadec arc system. Map showing the depth of the subducted slab beneath the Tonga–Kermadec arc system. Louisville seamount ages are after Koppers et al.49 ELSC, eastern Lau-spreading centre; DSDP, Deep Sea Drilling Programme; NHT, Northern Havre Trough; OT, Osbourn Trough; VFR, Valu Fa Ridge. Arrows mark total convergence rates.

  • Here is the oblique view of the slab from Green (2003).

  • Earthquakes and subducted slabs beneath the Tonga–Fiji area. The subducting slab and detached slab are defined by the historic earthquakes in this region: the steeply dipping surface descending from the Tonga Trench marks the currently active subduction zone, and the surface lying mostly between 500 and 680 km, but rising to 300 km in the east, is a relict from an old subduction zone that descended from the fossil Vitiaz Trench. The locations of the mainshocks of the two Tongan earthquake sequences discussed by Tibi et al. are marked in yellow (2002 sequence) and orange (1986 series). Triggering mainshocks are denoted by stars; triggered mainshocks by circles. The 2002 sequence lies wholly in the currently subducting slab (and slightly extends the earthquake distribution in it),whereas the 1986 mainshock is in that slab but the triggered series is located in the detached slab,which apparently contains significant amounts of metastable olivine

  • Here are figures from Richards et al. (2011) with their figure captions below in blockquote.
  • The main tectonic map

  • bathymetry, and major tectonic element map of the study area. The Tonga and Vanuatu subduction systems are shown together with the locations of earthquake epicenters discussed herein. Earthquakes between 0 and 70 km depth have been removed for clarity. Remaining earthquakes are color-coded according to depth. Earthquakes located at 500–650 km depth beneath the North Fiji Basin are also shown. Plate motions for Vanuatu are from the U.S. Geological Survey, and for Tonga from Beavan et al. (2002) (see text for details). Dashed line indicates location of cross section shown in Figure 3. NFB—North Fiji Basin; HFZ—Hunter Fracture Zone.

  • Here is the map showing the current configuration of the slabs in the region.

  • Map showing distribution of slab segments beneath the Tonga-Vanuatu region. West-dipping Pacifi c slab is shown in gray; northeast-dipping Australian slab is shown in red. Three detached segments of Australian slab lie below the North Fiji Basin (NFB). HFZ—Hunter Fracture Zone. Contour interval is 100 km. Detached segments of Australian plate form sub-horizontal sheets located at ~600 km depth. White dashed line shows outline of the subducted slab fragments when reconstructed from 660 km depth to the surface. When all subducted components are brought to the surface, the geometry closely approximates that of the North Fiji Basin.

  • This is the cross section showing the megathrust fault configuration based on seismic tomography and seismicity.

  • Previous interpretation of combined P-wave tomography and seismicity from van der Hilst (1995). Earthquake hypocenters are shown in blue. The previous interpretation of slab structure is contained within the black dashed lines. Solid red lines mark the surface of the Pacifi c slab (1), the still attached subducting Australian slab (2a), and the detached segment of the Australian plate (2b). UM—upper mantle;
    TZ—transition zone; LM—lower mantle.

  • Here is their time step interpretation of the slabs that resulted in the second figure above.

  • Simplified plate tectonic reconstruction showing the progressive geometric evolution of the Vanuatu and Tonga subduction systems in plan view and in cross section. Initiation of the Vanuatu subduction system begins by 10 Ma. Initial detachment of the basal part of the Australian slab begins at ca. 5–4 Ma and then sinking and collision between the detached segment and the Pacifi c slab occur by 3–4 Ma. Initial opening of the Lau backarc also occurred at this time. Between 3 Ma and the present, both slabs have been sinking progressively to their current position. VT—Vitiaz trench; dER—d’Entrecasteaux Ridge.

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

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

  • Here is a great summary of the fault mechanisms for earthquakes along this plate boundary (Yu, 2013).

  • Large subduction-zone interplate earthquakes (large open gray stars) labeled with event date, Mw, GCMT focal mechanisms, and GPS velocity vectors (gray arrows and black triangles labeled with station name). GPS velocities are listed in Table 3. Black lines indicate the Tonga–Kermadec and Vanuatu trenches. Note that the 2009/09/29 Samoa–Tonga outer trench-slope event (Mw 8.1) triggered large interplate doublets (both of Mw 7.8; Lay et al., 2010). The Pacific plate subducts westward beneath the Australian plate along the Tonga–Kermadec trench, whereas the Australian plate subducts eastward beneath the Vanuatu arc and North Fiji basin. The opposite orientation between the Tonga–Kermadec and Vanuatu subduction systems is due to complex and broad back-arc extension in the Lau and North Fiji basins (Pelletier et al., 1998).


    Regional map of moderate-sized (mb > 4:7) shallow-focus repeating earthquakes and background seismicity along the (a) Tonga–Kermadec and (b) Vanuatu (former New Hebrides) subduction zones. Shallow repeating earthquakes (black stars) and their available Global Centroid Moment Tensor (GCMT; Dziewoński et al., 1981; Ekström et al., 2003) are labeled with event date and doublet/cluster id where applicable. Colors of GCMT are used to distinguish nearby different repeaters. Source parameters for the clusters and doublets are listed in Tables 1 and 2. Background seismicity is shown as gray dots and large interplate earthquakes (moment magnitude, Mw > 7:3) since 1976 are shown as large open gray stars. Black lines indicate the trench (Bird, 2003) and slab contour at 50-km depth (Gudmundsson and Sambridge, 1998). Repeating earthquake clusters in the (a) T1 and T2 plate-interface regions in Tonga and (b) V3 plate-interface region in Vanuatu are used to study the fault-slip rate ( _d). A regional map of the Tonga–Kermadec–Vanuatu subduction zones is
    shown in the inset figure, with the gray dotted box indicating the expanded region in the main figure.

    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

  • Richards, S., Holm, R., and Barber, G., 2011. Skip Nav Destination When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region in Geology, c. 39, no. 8, p. 787-790, https://doi.org/10.1130/G31937.1
  • Timm, C., Bassett, D., Graham, I. et al. Louisville seamount subduction and its implication on mantle flow beneath the central Tonga–Kermadec arc. Nat Commun 4, 1720 (2013). https://doi.org/10.1038/ncomms2702

Return to the Earthquake Reports page.

Earthquake Report: M 7.6 Earthquake in Mexico

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 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.

    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

  • Franco, A., C. Lasserre H. Lyon-Caen V. Kostoglodov E. Molina M. Guzman-Speziale D. Monterosso V. Robles C. Figueroa W. Amaya E. Barrier L. Chiquin S. Moran O. Flores J. Romero J. A. Santiago M. Manea V. C. Manea, 2012. Fault kinematics in northern Central America and coupling along the subduction interface of the Cocos Plate, from GPS data in Chiapas (Mexico), Guatemala and El Salvador in Geophysical Journal International., v. 189, no. 3, p. 1223-1236. DOI: https://doi.org/10.1111/j.1365-246X.2012.05390.x
  • Franco, S.I., Kostoglodov, V., Larson, K.M., Manea, V.C>, Manea, M., and Santiago, J.A., 2005. Propagation of the 2001–2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico in Earth Planets Space, v. 57., p. 973-985.
  • Garcia-Casco, A., Projenza, J.A., Iturralde-Vinent, M.A., 2011. Subduction Zones of the Caribbean: the sedimentary, magmatic, metamorphic and ore-deposit records UNESCO/iugs igcp Project 546 Subduction Zones of the Caribbean in Geologica Acta, v. 9, no., 3-4, p. 217-224
  • Benz, H.M., Dart, R.L., Villaseñor, Antonio, Hayes, G.P., Tarr, A.C., Furlong, K.P., and Rhea, Susan, 2011 a. Seismicity of the Earth 1900–2010 Mexico and vicinity: U.S. Geological Survey Open-File Report 2010–1083-F, scale 1:8,000,000.
  • Franco, A., Lasserre, C., Lyon-Caen, H., Kostoglodov, V., Molina, E., Guzman-Speziale, M., Monterosso, D., Robles, V., Figueroa, C., Amaya, W., Barrier, E., Chiquin, L., Moran, S., Flores, O., Romero, J., Santiago, J.A., Manea, M., Manea, V.C., 2012. Fault kinematics in northern Central America and coupling along the subduction interface of the Cocos Plate, from GPS data in Chiapas (Mexico), Guatemala and El Salvador in Geophysical Journal International., v. 189, no. 3, p. 1223-1236 https://doi.org/10.1111/j.1365-246X.2012.05390.x
  • Manea, V.C., et al., 2013. A geodynamical perspective on the subduction of Cocos and Rivera plates beneath Mexico and Central America in Tectonophysics, http://dx.doi.org/10.1016/j.tecto.2012.12.039
  • Manea, M., and Manea, V.C., 2014. On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective in JGVR, v. 175, p. 459-471.
  • Mann, P., 2007. Overview of the tectonic history of northern Central America, in Mann, P., ed., Geologic and tectonic development of the Caribbean plate boundary in northern Central America: Geological Society of America Special Paper 428, p. 1–19, doi: 10.1130/2007.2428(01). For
  • McCann, W.R., Nishenko S.P., Sykes, L.R., and Krause, J., 1979. Seismic Gaps and Plate Tectonics” Seismic Potential for Major Boundaries in Pageoph, v. 117

Return to the Earthquake Reports page.

Earthquake Report: M 6.0 northeast Pacific Ocean

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.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000ilwt/executive

This is possibly one of the most mysterious earthquakes of the year. I forgot to write this up at the time so need to fill in more details after I am done working up my annual summary.

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 I include a large scale view of the magnetic anomaly data. These anomalies are formed at mid-ocean ridges, so are parallel to the ridges. When transform faults offset these anomalies, the anomalies get offset.
  • In the lower right corner is a map showing the USGS modeled intensity that uses the Modified Mercalli Intensity (MMI) data.
  • In the upper left center and the lower right center I include maps from two papers that show the magnetic anomaly data for the Pacific Ocean.
  • Here is the map with 3 month’s seismicity plotted.

    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

Return to the Earthquake Reports page.

Earthquake Report for M 6.9 Earthquake in Taiwan

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).

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Earthquake Report: M 6.9 Mid Atlantic Ridge

There have not been that many large earthquakes this year. This is good for one main reason, there is a lower potential for human suffering.

Therefore, there are fewer Earthquake Reports for this year.

This morning (my time) there was a magnitude M 6.9 earthquake along the Romanche transform fault, a right-lateral strike-slip fault system that offsets the Mid Atlantic Ridge in the equatorial Atlantic Ocean. The fault is part of the Romanche fracture zone.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000i53f/executive

The transform faults in this part of the Mid Atlantic Ridge plate boundary have a pattern of earthquakes that seem to max out in the lower 7 magnitudes. This may be (at least partly) due to the maximum length of these faults (?).

The Romanche fault is about 900 kilometers long. The Chain fault is about 250 km long. The St. Paul fault is about 350 km long.

    Earthquake magnitude is controlled by three things:

  1. the size of the earthquake slip area, for most events, this is basically the length of the fault (since the width of the fault is controlled by the thickness of the lithosphere, or the crust)
  2. the amount that the fault slipped
  3. physical properties of the lithosphere or crust on either side of the fault (how “elastic” the Earth is)

Using empirical (data) based relations between earthquake subsurface rupture length and earthquake magnitude (Wells and Coppersmith, 1994), I calculate the maximum earthquake magnitude we may get on these three faults listed above.

Here are the data that Wells and Coppersmith use to establish these relations.


(a) Regression of subsurface rupture length on magnitude (M). Regression line shown for all-slip-type relationship. Short dashed line indicates 95% confidence interval. (b) Regression lines for strike-slip relationships. See Table 2 for regression coefficients. Length of regression lines shows the range of data for each relationship.

Here are the magnitude estimates for each of these fault systems.


Looking at the interpretive poster, we can see that there have not been any temblors that approach the sizes listed in this table. The largest historic earthquake was M 7.1 (there were several).

So, we may ask ourselves one of the most common questions people ask regarding earthquakes. Was this M 6.9 a foreshock to a larger earthquake?

Obviously, we cannot yet know this. Nobody can predict the future (at least not yet).

However, based on the incredibly short historic record of earthquakes, we may answer this question: “no, probably not.” This answer is tempered by the very short seismic record. If magnitude 8 earthquakes occur, on average, every 1000 years, then our ~100 year record might be too short to “notice” one of these M 8 events.
If we continue to look at the historic record, we will see that there appear to be three instances where one of these M 6.5-7 earthquakes had a later earthquake of a similar magnitude.
When an earthquake fault slips, the crust surrounding the fault squishes and expands, deforming elastically (like in one’s underwear). These changes in shape of the crust cause earthquake fault stresses to change. These changes in stress can either increase or decrease the chance of another earthquake.
I wrote more about this type of earthquake triggering for Temblor here. Head over there to learn more about “static coulomb stress triggering.”
In the poster, I label these earthquakes as “Linked Earthquakes.” Perhaps the later of each earthquake pair (or triple) was triggered by the change in static coulomb stress.

    Here are the three sets of “Linked Earthquakes:”

  • In 1992, along the Chain fault, the 16 Feb M 6.6 appears to have triggered the 18 Feb M 6.6. More speculatively, about 6 months later, it seems that there was a triggered M 6.9 on 18 Aug. Static Coulomb triggering typically has a limit of about 2-3 times the rupture length (and this depends of the pre-existing stress on the receiver fault, the fault that may be having triggered slip). A M 6.6 may have a rupture length of 50 km, so could possibly affect faults as far as 100-150 km away. The M 6.9 is about 70 km from the easternmost M 6.6, so it seems possible that the M 6.9 was triggered by the M 6.6.
  • In 2003, along the Romanche fault, there were two M 6.6 earthquakes separated by about 6 weeks. These quakes are about 100 km apart, possibly close enough to be triggered.
  • In 2020, along the St. Paul transform fault, there was a pair of quakes about 3 weeks and 340 km apart. The first quake was M 6.5, so this pair of events seems to far apart to be related.

So, given the historic record, it sure seems likely that there may be another M6-7 earthquakes in the region of the fault sometime in the next couple of months. And, given our lack of knowledge about the long term behavior of these faults, it is also possible that there could be a larger M 8 event.
Since we cannot yet know the real answer to this question, we are reminded of the advice that educators and emergency response people provide: If one lives in Earthquake Country, get earthquake prepared. Just a little effort to get better prepared makes a major difference in the outcome.
Head over to Earthquake Alliance where there are some excellent brochures about how to be better prepared and more resilient to earthquake and tsunami hazards. Living on Shaky Ground is one of my favorites!

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 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. 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 age of the oceanic crust in the Atlantic Ocean. Oceanic crust (or lithosphere) is created at mid ocean ridges, where there is extension that allows upward movement of magma, leading to the formation of oceanic crust. The Mid Atlantic Ridge system is one of these types of plate boundaries.
  • Above the crust age map is an illustration showing the how the crust moving away from the ocean ridges leaves behind oceanic crust. The Earth’s magnetic polarity changes at times and the oceanic crust records these changes in magnetic polarity. These changes are the main reason why we know that the crust is formed along these ridge systems. Read more here.
  • In the upper left corner is a small scale map that shows the historic seismicity, the plate boundary fault systems, and the magnetic anomalies. Places with crust formed when the magnetic field is like today, is colored red (a.k.a. normal polarity) and crust formed when the poles were reversed relative to today is blue (i.e., reversed polarity).
  • In the upper right corner is a map that shows the earthquake intensity from this earthquake (using the modified Mercalli Intensity Scale). Intensity is a measure of how strongly the shaking is felt, not a measure of the earthquake size. So, the intensity gets smaller with distance (see how the highest intensity is nearest the earthquake epicenter).
  • In the lower left center there is a map from Heezen et al. (1964). Heezen was an oceanographer that contributed greatly to our knowledge of the oceans. In this study, one of the things that they were studying is the flow of deep water (deep water flows largely because of changes in density of the seawater, controlled by salinity and temperature). Because of this, they were mapping the shape of the seafloor to see where this deep water could flow. Ths location of this map is outlined by a dashed rectangle in the main map.
  • Here is the map with a month’s seismicity plotted.

Some Relevant Discussion and Figures

  • Here is the Müller et al. (2008) figure from the interpretive poster above.

  • Here a the Bonatti et al. (2001) figure showing the bathymetry of this area. I include the figure caption as a blockquote below.

  • A: Multibeam topography of Romanche region, showing north-south profiles where sampling was carried out. Black dots and red numbers indicate estimated age (in million years) of lithosphere south of Romanche Transform, assuming spreading half-rate of 17 mm/yr within present-day ridge and transform geometry. White dots indicate epicenters of teleseismically recorded 1970–1995 events (magnitude . 4). FZ is fracture zone. B: Topography and petrology at eastern intersection of Romanche Fracture Zone with Mid-Atlantic Ridge. Data were obtained during expeditions S-16, S-19, and G-96 (Bonatti et al., 1994, 1996). C: Location of A along Mid-Atlantic Ridge.

  • Dr. Stephen Hicks and their colleagues conducted a fascinating study of the 2016 M 7.1 earthquake. They hypothesize that the Romanche fault slipped in different parts of the fault at different times (during the earthquake).
  • This map shows the historic seismicity of the region.

  • Seismotectonic context. The map location is given by the red rectangle on the inset globe. Focal mechanisms are shown for events with Mw > 6 (ref. 30). Mw > 7.0 events are labelled. Stations of the PI-LAB ocean bottom seismometer network are indicated by triangles. Our relocated hypocentre and low-frequency RMT of the 2016 earthquake are shown by the red star and red beach ball, respectively. The orange beach ball is a colocated Mw 5.8 used for the Mach cone analysis. The black rectangle shows the location of the map in Fig. 2. ISC Bulletin, Bulletin of the International Seismological Centre.

  • Here is where Hicks et al. (2020) hypothesize that the slip slipped.

  • Interpretation of rupture dynamics for the 2016 Romanche earthquake. Top: perspective view of bathymetry along the Romanche FZ. Bottom: interpretive cross-section along the ruptured fault plane. Colours show a thermal profile based on half-space cooling. The green line denotes the predicted transition between velocity-strengthening and velocity-weakening frictional regimes (as expressed by the a – b friction rate parameter) from Gabbro data35. The numbers show the key stages of rupture evolution: (1) rupture initiation (star) in the oceanic mantle, (2) initiation phase has sufficient fracture energy to propagate upwards to the locked section of fault, (3) weak subshear rupture front travels east in the lower crust and/or upper mantle, (4) rupture reaches the locked, thinner crustal segment close to the weaker RTI (SE1), (5) sufficient fracture energy for a westward supershear rupture in the crust along the strongly coupled fault segment (SE2) and (6) rupture possibly terminated by a serpentinized and hydrothermally altered fault segment.

    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

  • Abercrombie, R.E. and Ekstrom, G., 2001. Earthquake slip on oceanic transform faults in Nature, v. 410, p. 74-77
  • Bonatti, E., Brunello, D., Fabretti, P., Ligi, M., Porcaro, R.A., and Sealer, M., 2001. Steady-state creation of crust-free lithosphere at cold spots in mid-ocean ridges in Geology, v. 29, no. 11, p. 979-982.
  • Hicks, S.P., Okuwaki, R., Steinberg, A., Rychert, C.A., Harmon, N. Abercrombie, R.E., Bogiatzis, P., Cataphors, D., Zahradnik, J., Kendall, J-M., Yagi, Y., Shimizu, K., and Sudhaus, H., 2020. Back-propagating supershear rupture in the 2016 Mw 7.1 Romanche transform fault earthquake in Nature Geoscience, v. 13, p. 647-653, https://doi.org/10.1038/s41561-020-0619-9
  • Heezen, B.C., Bunce, E.T., Hersey, J.B., and Tharp, M., 1964. Chain and Romanche fracture zones in Deep-Sea research, v. 11, p. 11-33
  • Müller, R.D., Sdrolias, M., Gaina, C., and Roest, W.R., 2008. Age, spreading rates and spreading symmetry of the world’s ocean crust in Geochem. Geophys. Geosyst., 9, Q04006, doi:10.1029/2007GC001743
  • Torsvik, T.H., Tousse, S., Labaila, C., and Smethurst, M.A., 2009. A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin in Geophysical Journal International, v. 177, p. 1315-1333.

Return to the Earthquake Reports page.


Earthquake Report M 7.0 Philippines

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.

https://earthquake.usgs.gov/earthquakes/eventpage/us6000i5rd/executive

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.

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

20220727_philippines_interpretation.pdf 16 MB pdf

    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

Social Media: Here is my thread for this event.

Return to the Earthquake Reports page.

Earthquake Report: M 6.2 along the Great Sumatra fault

There was a magnitude M 6.2 Gempa or Earthquake on 25 February 2022.

https://earthquake.usgs.gov/earthquakes/eventpage/us6000gzyg/executive

The plate boundary fault system that dominates the tectonics of the Island of Sumatera, Indonesia, is the complicated.

The oceanic India-Australia plate converges with the Eurasia plate to form the Sunda trench. This convergent plate boundary forms a subduction zone where the oceanic plate subducts beneath the continental plate.
Below is a low-angle oblique view cut into the Earth showing this plate configuration from the Earth Observatory Singapore.


However, the direction of plate convergence is not perpendicular to the plate boundary fault (the megathrust subduction zone). Why does this matter?
Because the convergence is at an angle oblique to the plate boundary, we can imagine that this convergence can be subdivided into two components of motion:

  1. the fault perpendicular motion
  2. and the fault parallel motion

The amount of plate convergence that is perpendicular to the plate boundary is accommodated by earthquake fault slip on the megathrust.

The amount of plate convergence that is parallel to the plate boundary is accommodated by earthquake fault slip on a different series of faults that we call sliver faults. The Great Sumatra fault is one of these [forearc] sliver faults.

Here is a figure from Lange et al. (2008) that shows how oblique plate convergence forms both a subduction zone and a forearc sliver fault system.


The M 6.2 earthquake is a strike-slip earthquake along the Great Sumatra fault, one of these forearc sliver faults.

Based on our knowledge of this fault system and the earthquake mechanism, we can easily interpret this to be a right-lateral strike-slip fault.

There are numerous historical analogies from the past century. Most of the events in the past few decades have been in the M 6-7 range, though there have been events of larger magnitude in the past centuries.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past 3 months, with diameter representing magnitude (see legend). I include earthquake epicenters from 1922-2022 with magnitudes M ≥ 6.0.
  • 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 that shows the major plate boundaries with tectonic strain shown in red and blue. Areas that are red have a higher rate of tectonic deformation due to the motion of these plates and the orientation of the plate boundaries.
  • In the lower center is a low angle oblique view of the Sumatra subduction zone that forms the Sunda Trench. I placed a red circle in the location of the M 6.2 earthquake.
  • In the upper left center there is a map that shows the earthquake shaking intensity. Read more about this further down in this report.
  • In the upper right center is a plot showing earthquake shaking intensity (vertical axis) relative to distance from the earthquake (horizontal axis). This shows a comparison between the USGS shaking models as colored lines (also shown on the map to the left) relative to real reports from real people (the Did You Feel It? (dyfi) points).
  • Below the earthquake intensity map is a map that shows the mapped active faults, along with their slip rates from Natawidjaja (2018).
  • On the right margin are two maps that show models of earthquake triggered landslides and earthquake induced liquefaction. I describe these phenomena later in this report.
  • In the upper left corner are two maps from the Global Earthquake Model program: Seismic Hazard and Seismic Risk. Read more about this later in this report.
  • Here is the map with 3 month’s seismicity plotted.

Some Relevant Discussion and Figures

  • Here is a part of the poster “Seismicity of the Earth 1900-2012, Sumatra and Vicinity” (Hayes et al., 2013). Note the location of Padang, which is southwest of the M 6.2 earthquake.
  • The map shows the location of the seismicity cross section (the next figure). The cross section includes earthquakes from locations within the rectangle. These events are plotted along the line C-C.’
  • See that the Sumatra fault crosses this cross section just to the east of the center.

  • Here is the seismicity cross section C-C’. Many of the earthquakes plotted here follow the subducting slab of the India-Australia plate.
  • However, there are some shallow earthquakes in the upper plate that represent slip along Sumatra fault zone faults (like yesterday’s M 6.2).

  • Further to the north there was a pair of subduction zone earthquakes in 2004 and 2005. The 2004 Sumatra-Andaman subduction zone earthquake was devastating and the earthquake and tsunami led to almost a quarter of million deaths.
  • There are several earthquake report pages on these two events. Here is one of them:/li>
  • This is my poster from that report.

  • Here is a cross section showing where the earthquake hypocenter is compared to where we think the mantle exists. We have not been here, so nobody actually knows… These interpretations are based on industry deep seismic data (Singh et al., 2008 ).

  • Here is the historic rupture map showing the locations of historic subduction zone earthquakes. I include a figure caption below that I wrote as blockquote.

  • 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).

  • One of my favorite interpretive posters for this part of the world is from the 2012 outer rise sequence offshore of northern Sumatra. This poster includes additional details about the structure of the India-Australia plate.
  • The India-Australia plate is important as it appears to control some of the tectonics of the megathrust, as well as for the upper plate faults like the Sumatra fault.
  • Read more about the 2012 sequence here.

  • This is a video showing a visualization of the seismic waves transmitted from the 2004 SASZ earthquake from IRIS and others.
  • This movie illustrates simulation of seismic wave propagation generated by Dec. 26 Sumatra earthquake. Colors indicate amplitude of vertical displacement at the surface of the Earth. Red is upward and blue is downward. Total duration of this simulation is 20 minutes. Source model we used is that of Chen Ji of Caltech. Simulation was performed by using the Earth Simulator of JAMSTEC.

  • 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.

  • Now let’s take a closer look at the Sumatra fault. Here is a map that is from Natawidjaja (2017). Dr. Natawidjaja worked with Dr. Kerry Sieh on the Sumatra fault for his Ph.D. research. This map is an updated version showing the different fault segments of the Sumatra fault system. I inlcude their figure caption in blackquote.

  • New revised (simplified) active fault map of the Sumatran Fault Zone (SFZ) according to the PuSGeN Team for Updating Indonesia Seismic Hazard Map (2016) with new slip rates from geological and geodetical (GPS) recent studies.

  • Here is a figure from their dissertation showing these fault segments in greater detail (Natawidjaja , 2002). The M 6.2 earthquake is near the southern boundary of the Barumun segment.

  • Map of 20 geometrically defined segments of the Sumatran fault system and their spatial relationships to active volcanoes, major graben, and lakes.

  • This map shows what the slip rate would be on an hypothetical forearc sliver fault in the location of the Sumatra fault, given plate convergence rates and coral uplift rates (Natawidjaja, 2017).

  • Tectonic modelling based on continuous GPS – SuGAr 9 Sumatran GPS Array) and coral uplift rates,

  • This figure shows a comparison of fault slip rates for different parts of the Sumatra fault (Natawidjaja, 2017).

  • Comparison of GPS velocity profiles across the Sumatran fore arc inferred from (left) kinematic block models (right) with previously published velocity profiles. Modeling all fore-arc site velocities with a single strike-slip fault results in anomalously high inferred slip-rates (>22mm/yr) and missing the Sumatran Fault trace by up to 40km. Incorporating the effect of oblique locking of the Sunda megathrust results in lower inferred slip – rates for the Sumatran Fault (~15mm/yr) that are more consistent with updated geological slip rates.

  • And finally here is an interpretive figure showing how Natawidjaja(2002) interpret the formation of the Sumatra fault system.

  • A plausible (but nonunique) history of deformation along the obliquely convergent Sumatran plate margin, based upon our work and consistent with GPS results and the timing of deformation in the forearc region. (a) By about 4 Ma, the outer-arc ridge has formed. The former deformation front and the Mentawai homocline provide a set of reference features for assessing later deformations. From 4 to 2 Ma, partitioning of oblique plate convergence occurs only north of the equator. Dextral-slip faults on the northeast flank of the forearc sliver plate parallel the trench in northern Sumatra but swing south and disarticulate the forearc basin and outer-arc ridge north of the equator. (b) Slip partitioning begins south of the equator about 2 Ma, with the creation of the Mentawai and Sumatran faults. Transtension continues in the forearc north of the equator. (c) In perhaps just the past 100 yr, the Mentawai fault has become inactive, and the rate of slip on the Sumatran fault north of 2°N has more than doubled. This difference in slip rate may be accommodated by a new zone of transtension between the Sumatran fault and the deformation front in the forearc and outer-arc regions.

  • What about further back in time? Hurukawa et al., 2014 prepared a summary of the earthquakes along the Sumatra fault. Below is a map showing the location of these ruptures.

  • Relocated MJHD epicenters. (a) Northern Sumatra. (b) Central Sumatra. (c) Southern Sumatra. Solid lines with names indicate segments of the Sumatran fault (Sieh and Natawidjaja, 2000). Symbols are as in Figure 2. The thick solid line (see Fig. 4c) indicates the Ranau–Suwoh area, which was severely damaged by the 1933 Liwa earthquake (Berlage, 1934;Widiwijayanti et al., 1996). The slip rates of the Sumatran fault in northern, central, and southern Sumatra are taken from Ito et al. (2012) and Genrich et al. (2000) for Global Positioning System (GPS) and Bellier and Sebrier (1995) for Satellite Pour l’Observation de la Terre (SPOT).

  • This shows these records on a space-time diagram.

  • Earthquake history along the Sumatran fault since 1892. Fault planes estimated in this study are shown by thick lines. SG: Seismic gap.

    Earthquake Stress Triggering

    • When an earthquake fault slips, the crust surrounding the fault squishes and expands, deforming elastically (like in one’s underwear). These changes in shape of the crust cause earthquake fault stresses to change. These changes in stress can either increase or decrease the chance of another earthquake.
    • I wrote more about this type of earthquake triggering for Temblor here. Head over there to learn more about “static coulomb stress triggering.”
    • There are two kinds of earthquake triggering.
      1. Dynamic Triggering – When seismic waves travel through the Earth, they change the stresses in the crust. IF the faults are “locked and loaded” (i.e. they are just about ready to slip in an earthquake), there may be an earthquake on the “receiver” fault. Generally, once the seismic waves are done travelling, this effect is over. Though, some suggest that this affect on the stress changes may last longer (but not much longer).
      2. Static Triggering – When an earthquake fault slips, it deforms (changes the shape) of the crust surrounding that earthquake. These changes can cause increases and decreases in the stress on faults (either increasing or decreasing the chance for an earthquake). Just like for dynamic triggering, the fault needs to be about ready to slip. The effect on fault slip changes in “static coulomb stress” generally extend a distance of about 2-3 times the fault length of the “source” fault.
    • Raffie et al. (2021) calculated static coulomb stress changes on the central Sumatra fault segments as imposed by several megathrust subduction zone earthquakes.
    • Below is a series of maps that show the results from their analyses.

    • Coulomb stress models resolved on receiver faults of central part of GSF from coseismic slip model of each large interplate earthquakes. The color represents the maximum stress changes at 10 km depth with a scale saturated at 1 bar.

    • Here are the results when they consider all sources in a cumulative manner.

    • Cumulative ΔCFF of each earthquake listed in Table 1 (a) and cumulative ΔCFF of 1797, 1833, and 1861 earthquakes (b). The cyan ellipses are the damage area of large intraplate earthquakes marked as green star. The ΔCFF is calculated at 10 km depth with a scale saturated at 1 bar.

    Shaking Intensity

    • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
    • In the upper panel is the USGS Did You Feel It reports map, showing reports as colored dots using the MMI color scale. Underlain on this map are colored areas showing the USGS modeled estimate for shaking intensity (MMI scale).
    • In the lower panel is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orange and purple dots. Note how well the reports fit the green line (the model that represents how MMI works based on quakes in California).
    • Below the lower plot is the USGS MMI Intensity scale, which lists the level of damage for each level of intensity, along with approximate measures of how strongly the ground shakes at these intensities, showing levels in acceleration (Peak Ground Acceleration, PGA) and velocity (Peak Ground Velocity, PGV).

    Potential for Ground Failure

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

      FOS = Resisting Force / Driving Force

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

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

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

    • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides.
    • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.

    Seismic Hazard and Seismic Risk

    • Here is a map that shows the seismic hazard in southeast Asia, including Sumatra (Hayes et al., 2013). The plate convergence vectors, showing the direction of plate convergence and the rate of plate convergence in mm per year. Note how the plate convergence vectors are not perpendicular to the plate boundary.

    • 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).

      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
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    • 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
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