Category Archives: mexico

Earthquake Report: M 7.6 Earthquake in Mexico

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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

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Earthquake Report Lite: M 7.0 near Acapulco, Mexico

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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.
Last afternoon (my time) there was an M 7.0 earthquake near Acapulco, Mexico. This event generated a tsunami, landslides, building damage, casualties (one fatality as I write this), and many emotions.
https://earthquake.usgs.gov/earthquakes/eventpage/us7000f93v/executive
I present my interpretive poster and a few figures. Read more about the tectonics of this region here, in a report for an M 7.4 earthquake in 2020.

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 ≥ 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.

  • In the upper left corner is a small scale map showing the major plate boundaries.
  • Below the plate tectonic map is a plot showing the tide gage data from Acapulco, Mexico. Note the clear tsunami signal.
  • To the right of the plate tectonic map is a large scale map showing aftershocks in the region of the M 7.1 mainshock. Note that these aftershocks are from the Servicio Sismológico Nacional (SSN) Catálogo de sismos and that there are two mainshock locations (USGS M 7.0 and SSN M 7.1).
  • In the lower right corner is a map that shows a comparison of earthquake intensity between the USGS models and the Did You Feel It observations.
  • Above the intensity comparison map is a plot showing these same data, intensity is on the vertical axis an distance from the earthquake [Hypocenter] is on the horizontal axis.
  • In the upper right corner is a map that shows the results of an earthquake induced liquefaction model. Read more about this model here.
  • Here is the map with a week’s seismicity plotted.


    •

Tide Gage Data – Acapulco

Earthquake Intensity

  • Below is a comparison of earthquake shaking intensity between the USGS Model results and the Did You Feel It observations.


    •

    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 (and Tsunami) Oaxaca, Mexico

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Well, it has been a busy couple of weeks.

  • On 18 June, here was a M 7.4 earthquake in the Pacific plate along the Kermadec trench north of New Zealand which generated a small tsunami, even though it was a strike-slip earthquake (hopefully I can get to write that up, people are often surprised by tsunami generated by strike-slip earthquakes, but they are not that uncommon).
  • Then, on 19 June, there was a M 4.6 event as part of the Monte Cristo Earthquake Sequence. I put together a poster, but no report. See more on this sequence here. My coworker is developing an earthquake Quick Reporting program to provide earthquake information to the state geologist (Steven Bohlen), the director of the Department of Conservation (David Shabazian), and the Secretary of the Natural Resources Agency (Wade Crowfoot). Cindy has been doing some of this in various roles for many years, but we are formalizing the process. I have been supporting Cindy by preparing maps. Even though this event was in Nevada, it satisfied the [draft] minimum criteria to prepare a Quick Report.
  • Then, on 23 June (Monday my time), there was a M 4.6 south of Lone Pine, CA. This happened late in the day, but Cindy prepared a Quick Report, along with my map and other information from the Strong Motion Instrument Program (seismometers in CA) run by Hamid Haddadi.
  • Then, on 23 June (Tuesday my time), right before work started, there was an earthquake in Oaxaca, Mexico. The Tsunami Unit at CGS was having our meeting and we all made observations and interpreted the earthquake and tsunami in real time. This is what I am writing about here.
  • Today, on 24 June (Wednesday my time), the CGS was having an all staff meeting. During the meeting, there was a M 5.8 earthquake near where the M 4.6 happened, near Lone Pine and Keeler. I will write about that earthquake next.

https://earthquake.usgs.gov/earthquakes/eventpage/us6000ah9t/executive

The west coast coastline of southern Mexico, Central America, and South America is formed by a convergent plate boundary where oceanic tectonic plates dive eastwards beneath the continents. The fault formed at this plate boundary is called a subduction zone and the dynamics of subduction zones form deep sea trenches. I spend a few paragraphs discussing the different faults that form at different plate boundaries here.

Offshore of southern Mexico the Middle America trench shows us the location of the subduction zone megathrust fault. This fault system has a long history of damaging earthquakes, including some events that affect areas hundreds of kilometers from the source earthquake (e.g. the 1985 magnitude M 8 Mexico City earthquake).

In the past few years, evidences this megathrust is active continue to present themselves. There is a list of some earthquake reports at the bottom of this page.
The M 7.4 Oaxaca, Mexico Earthquake occurred along the megathrust fault interface (an “interplate” earthquake) based on our knowledge of the location of the fault, our calculation of the earthquake location, and the earthquake mechanisms prepared by seismologists (i.e. focal mechanisms or moment tensors).

The earthquake generated seismic waves that travelled around the world, including some that caused strong shaking in Mexico City. Mexico City was built where the Aztec Civilization had once constructed a great city. This city was built next to a lake where the Aztec constructed floating gardens. Eventually, these gardens filled the lake and the lake filled with sediment (I am simplifying what happened over a long time).

So, Mexico City is built in a sedimentary basin. Sedimentary basins can amplify shaking from seismic waves. These basins can also focus seismic waves and these waves can resonate within the basin, causing further amplification. This is why there was so much damage in Mexico City from the 1985 subduction zone earthquake.

The same thing happened a couple years ago for a recent earthquake there.

Well, when subduction zone earthquakes happen, the crust around the fault can flex like the elastic on one’s waist band. As the crust moves, if that crust is beneath the water, this crust motion moves the water causing a tsunami.

There are a number of organizations that monitor the Earth for earthquakes that may cause tsunami. These organizations alert officials in regions where these tsunami may inundate so that residents and visitors to the coast can take action (e.g. head to high ground). These programs save lives.

This M 7.4 earthquake generated a tsunami that was recorded along the coastline, but not at all tide gage stations. The Salina Cruz station has a great record of this tsunami and is located >80 km from the epicenter. The Acapulco station also recorded a tsunami, but those data were not uploaded to the IOC website (they are working this out now). It appeared that the Acapulco data were being streamed in real time, but I noticed that they were the same data as posted for the Salina Cruz station.

Here I plot the water surface elevations observed at the Salina Cruz tide gage. I mark the earthquake event time and the tsunami arrival time, then calculate the tsunami travel time.
I noticed that there is a down-first wave prior to the tsunami. This was observed at both stations (Acapulco and Salina Cruz). Dr. Costas Synolakis (USC) informed me that this is a well known phenomena called a “Leading Depression N-wave.” I mark the location of the Salina Cruz gage on the interpretive poster below.

The Wave Height of the tsunami is the vertical distance measured between the peak and the trough. These data show a Maximum Wave Height of 1.4 meters.


The strong ground shaking from an earthquake can also cause landslides and liquefaction. I discuss these further down in this report and include maps in the poster.

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 1920-2020 with magnitudes M ≥ 7.0.
  • 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. In the map below I include the magnetic anomaly data, also explained on this web page.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

  • In the upper left corner is a global scale map showing the major plate boundary faults and arrows show relative plate motions for these fault systems. The spreading ridge (orange arrows) between the Pacific and Cocos plates interacts with a flipping magnetic pole to form these magnetic anomalies. Because of this, they form parallel to the spreading ridge. Note how the anomalies are parallel to the East Pacific Rise spreading center (not labelled on this map, but look at figures lower in this report).
  • In the lower right corner is a map showing the earthquake intensity using the Modified Mercalli Intensity Scale (MMI) as modeled by the California Integrated Seismic Network (CISN). I also include observations from the USGS “Did You Feel It?” observations (these are from reports from the public).
  • To the left of this map is a plot showing how shaking intensity lowers with distance from the earthquake. The models that were used to produce the Earthquake Intensity map to the right are the same model results represented by the orange and green lines. However, on this plot, there are also observations from real people! The USGS Did You Feel It? questionnaire lets people report their observations from the earthquake and these data are plotted here. We can then compare the model with the observations.
  • In the upper and center right are maps that shows the liquefaction susceptibility and landslide probability models from the USGS. These are models and not based on direct observation, however, they could be used to help direct field teams to search for this type of effect.
  • To the left of these slope failure maps is a map and cross section from Benz et al. (2011). The circles represent earthquake locations and the diameter represents earthquake magnitude. The cross section B-B’ location in shown on this inset map and the main map as blue lines.
  • To the right of the main legend is the tide gage record from Salina Cruz.
  • Here is the map with 3 month’s seismicity plotted.



Other Report Pages

Some Relevant Discussion and Figures

  • 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 the figure that shows how the upper and lower plate structures interplay.


    • Kinematic model (mantle reference frame) of the subducting Cocos slab along the MAT in the vicinity of Cocos–Caribbe–North America triple junction since Early Miocene. The evolution of Caribbean–North America tectonic contact is based on the model of Witt et al. (2012). The blue strips represent markers on the Cocos plate. Note how trench roll forward is associated with steep slab in Central America, whereas trench roll back is associated with flat slab in Mexico.

  • 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 are 2 different figures from Mann (2007). First we see a map that shows the structures in the Cocos plate. Note the 3 profile locations labeled 1, 2, and 3. These coincide with the profiles in the lower panel.


    • Present setting of Central America showing plates, Cocos crust produced at East Pacifi c Rise (EPR), and Cocos-Nazca spreading center (CNS), triple-junction trace (heavy dotted line), volcanoes (open triangles), Middle America Trench (MAT), and rates of relative plate motion (DeMets et al., 2000; DeMets, 2001). East Pacifi c Rise half spreading rates from Wilson (1996) and Barckhausen et al. (2001). Lines 1, 2, and 3 are locations of topographic and tomographic profi les in Figure 6.

  • Here are 2 different views of the slabs in the region. These were modeled using seismic tomography (like a CT scan, but using seismic waves instead of X-Rays). The upper maps show the slabs in map-view at 3 different depths. The lower panels are cross sections 1, 2, and 3. Today’s M=6.6 earthquake happened between sections 1 & 2.


    • (A) Tomographic slices of the P-wave velocity of the mantle at depths of 100, 300, and 500 km beneath Central America. (B) Upper panels show cross sections of topography and bathymetry. Lower panels: tomographic profi les showing Cocos slab detached below northern Central America, upper Cocos slab continuous with subducted plate at Middle America Trench (MAT), and slab gap between 200 and 500 km. Shading indicates anomalies in seismic wave speed as a ±0.8% deviation from average mantle velocities. Darker shading indicates colder, subducted slab material of Cocos plate. Circles are earthquake hypocenters. Grid sizes on profi les correspond to quantity of ray-path data within that cell of model; smaller boxes indicate regions of increased data density. CT—Cayman trough; SL—sea level (modifi ed from Rogers et al., 2002).

  • Below is a video that explains seismic tomography from IRIS.

  • 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.

  • Here are some figures that show how the subduction zone varies across the Tehuantepec Ridge. More about this in my initial report, as well as in my reports for the M 8.1 earthquake.
  • This is a figure showing the location of the Tehuantepec Ridge (Quzman-Speziale and Zunia, 2015).


    • Tectonic framework of the Cocos plate convergent margin. Top- General view. Yellow arrows indicate direction and speed (in cm/yr) of plate convergence, calculated from the Euler poles given by DeMets et al. (2010) for CocoeNoam (first three arrows, from left to right), and CocoeCarb (last four arrows). Length of arrow is proportional to speed. Red arrow shows location of the 96 longitude. Box indicates location of lower panel. Bottom- Location of features and places mentioned in text. Triangles indicate volcanoes of the Central American Volcanic Arc (CAVA) with known Holocene eruption (Siebert and Simkin, 2002).

  • Here is another figure, showing seismicity for this region (Quzman-Speziale and Zunia, 2015).


    • Seismicity along the convergent margin. Top: Map view. Blue circles are shallow (z < 60 km) hypocenters; orange, intermediate-depth (60 < z < 100 km); yellow, deep (z > 100 km). Next three panels: Earthquakes as a function of longitude and magnitude for shallow (blue dots), intermediate (orange), and deep (yellow) hypocenters. Numbers indicate number of events on each convergent margin, with average magnitude in parenthesis. Gray line in this and subsequent figures mark the 96 deg longitude.

  • This shows the location of the cross sections. The cross sections show how the CP changes dip along strike (from north to south) (Quzman-Speziale and Zunia, 2015).


    • Location of hypocentral cross-sections. Hypocentral depths are keyed as in previous figures.

  • Here are the cross sections showing the seismicity associated with the downgoing CP (Quzman-Speziale and Zunia, 2015).


    • Hypocentral cross-sections. Depths are color-coded as in previous figures. Dashed lines indicate the 60-km and 100-km depths. Tick marks are at 100-km intervals, as shown on the sections. There is no vertical exaggeration and Earth’s curvature is taken into account. Number of sections refers to location on Fig. 3.

  • This figure shows thrust and normal earthquakes for three ranges of depth (Quzman-Speziale and Zunia, 2015).


    • Earthquake fault-plane solutions from CMT data. a. Shallow (z < 60 km), thrust-faulting mechanisms. b. Intermediate-depth (60 < z < 100 km) thrust-faulting events. c. Deep (z > 100 km), thrust-faulting earthquakes. d. to f. Normal-faulting events, in same layout as for thrust-faulting events.

  • Here is an educational animation from IRIS that helps us learn about how different earth materials can lead to different amounts of amplification of seismic waves. Recall that Mexico City is underlain by lake sediments with varying amounts of water (groundwater) in the sediments.

  • Here is an educational video from IRIS that helps us learn about resonant frequency and how buildings can be susceptible to ground motions with particular periodicity, relative to the building size.

    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, 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

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Earthquake Report: Guatemala and Mexico

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This morning (my time) there was a moderately deep earthquake along the coast of southern Mexico and northern Guatemala. Here is my Temblor article about this M=6.6 earthquake and how it might relate to the 2017 M=8.2 quake.
https://earthquake.usgs.gov/earthquakes/eventpage/us2000jbub/executive
Offshore of Guatemala and Mexico, the Middle America trench is formed by the subduction of the oceanic Cocos plate beneath the North America and Caribbean plates.
To the east of Guatemala and Mexico, the North America and Caribbean plates are separated by a left lateral (sinistral) strike-slip plate boundary fault (that forms the Cayman Trough beneath the Caribbean Sea).
As this plate boundary comes onshore, this fault forms multiple splays, including the Polochi-Montagua fault. As this system trends westwards across Central America, it joins another strike-slip plate boundary associated with the subduction zone (the Volcanic Arc fault).
South of about 15°N, the relative plate motion between the Caribbean and Cocos plates is oblique (they are not moving towards each other in a direction perpendicular to the subduction zone fault). At plate boundaries where plate convergence is oblique (like also found in Sumatra), the strain is partitioned onto the subduction zone (for fault normal component of the relative plate motion) and a forearc sliver fault (for the fault parallel relative motion).
The Tehuantepec fracture zone (TFZ) is a major structure in the Cocos plate. Coincidentally, the strike-slip fault systems trend towards where the TFZ intersects the trench.
There is left-lateral offset of the seafloor across the TFZ so the crust is about 10 million years older on the north side of the eastern TFZ. This age offset changes the depth of the crust across the TFZ and also may affect the megathrust fault properties on either side of the TFZ.
In addition, the TFZ may have geological properties that also affect the fault properties when this part of the plate subducts (affecting where, when, and how the fault slips).
There are so many things going on, but I will mention one more thing. Something that also appears to be happening in this part of the subduction zone is that there may be gaps in the slab beneath the megathrust. If this is true (Mann, 2007), then there may be changes in slab pull tension along strike as a result of different widths of attached downgoing slab.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 6.5 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

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

    Magnetic Anomalies

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

    Age of Oceanic Lithosphere

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

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

  • In the lower left corner is a pair of figures from Manea et al. (2013). On the left is a map showing some major plate boundary faults and other fault systems relevant to this region. On the right is a low angle oblique visualization of the Cocos plate. North is to the lower right. The depth of the slab is shown in shades of blue (see legend). Note the offset of blue color across the TFZ.
  • In the upper right corner is another low angle oblique visualization of the structures (Manea et al., 2013). Note the difference in depth of the slab across the TFZ and how the forearc sliver and North America / Caribbean strike-slip faults cross the upper plate. Read more about the forearc sliver in this report about an earthquake in El Salvador.
  • In the lower right corner is a map of the region showing details of the structures in the Cocos plate (Mann, 2007). There are an abundance of faults associated with the spreading ridges and offsets of these by numerous fracture zones. Note how the Cocos plate is formed by 2 different spreading ridges.
  • Here is the map with a century’s seismicity plotted.


    •

  • Here is the map with a century’s seismicity plotted, using the age of the crust as an overlay.


    •

There are also some interesting relations between different historic earthquakes.
In 2017 there was a series of large magnitude earthquakes in the region of today’s M=6.6 and further to the south. These quakes are highlighted in the posters above, notable are the 6 Jun M=6.9 and 22 Jun M=6.8. The first quake was a deep extensional event, followed by a thrust event (possibly triggered by the M=6.9). In addition, there was a M=6.9 extensional earthquake in 2014 that also may have been a player.
I presented an interpretive poster showing the zone of aftershocks associated with the June sequence. Later, in Sept, there was a M=8.2 extensional tsunamigenic earthquake to the north of the June sequence. If we look at the aftershock zone for the M=8.2 quake, it looks like a sausage link adjacent to the sausage link formed by the June aftershocks. mmmm veggie sausages.
However there was no megathrust earthquake in the area of the M=8.2 sequence.

  • Here is an interpretive poster showing how the 2017 June and September sequences spatially relate.


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  • Here is a report where I discuss the June 2017 sequence in greater detail.


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Other Report Pages

Some Relevant Discussion and Figures

  • 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 the figure that shows how the upper and lower plate structures interplay.


    • Kinematic model (mantle reference frame) of the subducting Cocos slab along the MAT in the vicinity of Cocos–Caribbe–North America triple junction since Early Miocene. The evolution of Caribbean–North America tectonic contact is based on the model of Witt et al. (2012). The blue strips represent markers on the Cocos plate. Note how trench roll forward is associated with steep slab in Central America, whereas trench roll back is associated with flat slab in Mexico.

  • Here are 2 different figures from Mann (2007). First we see a map that shows the structures in the Cocos plate. Note the 3 profile locations labeled 1, 2, and 3. These coincide with the profiles in the lower panel.


    • Present setting of Central America showing plates, Cocos crust produced at East Pacifi c Rise (EPR), and Cocos-Nazca spreading center (CNS), triple-junction trace (heavy dotted line), volcanoes (open triangles), Middle America Trench (MAT), and rates of relative plate motion (DeMets et al., 2000; DeMets, 2001). East Pacifi c Rise half spreading rates from Wilson (1996) and Barckhausen et al. (2001). Lines 1, 2, and 3 are locations of topographic and tomographic profi les in Figure 6.

  • Here are 2 different views of the slabs in the region. These were modeled using seismic tomography (like a CT scan, but using seismic waves instead of X-Rays). The upper maps show the slabs in map-view at 3 different depths. The lower panels are cross sections 1, 2, and 3. Today’s M=6.6 earthquake happened between sections 1 & 2.


    • (A) Tomographic slices of the P-wave velocity of the mantle at depths of 100, 300, and 500 km beneath Central America. (B) Upper panels show cross sections of topography and bathymetry. Lower panels: tomographic profi les showing Cocos slab detached below northern Central America, upper Cocos slab continuous with subducted plate at Middle America Trench (MAT), and slab gap between 200 and 500 km. Shading indicates anomalies in seismic wave speed as a ±0.8% deviation from average mantle velocities. Darker shading indicates colder, subducted slab material of Cocos plate. Circles are earthquake hypocenters. Grid sizes on profi les correspond to quantity of ray-path data within that cell of model; smaller boxes indicate regions of increased data density. CT—Cayman trough; SL—sea level (modifi ed from Rogers et al., 2002).

  • 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.




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  • Franco et al. (2012) used GPS observations to evaluate the kinematics (how the plates move and interact relative to each other) of this region. Below is a map that shows earthquake mechanisms that reveal the strike-slip faults as they converge. The forearc sliver (the block between the megathrust and the forearc sliver fault) is shaded gray.
  • These authors also use a model to estimate how much the megathrust is locked and accumulating elastic strain. They evaluate a range of possible physical properties of the find that the megathrust north of the forearc sliver is more highly locked (seismogenically coupled).


    • Proposed model of faults kinematics and coupling along the Cocos slab interface, revised from Lyon-Caen et al. (2006). Numbers are velocities relative to CA plate in mmyr−1. Focal mechanisms are for crustal earthquakes (depth ≤30 km) since 1976, from CMT Harvard catalogue.

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


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  • Below is a video that explains seismic tomography from IRIS.

Geologic Fundamentals

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

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


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

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

    • Strike Slip:

    Compressional:

    Extensional:

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


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

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


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

    Social Media

Return to the Earthquake Reports page.


Earthquake Report: 2018 Summary

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


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

Below is my summary poster for this earthquake year

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


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

2018 Earthquake Report Pages

Other Annual Summaries

2018 Earthquake Reports

    General Overview of how to interact with these summaries

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

    Background on the Earthquake Report posters

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

    Magnetic Anomalies

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

2018.01.10 M 7.6 Cayman Trough

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

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


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


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


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2018.01.14 M 7.1 Peru

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


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

2018.01.23 M 7.9 Gulf of Alaska

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

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

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


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


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    • Large Scale Interpretive Map (from update report)


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


    2018.02.16 M 7.2 Oaxaca, Mexico

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

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


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    2018.02.25 M 7.5 Papua New Guinea

  • 2018.02.26 M 7.5 Papua New Guinea Update #1

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

  • The same map without historic seismicity.



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

    • Here is the “update” map with aftershocks


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    2018.03.08 M 6.8 New Ireland

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

    Main Interpretive Poster with emag2


    Earthquakes M≥ 6.5 with emag2


    2018.03.26 M 6.6 New Britain

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

    2018.03.26 M 6.9 New Britain

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

    2018.04.02 M 6.8 Bolivia

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

    2018.05.04 M 6.9 Hawai’i

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

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

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

    • Hilo, Hawaii


    • Kawaihae, Hawaii


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    Temblor Reports:

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

    2018.08.05 M 6.9 Lombok, Indonesia

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

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


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    2018.08.15 M 6.6 Aleutians

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

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


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


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    2018.08.18 M 8.2 Fiji

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

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


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    2018.08.19 M 6.9 Lombok, Indonesia

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

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


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


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


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    2018.08.21 M 7.3 Venezuela

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

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


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    2018.08.24 M 7.1 Peru

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

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


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    2018.09.05 M 6.6 Hokkaido, Japan

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

    • Here is the map with a centuries seismicity plotted.


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    Temblor Reports:

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

    2018.09.09 M 6.9 Kermadec

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

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


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    • Here is the map with a centuries seismicity plotted.


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    2018.09.28 M 7.5 Sulawesi

  • 2018.10.16 M 7.5 Sulawesi UPDATE #1

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

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




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


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


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

    My 2018.10.01 BC Newshour Interview

    InSAR Analysis

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

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


    M 7.5 Landslide Model vs. Observation Comparison

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

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


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

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


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

    Temblor Reports:

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

    2018.10.10 M 7.0 New Britain, PNG

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

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


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    Temblor Reports:

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

    2018.10.22 M 6.8 Explorer plate

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

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


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    2018.10.25 M 6.8 Greece

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

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


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


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    Temblor Reports:

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

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

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

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


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


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    2018.11.30 M 7.0 Alaska

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

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


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    Temblor Reports:

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

    2018.12.05 M 7.5 New Caledonia

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

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


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


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    2018.12.20 M 7.4 Bering Kresla

  • 2018.12.20 M 7.3 Bering Kresla UPDATE #1

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

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


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


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    UPDATE #1

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


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


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    2018.12.29 M 7.0 Philippines

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

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


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    Geologic Fundamentals

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

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


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

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

      • Strike Slip:

      Compressional:

      Extensional:

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


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

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


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

    Return to the Earthquake Reports page.

    Earthquake Report: Gulf of California

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    Today we had an earthquake with magnitude M 6.3 in the Gulf of California (GOC). The GOC is formed by transtension (extension along a strike-slip fault system) along the North-America-Pacific plate boundary. Transtension happens when the plate motion across a fault is not oriented parallel to the fault. This non-optimal relation (plate motion vs fault direction) can generally happen as a result of (1) a bending fault or (2) due to stepped offsets of the fault. The dextral (right-lateral) strike-slip faults here make “right-steps” and pull apart basins form in these locations.
    The strike-slip faults are offset by oceanic spreading ridges. These spreading ridges are connected the East Pacific Rise to the south (and the Juan de Fuca Ridge and Gorda Rise to the north, via the San Andreas fault).
    The geology of this region is much more complicated than this, but this is a good place to start when trying to understand the tectonics here. This M 6.3 earthquake happened on the southern boundary of the Guamas Basin, one of these pull apart basins.
    So far there has been a single aftershock (M 4.5).
    This M 6.3 earthquake appears to be pretty typical of this part of the Gulf. There have been several M 6 earthquakes in the last century. There have been a couple M 7 earthquakes, to the north.

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5.
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.3 earthquakes, in addition to some relevant historic earthquakes.
    I labeled the pull-apart basins in cyan and the faults in light orange. CdBF – C. de Ballenas fault; GF – Guaymas fault; CF – Carmen fault; FF – Farallon fault; PF – Pescadero fault; AF – Alcaron fault; AR – Alcaron Ridge (from Aragón-Arreola, M. and Martín-Barajas, A., 2007).

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

      I include some inset figures.

    • In the lower left corner I include a map that shows the tectonic setting of this region, with the geological units colored relative to their age and type (marine or continental). This is from a paper that discusses the interaction between spreading ridges and subduction trenches (Fletcher et al., 2007). I place a blue star in the general location of today’s earthquake.
    • In the upper left corner is the plate tectonic history for the past 12.3 Ma from Fletcher et al. (2007). I place a blue star in the general location of today’s earthquake.
    • Between the 2 Fletcher et al. (2007) figures is a figure that shows how there can be extension and compression along strike-slip figures.
    • Along the right side is the tectonic history of the GOC as interpreted by Bennett et al. (2013). I place a blue star in the general location of today’s earthquake. I used to think that the spreading ridges in the GOC were from the ridge that originally formed the Farallon plate. However, I have learned that the GOC extension formed as a result of the misfit of the plate boundary relative to plate motions (causing transtension).


    • On 2017.03.29 there was an M 5.7 earthquake in this same region as today’s M 6.3 earthquake. Here is my report and below is my interpretive poster.


      •

    • There was an earthquake on 2015.09.13 (M 6.6) located in the area of the Farallon Basin (here is my earthquake report page and the update page). Below is a map I prepared that shows the general location of the pull-apart basins along this plate boundary.


      •

    • Here is a fantastic animation showing the tectonic history of the GOC for the past 11 million years (Bennet et al., 2016). This animation was created using existing geological maps and fault data (geometry, changing or static slip-rates) and backing out the motion on the faults to create a map of what the geology looked like in the past.
    • There are several ways to do this and Bennett et al. (2016) use a Palinspastic reconstruction technique. These authors hypothesize that, based upon their observations, the transtension along this plate boundary promoted subsidence in the GOC. Read more about the Colorado River and how it responded to this tectonic forcing in their paper.

    • This is the Bennet et al. (2013) figure from the poster, which shows their interpretation for the tectonic history of the GOC. This is based largely on their tectonic reconstructions in the northern GOC.


      • Modified integrated transtensional shear model for the tectonic evolution of the Gulf of California. North America plate fixed. (A) Prior to 28 Ma, the spreading center between Pacific and Vancouver-Farallon tectonic plates approached the subduction zone between North America and Vancouver-Farallon plates. (B) By ca. 20 Ma, contact between the Pacific and North America plates created early dextral transform relative plate motion. The Basin and Range extensional province (light gray) accommodated moderate extension throughout western North America. (C) The Rivera triple junction migrated the full length of the Baja California Peninsula by ca. 12.5 Ma, lengthening the Pacific–North American transform plate boundary. The proto–Gulf of California period commenced (ca.12.5 Ma) with transtensional strain distributed across two distinct transtensional deformation belts, west and east of the stable Baja California microplate. (D) In late proto–Gulf of California time, shear deformation gradually localized within a narrow belt of focused en- echelon dextral shear zones embedded within the greater Mexican Basin and Range extensional province. These shear zones and intervening extensional regions both experienced high-magnitude strain. (E) By ca. 6 Ma, Pacific–North America plate boundary strain was localized and focused crustal thinning and subsidence in transtensional pull-apart basins that formed the Gulf of California. Faults shown represent primary structures active during Quaternary time. RP—Rivera plate, JDFP—Juan de Fuca plate, RTJ—Rivera triple junction, MTJ—Mendocino triple junction, SAF— San Andreas fault, CP—Colorado Plateau, SMO—Sierra Madre Occidental.

    • This is the Sutherland et al. (2012) interpretation of the geological history for the plate boundary in this region. These authors used seismic reflection data (multi-channel seismic) from a profile oriented parallel to the Alarcón Basin (perpendicular to the spreading ridge).


      • The model for the tectonic evolution of the Gulf of California (GOC). (A) The starting point of the evolution of the GOC began 14–12 Ma, where the Magdalena rise stalled off the west coast of Baja California; there was also a marked changed in the style of volcanism. Plate motion was split between the dying spreading ridge, subduction zone, and the new highly oblique extension in the proto–GOC. During this time, the dipping part of the subducted plate appears to have broken off, opening a slab window beneath the southern Baja peninsula. NAM—North American plate; PAC—Pacific plate. (B) Another major change in the system occurs near 8–7 Ma, where the volcanic style changes once again with many lavas of unusual composition deposited. Any minor component of spreading finally ceases and the Tosco-Abreojos fault forms within the borderlands west of Baja. Oblique extension continues in the GOC. (C) Seafloor spreading begins at the Alarcón Rise between 4 and 3 Ma. Small amounts of movement continue along the Tosco-Abreojos fault (TAF); even today the Baja peninsula is not fully transferred to the Pacific plate.

    • This shows the location of the Sutherland et al. (2012) seismic reflection profile.


      • Map showing location of Alarcón transect and the major basins along the profile.

    • This is the overview of the Sutherland et al. (2012) seismic reflection data.


      • An overview of multichannel seismic transect data presented in this paper. Seismic data are post-stack time migrated. TWTT—two-way travel time.

    • This is an example of the Sutherland et al. (2012) seismic reflection data, showing their interpretation in the lower panel. Note the extensive normal (and perhaps strike-slip) faulting (remember, we are in an extensional basin).


      • East Cerralvo basin. The uninterpreted (top) and interpreted (bottom) seismic profi les are shown. TWTT—two-way traveltime. Basement reflections are highlighted in blue, and sedimentary sequence boundaries are separated by green lines, with faults shown in red. Basement has a reflective discontinuous appearance. Unit 1 (divided into 1a and 1b) is a synrift deposit, with a chaotic character; unit 2 (divided into 2a and 2b) exhibits rotation and divergence and appears to be synrift; unit 3 consists of postrift, layered deeper water marine sediments. Two surface-cutting normal faults at the southeast end appear younger than the main basin, although the exact relationship of faulting is unclear and they appear to be overprinted by current-controlled erosion.

    • Here is a great diagram showing the major faults in the region (Umhoefer et al., 2002). I include their figure caption below.


      • (A) Simplified map of the Gulf of California region and Baja California peninsula showing the present plate boundary and some major tectonic features related to the plate-tectonic history since 12 Ma. The Gulf extensional province in gray is bounded by the Main Gulf Escarpment (bold dashed lines), which runs through the Loreto area and is shown in Figure 3. The Salton trough in southern California is merely the northern part of the Gulf extensional province. (B) Map of part of the southern Gulf of California and Baja California peninsula showing bathymetry (in meters), the transform–spreading-ridge plate boundary, and the location of subsequent figures with maps. The bathymetry is after a map in Ness and Lyle (1991) and the transform–spreading-ridge plate boundary is from Lonsdale (1989). The lines with double arrows are the three proposed rift segments modified here after Axen (1995); MS—Mulege´ segment, LS—Loreto segment, TS—Timbabichi
      segment.

    • This map shows the magnetic anomalies and the geologic map for the land and the youngest oceanic crust.


      • (A) Tectonic map of the southern Baja California microplate (BCM) and Gulf of California extensional province (GEP). The Magdalena fan is deposited on oceanic crust of the Farallon-derived Magdalena microplate located west of Baja California. Deep Sea Drilling Project Site 471 is shown as black dot on the Magdalena fan. Abbreviations: BCT—Baja California trench, BM—Bahia Magdalena, LC—Los Cabos block, T—Trinidad block, LP—La Paz, PV—Puerto Vallarta, SMSLF—Santa Margarita–San Lazaro fault, TAF—Tosco-Abreojos fault, TS—Todos Santos, V—Vizcaino peninsula. Geology is simplifi ed from Muehlberger (1996). Interpretation of marine magnetic anomalies, with numbers denoting the chron of positively magnetized stripes, is from Severinghaus and Atwater (1989) and Lonsdale (1991).

    • This map shows a more broad view of the magnetic anomalies through time.


      • Map-view time slices showing the widely accepted model for the two-phase kinematic evolution of plate margin shearing around the Baja California microplate. (A) Configuration of active ridge segments (pink) west of Baja California just before they became largely abandoned ca. 12.3 Ma. (B) It is thought that plate motion from 12.3 to 6 Ma was kinematically partitioned into dextral strike slip (325 km) on faults west of Baja California and orthogonal rifting in the Gulf of California (90 km). This is known as the protogulf phase of rifting. (C) From 6 to 0 Ma faults west of Baja California are thought to have died and all plate motion was localized in the Gulf of California, which accommodated ~345 km of integrated transtensional shearing. Despite its wide acceptance, our data preclude this kinematic model. In all frames, the modern coastline is blue. Continental crust that accommodated post–12.3 Ma shearing is dark brown. Unfaulted microplates of continental crust are light tan. Farallon-derived microplates are light green. Middle Miocene trench-filling deposits like the Magdalena fan are colored dark green. Deep Sea Drilling Project Site 471 is the black dot on the southern Magdalena microplate. Yellow line (296 km) in the northern Gulf of California connects correlated terranes of Oskin and Stock (2003). Maps have Universal Transverse Mercator zone 12 projection with mainland Mexico fixed in present position.

    • This is a nice simple figure, from the University of Sydney here, showing the terminology of strike slip faulting. It may help with the following figures.

    • Here is a fault block diagram showing how strike-slip step overs can create localized compression (positive flower) or extension (negative flower). More on strike-slip tectonics (and the source of this image) here.


      •

    • This is an animation from Tanya Atwater. Click on this link to take you to yt (if the embedded video below does not work).

    Update

    • I just found a paper that includes a map of the pull-apart basins in the northern GOC (Aragón-Arreola and Martín-Barajas, 2007). Today’s M 6.3 earthquake happened along the Carmen fault zone (labelled 11), close to where the letter L is located..


      • Eastern Gulf of California contains abandoned rift basins, while active rifting occurs in the western Gulf (Lonsdale, 1989; Fenby and Gastil, 1991; Persaud et al., 2003; Aragón-Arreola et al., 2005; this study). Eastern Gulf constitutes abandoned rift margin (see inset). PA—Pacifi c plate; GC—Gulf of California; GEP—Gulf Extensional Province; B&R—Basin
      and Range Province; SMO—Sierra Madre Occidental; CP—Colorado Plateau: ITI—Isla Tiburón; IAG—Isla Ángel de la Guarda.

    References:

    Earthquake Report: Chiapas Earthquake Update #2

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    Well, we had a really interesting earthquake today. There was a M 6.1 earthquake in the North America plate (NAP) to the north of the sequence offshore of Chiapas, with the M 8.1 mainshock. Here is the USGS website for the M 6.1 earthquake. There was also an M 5.8 earthquake that was a more typical aftershock (USGS website).

    Why is this earthquake interesting? It is outside the region of aftershocks from the M 8.1 earthquake and it is in the upper plate (the NAP). This is not altogether groundbreaking (pardon the pun) as there are many examples of earthquakes in one plate triggering earthquakes in other plates. For example, the recent sequence just to the south of the M 8.1 sequence (which may have led partly to the M 8.1 earthquake).

    This earthquake also triggered (sorry for the pun, another one) a debate about the difference between triggered earthquakes and aftershocks. This discussion is largely semantic and does not really matter from a natural hazards perspective. The rocks behave to physics, not how we classify them. So, we don’t need to get caught up in this lexicon (as long as we all have a general understanding of what is happening). In the classic sense, I interpret this M 6.1 (and the few nearby earthquakes) to be triggered, but they are in the region that may have an increased coulomb stress.

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS epicenters from 1917-2017 for magnitudes M ≥ 8.0. I include fault plane solutions for the 1985 and 1995 earthquakes (along with the MMI contours for those earthquakes, see below for a discussion of MMI contours).

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

      I include some inset figures in the poster.

    • In the left center I include a generalized plate tectonic map from Wikimedia Creative Commons here.
    • In the lower left corner I include a map from Dr. Jascha Polet. Dr. Polet plots focal mechanisms for historic earthquakes. Dr. Polet notes the M 8.1, M7.1, M 6.1, and M 5.8 earthquakes too. Purple dots are epicenters after the M 8.1 earthquake and black dots are SSN (the Mexico seismic network data).
    • In the upper right corner is a map and cross section from Dr. Gavin Hayes. The upper cross section, oriented perpendicular to the subduction zone fault, shows focal mechanisms. Note how the M 8.1 (large green symbol) is in the downgoing Cocos plate and the M 6,.1 (small red symbol at about 300 km) is in the North America plate.
    • In the upper left corner is a figure Dr. Hayes also prepared. This shows the change in static coulomb stress associated with the M 8.1 earthquake. Tremblor prepared this analysis and I presented that in the M 7.1 earthquake report here. Basically, areas of warm color show an increased stress and regions of cool color show a decreased stress. So, areas in warm color are more likely to trigger an earthquake. Though this is a simple run as different faults can respond differently.


    • Here is the initial report poster as presented in my initial Earthquake Report here.


      •

    • Here is the update #1 report poster as presented in my initial Earthquake Report here.


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    • Here is the update #1 report poster for the M 7.1 Puebla, Mexico earthquake (which shows the coulomb stress modeling from Tremblor).


      •

    • Here is Dr. Polet’s tweet of this map.

    • AND an updated map and cross section.

    • Here is Dr. Hayes’ tweet of his map and cross section.

    • Here is Dr. Hayes’ tweet of his static coulomb change map.

    • The discussion about what is a triggered earthquake and what is an aftershock, as I mentioned above, is the topic of discussion between experts in the field. The debate will probably be enduring for a quite a while, especially since classification systems are a social construct and have no real basis in reality (or physics). Classification systems are an excellent example of how science is subjective (science is fundamentally subjective, but that is a longer discussion, read some Karl Popper for more insight). This discussion led me to a web post following the 2011 Christchurch earthquake sequence. Here is a website with one view on this debate, prepared by Dr. Chris Rowan. I present two of their figures below.




      •

    References:

    • 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.
    • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011 b. Seismicity of the Earth 1900–2010 Caribbean plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-A, scale 1:8,000,000.
    • 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
    • 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.
    • Lay et al., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
    • 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
    • Symithe, S., E. Calais, J. B. de Chabalier, R. Robertson, and M. Higgins, 2015. Current block motions and strain accumulation on active faults in the Caribbean in J. Geophys. Res. Solid Earth, v. 120, p. 3748–3774, doi:10.1002/2014JB011779.

    Earthquake Report: Puebla, Mexico Update #1

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    Well, the responses of people who are in the midst of a deadly disaster have been inspiring, bringing tears to my eyes often. Watching people searching and helping find survivors. This deadly earthquake brings pause to all who are paying attention. May we learn from this disaster with the hopes that others will suffer less from these lessons.
    I have been discussing this earthquake with other experts, both online (i.e. the twitterverse, where most convo happens these days) and offline. Many of these experts are presenting their interpretations of this earthquake as it may help us learn about plate tectonics. While many of us are interested in learning these technical details, I can only hope that we seek a similar goal, to reduce future suffering. Plate tectonics is a young science and we have an ultra short observation period (given that the recurrence of earthquakes can be centuries to millenia, it may take centuries or more to fully understand these processes).
    Here I present a review of the material that I have seen in the past day and how I interpret these data. The main focus of the poster is a comparison of ground shaking for three earthquakes. Also of interest is the ongoing discussion about how the 2019.09.08 M 8.1 Chiapas Earthquake and this M 7.1 Puebla Earthquake relate to each other. My initial interpretation holds, that the temporal relations between these earthquakes is coincidental (but we now have the analysis to support this interpretation!).

    • There are some reasons why these earthquakes are unrelated.
      1. They are too distant (static triggering is often limited to 1-2 fault lengths from the first earthquake).
      2. The Cocos plate (CP) changes shape between these two earthquakes, so it is complicated. The CP dips at a steep angle in the Chiapas region, while it dips at a shallow angle (about flat in places) further north. The Tehuantepec Ridge (TR) has an age offset and this may affect how the CP behaves differently on either side of the TR (mostly a fracture zone, but I need to look into this more, it may be thickened crust for some reason other than simply due to the fracture zone here).
      3. Dynamic triggering is when faults slip because they have increased stress as seismic waves travel through them. There is some work suggesting that these seismic waves can change the fluid pressures for a transient time period, possibly triggering earthquakes for a period after the seismic waves have already passed. The M 7.1 did not happen while the seismic waves were traveling following the M 8.1, so the M 7.1 is probably not due to dynamic triggering.
    • There is one major reason the ground shaking is amplified in the region of Mexico City. Prior to the arrival of the Spanish, the first peoples here lived at the shores of a large lake. They farmed on floating islands made of reeds and other material. Eventually the lake filled with sediment and turned into land, until the lake was gone. Given that Mexico City has the largest population of any city on Earth, as it was developed, the ground water was probably drained to facilitate the construction of large buildings (but I don’t know as much about this part of the history). I include a video about why water saturated sediments (i.e. sand and mud) can amplify seismic waves and intensify ground shaking.

    Below is my interpretive poster for this earthquake

    I plot the USGS seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS epicenters from 1917-2017 for magnitudes M ≥ 7.0. I include the USGS fault plane solution for the 1985 earthquake. I also include the USGS moment tensor for the 2017.09.08 M 8.1 earthquake.

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

      I include some inset figures in the poster.

    • In the upper left corner I include a generalized plate tectonic map from Wikimedia Creative Commons here.
    • In the upper right corner are two map insets. The upper one is a map that includes the USGS MMI contours for the M 7.1 earthquake and the lower one is the same for the 1985 M 8.0 earthquake. I have outlined the area of Mexico City with a white dashed line. I created polygons for the higher MMI contours in the region of Mexico City and colored them with respect to these MMI valaues. For the M 7.1 earthquake, MMI VI is shown in yellow and MMI VI.5 is shown in darker yellow. For the 1985 earthquake, MMI VI is shown, but MMI VI.5 is not modeled for Mexico City. The take away: the M 7.1 potentially caused greater ground shaking in the Mexico City region than did the 1985 earthquake.
    • In the lower left corner is a comparison of three ground motion model results from the Instituto De Ingenieria. From left to right are the 1985 M 8.0, the 2017 M 8.1, and the 2017 M 7.1 earthquakes. There are a variety of model results for these earthquakes, but I selected the results shown for a 1 second period (the period of seismic waves) because this is a frequency of seismic waves that multi story buildings can be sensitive to (see educational video about resonance below for more on this). Note that the largest ground motions are from the M 7.1 earthquake. The 1985 was quite deadly and damaging, with between 6,000 and 12,000 deaths. If this M 7.1 earthquake had occurred in 1985, there probably would have been even more damage and a higher casualty number.
    • Above these comparison maps is a figure prepared by Temblor here, a company that helps people learn and prepare to be more resilient given a variety of natural hazards. This figure is the result of numerical modeling of static coulomb stress changes in the lithosphere following the 2017 M 8.1 earthquake. This basically means that regions that are red have an increased stress (an increased likelihood for an earthquake) following the earthquake, while blue represents a lower stress, or likelihood. The change in stress are very very small compared to the overall stress on any tectonic fault. This means that an earthquake may be triggered from this change in stress ONLY IF the fault is already highly strained (i.e. that the fault is about ready to generate an earthquake within a short time period, like a day, month, or year or so). The take away: the M 8.1 earthquake did not increase the stress on faults in the region of the M 7.1 (Temblor suggests the amount of increased stress near the M 7.1 is about the amount of force it takes to snap one’s fingers.


    • Here is my original interpretive poster.


      •

    • As I mentioned the lake basin, here are some figures addressing that.
    • Here is a figure showing the thickness of the lake sediments here (Cruz-Atienza et al., 2016).


      • Topographic setting of Mexico City (MC) and the Valley of Mexico. Color scale corresponds to the basin thickness (i.e., the basin contact with the Oligocene volcanics of the Transmexican Volcanic Belt, TMVB). Stars show the epicenters for the vertical body forces applied at the free surface (green) and the magnitude 3.4 earthquake of December 1, 2014 (red). This figure has been created using the Generic Mapping Tools (GMT) Version 5.3.0, http://gmt.soest.hawaii.edu.

    • This is also from Cruz-Atienza et al. (2016) which shows their modeled seismic waves traveling through the basin.


      • Snapshots of the Green’s function for the vertical body force S6 (see Fig. 1) described by the inset time history with flat spectrum up to 1 Hz. Notice the topographic scattering, the generation and propagation of wave trains at different speeds within the basin, and their multiple diffractions. This figure has been created using the Matlab software Version R2016a, http://www.mathworks.com/.

    • Finally, here is a compilation of their model results showing how the lake basin sediments both amplify the ground motions (upper right panel) and increase their duration (lower right panel). Basically, the lake acts like a bowl of Jello.


      • (a,c) Comparison of average eigenfunctions for the 8 sources with standard deviation bars for both elastic (blue solid) and viscoelastic (red solid) simulations at two representative sites, P1 and P2, and different frequencies. Dashed lines show theoretical eigenfunctions for the vertical component of Rayleigh waves in the model of Figure A1a (Table A1) for the fundamental mode (blue) and the first (red) and second (green) overtones. Normalized peak vertical displacements observed in different boreholes (green dots in Fig. 1) are shown with black circles and error bars (after Shapiro et al., 2001). (b) Fourier spectral amplifications (geometric mean of both horizontal components) at 0.5 Hz with respect to the CUIG site (Fig. 1) averaged for the 8 sources. The black contour corresponds to the 2 s dominant-period. (d) Duration of the strong shaking phase for f < 1 Hz averaged for the 8 sources.

    • Here is an educational animation from IRIS that helps us learn about how different earth materials can lead to different amounts of amplification of seismic waves. Recall that Mexico City is underlain by lake sediments with varying amounts of water (groundwater) in the sediments.

    • Here is an educational video from IRIS that helps us learn about resonant frequency and how buildings can be susceptible to ground motions with particular periodicity, relative to the building size.

    • So, bringing this work as applied to this earthquake, Dr. Jascha Polet prepared this map that shows the outline of the lake and the locations of damaged and collapsed buildings. Note the correlation. Below the map, I include her tweet.


    • Here are some figures that show how the subduction zone varies across the Tehuantepec Ridge. More about this in my initial report, as well as in my reports for the M 8.1 earthquake.
    • This is a figure showing the location of the Tehuantepec Ridge (Quzman-Speziale and Zunia, 2015).


      • Tectonic framework of the Cocos plate convergent margin. Top- General view. Yellow arrows indicate direction and speed (in cm/yr) of plate convergence, calculated from the Euler poles given by DeMets et al. (2010) for CocoeNoam (first three arrows, from left to right), and CocoeCarb (last four arrows). Length of arrow is proportional to speed. Red arrow shows location of the 96 longitude. Box indicates location of lower panel. Bottom- Location of features and places mentioned in text. Triangles indicate volcanoes of the Central American Volcanic Arc (CAVA) with known Holocene eruption (Siebert and Simkin, 2002).

    • Here is another figure, showing seismicity for this region (Quzman-Speziale and Zunia, 2015).


      • Seismicity along the convergent margin. Top: Map view. Blue circles are shallow (z < 60 km) hypocenters; orange, intermediate-depth (60 < z < 100 km); yellow, deep (z > 100 km). Next three panels: Earthquakes as a function of longitude and magnitude for shallow (blue dots), intermediate (orange), and deep (yellow) hypocenters. Numbers indicate number of events on each convergent margin, with average magnitude in parenthesis. Gray line in this and subsequent figures mark the 96 deg longitude.

    • This shows the location of the cross sections. The cross sections show how the CP changes dip along strike (from north to south) (Quzman-Speziale and Zunia, 2015).


      • Location of hypocentral cross-sections. Hypocentral depths are keyed as in previous figures.

    • Here are the cross sections showing the seismicity associated with the downgoing CP (Quzman-Speziale and Zunia, 2015).


      • Hypocentral cross-sections. Depths are color-coded as in previous figures. Dashed lines indicate the 60-km and 100-km depths. Tick marks are at 100-km intervals, as shown on the sections. There is no vertical exaggeration and Earth’s curvature is taken into account. Number of sections refers to location on Fig. 3.

    • This figure shows thrust and normal earthquakes for three ranges of depth (Quzman-Speziale and Zunia, 2015).


      • Earthquake fault-plane solutions from CMT data. a. Shallow (z < 60 km), thrust-faulting mechanisms. b. Intermediate-depth (60 < z < 100 km) thrust-faulting events. c. Deep (z > 100 km), thrust-faulting earthquakes. d. to f. Normal-faulting events, in same layout as for thrust-faulting events.

    • Here are three figures from Tremblor.net, one of which is in the interpretive poster. These are the analyses I was discussing that we needed to see in my initial report. More detailed discussion can be found here.


    • This figure shows that there are not many earthquakes in the region between the M 8.1 and M 7.1 earthquakes. This is supporting evidence that there was not a significant increase in stress in this region (independent negative evidence for static triggering of the M 7.1 from the M 8.1).


    • This figure shows their modeling of the subduction zone in the region of the M 8.1 earthquake. I queried whether the megathrust had an increased stress following the M 8.1 earthquake. Part of the megathrust here ruptured in 1902, but the rest of the “Tehuantepec Gap” does not have an historic record (since ~1600 AD). Note how the megathrust is mostly blue, suggesting a lower likelihood of rupture. There is a narrow band of increased stress (in red). This model uses the finite fault model from Dr. Gavin Hayes (USGS).


      •

    • As far as the likelihood of dynamic triggering (increased stress on faults while seismic waves are travelling through them), here is an analysis that helps us visualize this. This analysis (Pollitz et al., 2012) shows regions of increased dynamic stress following the 2012 Wharton Basin earthquakes. The lower spheres show seismicity for a time period following the earthquakes and note how they align with the red areas, areas of increased dynamic stress.


      • The 2012 M = 8.6 mainshock and M = 8.2 aftershock fault ruptures and maps of strain duration tstrain at a threshold value of 0.1 microstrain. a, Inferred fault ruptures of the 11 April 2012 M = 8.6 east Indian Ocean earthquake and an M = 8.2 aftershock that occurred 2 h later. Superimposed are the first 20 d of M > 4.5 aftershocks of 0–100-km depth. These earthquakes probably ruptured a complex set of subparallel and conjugate faults with the indicated sense of motion (arrows). Parts of the rupture areas of the 2004 M = 9.2 and 2005 M = 8.7 Nias earthquakes on the Sunda megathrust are indicated. b, c, Global maps of tstrain (colour scale). Superimposed are the epicentres of M>5.5 events that occurred during the 6 d preceding the mainshock (2 epicentres) and following the mainshock (24 epicentres, 16 of which are remote, that is, .1,500km from the mainshock). Focal mechanisms of six post-mainshock events with near-vertical strike-slip mechanisms (plunge of neutral axis, >60 deg) are indicated with red beachballs. The 9:00:09 11 April 2012 M = 5.5 event (in the western Aleutian Islands) occurred 21 min 33 s after the mainshock between the direct P- and S-wave arrivals from the mainshock; all others are delayed by hours to days. The focal mechanism of the mainshock is plotted at its epicentre.

    • Here is the comparison I put together for the ground motion modeling presented in the poster above.


      •

    • Here is a really cool video that shows the seismic record of Hurricane Maria and the M 7.1 earthquake are recorded by seismometers (prepared by . The top panel shows the seismograph. The middle panel shows a spectrogram of these seismic data (showing the frequency content of the seismic waves). The lower panel shows the position of the Hurricane and M 8.1 earthquake epicenter (they should have shown the M 7.1, but that is not important. The audio is a conversion of the seismic data into sound. Here is the 1 MB mp4 file for downloading. This was prepared by Zhigang Peng from Georgia Tech for the station IU.SJG — San Juan, Puerto Rico. This is posted on the IRIS special event page. note: the hurricane and this earthquake are NOT RELATED!

    References:

    • 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.
    • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011 b. Seismicity of the Earth 1900–2010 Caribbean plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-A, scale 1:8,000,000.
    • Cruz-Atienza et al., 2016. Long Duration of Ground Motion in the Paradigmatic Valley of Mexico in Scientific Reports, v. 6, DOI: 10.1038/srep38807
    • 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
    • Gérault, M., Husson, L., Miller, M.S., and Humphreys, E.D., 2015. Flat-slab subduction, topography, and mantle dynamics in southwestern Mexico in Tectonics, v. 34, p. 1892-1909, doi:10.1002/2015TC003908.
    • Quzman-Speziale, M. and Zunia, F.R., 2015. Differences and similarities in the Cocos-North America and Cocos-Caribbean convergence, as revealed by seismic moment tensors in Journal of South American Earth Sciences, http://dx.doi.org/10.1016/j.jsames.2015.10.002
    • 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.
    • Lay et al., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
    • 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
    • Pérez-Campos, Z., Kim, Y., Husker, A., Davis, P.M. ,Clayton, R.W., Iglesias,k A., Pacheco, J.F., Singh, S.K., Manea, V.C., and Gurnis, M., 2008. Horizontal subduction and truncation of the Cocos Plate beneath central Mexico in GRL, v. 35, doi:10.1029/2008GL035127
    • Polltz, F.F., Stein, R.S., Sevigen, V., Burgmann, R., 2012. The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide in Nature, v. 000, doi:10.1038/nature11504
    • Symithe, S., E. Calais, J. B. de Chabalier, R. Robertson, and M. Higgins, 2015. Current block motions and strain accumulation on active faults in the Caribbean in J. Geophys. Res. Solid Earth, v. 120, p. 3748–3774, doi:10.1002/2014JB011779.

    Earthquake Report: Chiapas, Mexico Update #1

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    Well, after about 4 hours sleep, my business partner woke me up to talk about the fire alarms we were installing in a rental (#safetyfirst). Now that I have had some breakfast, I here provide some additional observations that people have made since I prepared my initial report.
    Below I present some figures about the Tehuantepec Seismic Gap (as before, but with additional figures). The impetus for this is two fold: (1) it is interesting for earthquake geologists as they consider earthquake recurrence patterns, globally and (2) that the M 8.1 earthquake was not a subduction zone earthquake and may have loaded the megathrust.

    1. The topic of seismic gaps is a long conversation that I don’t currently have the time to delve into (need to grout some tile, paint some trim, edit a conference paper, etc.). But I will revisit this later. If one wants to read the latest science about seismic gaps, check out the papers regardign the Shumagin Gap. There are now papers that suggest the gap exists and that the gap does not exist. More on that some other time.
    2. The M 8.1 earthquake was an extensional earthquake in the downgoing Cocos plate. While a formal analysis needs to occur, I hypothesize that the megathrust fault probably has an increased coulomb stress following the M 8.1 earthquake. I present a figure below from Lay et al. (2011) that is an imperfect analogy. Others will need to do the modeling. Either way, the region will hopefully be continued to be prepared for a subduction zone earthquake in this region. It is possible that this region may not have Great subduction zone earthquakes (M > 8), but as always, hope for the best and plan for the worst.

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS epicenters from 1917-2017 for magnitudes M ≥ 8.0. I include fault plane solutions for the 1985 earthquake (along with the MMI contours for that earthquake, see below for a discussion of MMI contours).
    I outline the region of seismicity from June 2017. I include posters and links to the reports from that sequence below.
    I also outline the region of the megathrust where the Tehuantepec Seismic Gap is located, generally in the region of the M 7.8 1902 megathrust earthquake. The Tehuantepec Ridge is a player in the regional tectonics as I discussed on my report earlier today.

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. Note how much larger the MMI intensity is for this earthquake, compared to the 1985 and 1995 earthquakes.
    • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

      I include some inset figures in the poster.

    • In the lower left corner is a figure from Franco et al. (2012) that shows the tectonic plate boundaries in this region. I place a blue star in the general location of this M 8.1 earthquake (as below).
    • In the upper right corner is a figure from McCann et al. (1979) showing the major subduction zone earthquakes associated with the subduction zone that forms the Middle America Trench. This is the first (?) acknowledgement for the potential of a seismic gap in the region of today’s M 8.1 earthquake.


    • Here is the initial report poster as presented in my initial Earthquake Report here.


      •

    • Here are two views of the earthquake as recorded on Humboldt State University Department of Geology’s Baby Benioff seismometer. The photos are from Dr. Lori Dengler and were taken in the hallway in Van Matre Hall. Click on the image for a high res version (2 and 5 MB files).




      •

    • Here is the Franco et al. (2012) tectonic map.


      • 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.

    • Here is an updated list of observations of the trans-Pacific tsunami.


      •

    • This is an update of the tide gage record at Salina Cruz in Oahaca, Mexico. It has been 15 hours and the tsunami waves are still significant (not as large amplitude as the initial few waves, but still potentially dangerous).


      •

    • Here is the McCann et al. (1979) summary figure.


      • 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.
    • In 1902, there was an M 7.8 earthquake in the same region as tonight’s M 8.1. 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.

    • Here is a cross section showing focal mechanisms for this region, prepared by Dr. Mike Brudzinski.


      •

    • Below are a map and cross section showing focal mechanisms for this region from Dr. Jascha Polet.




    • Late breaking update from Jascha. Dr. Polet plotted the aftershocks to find that there is a clear NW rupture trend. Dr. Polet also points out that the aftershocks are in a narrow band, suggesting a steeply dipping fault. Also, they suggest that this is a short rupture (length ~200km) for a M 8.1 earthquake.


      •

    • Here is a figure from Lay et al, (2011) that shows how stresses change on various types of faults given slip on the megathrust. A very simplistic way of applying this to today’s scenario would be that instead of an increasted stress on normal faults imparted by the megathrust earthquake (as in Japan), that there may be an increased coulomb stress on the megathrust imparted by the normal fault earthquake (as in Mexico). This is pure arm waving (as one would expect from a geologist), but I hope that someone does the analysis soon. I suspect that Tremblor.com will have something on this very soon.


      •

    References:

    • 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.
    • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011 b. Seismicity of the Earth 1900–2010 Caribbean plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-A, scale 1:8,000,000.
    • 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
    • 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.
    • Lay et al., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
    • 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
    • Symithe, S., E. Calais, J. B. de Chabalier, R. Robertson, and M. Higgins, 2015. Current block motions and strain accumulation on active faults in the Caribbean in J. Geophys. Res. Solid Earth, v. 120, p. 3748–3774, doi:10.1002/2014JB011779.

    Earthquake Report: Chiapas, Mexico

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    While I was spending time with my friend Steve Tillinghast (he is getting married on Saturday), there was a Great Earthquake offshore of Chiapas, Mexico. This is one of four M 8 or greater earthquakes ever recorded along the subduction zone forming the Middle American Trench. There has recently been some seismic activity to the east of this current M 8.1 earthquake. These earthquakes happened near the boundary between the North America (NAP) and Caribbean (CP) upper plates.
    This M 8.1 earthquake happened in a region of the subduction zone that is interpreted to have a higher coupling ratio than further to the south (higher proportion of the plate convergence rate is accumulated as elastic strain due to seismogenic coupling of the megathrust fault). Faults that are aseismic (fully slipping) have a coupling ratio of zero. The Polochic-Motagua fault zone marks this NAP-CP boundary. The recent seismicity offshore of Guatemala (June 2017) comprised a series of thrust earthquakes along the upper megathrust, along with some down-dip extensional faulting.
    Tonight’s earthquake will be a very damaging and deadly earthquake and, based upon the shake map, possibly more damaging than either the 1985 or 1995 earthquakes. The 1985 earthquake caused severe damage in Mexico City. The PAGER alert shows an estimate of 34% probability for between 1000 and 10,000 fatalities. However, please read below about the PAGER alert and go to the USGS website about PAGER alerts (link below). These are just model based estimates of damage, so we won’t really know the damage until this is evaluated with “boots on the ground.” One might consider PAGER alerts to be the “armchair estimate” of damage. Thanks to Dr. Lori Dengler for reviewing my report (though any mistakes are only to be credited to me).
    This M 8.1 earthquake is deeper than the megathrust fault and has an extensional moment tensor. This is not a megathrust earthquake, but is related to slip on a fault in the downgoing Cocos plate. At this depth, it may be due to bending in the downgoing oceanic lithosphere.
    There is no danger of a tsunami here along the west coast of the U.S. West Coast, British Colombia, or Alaska. There have been some tsunami observations

    • Here are the USGS web pages for these four Great Earthquakes.
    • 1932.06.03 M 8.1 Jalisco, Mexico
    • 1985.09.19 M 8.0 Colima, Mexico
    • 1995.10.09 M 8.0 Colima, Mexico
    • 2017.09.08 M 8.1 Chiapas, Mexico

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS epicenters from 1917-2017 for magnitudes M ≥ 8.0. I include fault plane solutions for the 1985 and 1995 earthquakes (along with the MMI contours for those earthquakes, see below for a discussion of MMI contours).

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. Note how much larger the MMI intensity is for this earthquake, compared to the 1985 and 1995 earthquakes.
    • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

      I include some inset figures in the poster.

    • In the lower left corner is a map from Mann (2007) that shows the regional tectonics. Plate boundary faults are in bold line, while lineations representing the spreading history are represented by thinner lines. I place a blue star in the general location of tonight’s M 8.1 earthquake (also in other inset maps).
    • in the upper right corner is a map showing USGS seismicity in this region (Benz et al., 2011 a). To the left I present their cross section B-B’ showing USGS hypocenters. This cross section is shown as a blue line on the map.
    • In the upper left corner is the USGS fault slip model.
    • In the lower right corner is a map that shows the Tehuantepec Ridge and how the oceanic lithosphere varies across this boundary. The crust has different ages on either side of this transform fault, which can control the stresses imposed upon the fault on either side of this fault.


    • Here is the same poster, but shows seismicity with earthquakes of smaller magnitudes (M 7).


      •

    • As mentioned above, this earthquake has the potential to cause much harm to people and their belongings and infrastructure. Below is the USGS report that includes estimates of damage to people (possible fatalities) and their belongings from the Rapid Assessment of an Earthquake’s Impact (PAGER) report. More on the PAGER program can be found here. An explanation of a PAGER report can be found here. PAGER reports are modeled estimates of damage. On the top is a histogram showing estimated casualties and on the right is an estimate of possible economic losses. This PAGER report suggests that there will be quite a bit of damage from this earthquake (and casualties). This earthquake has a high probability of damage to people and their belongings.


      •

    • Here is the threat forecast from the Pacific Tsunami Warning Center. The below all come from this report.

      • TSUNAMI THREAT FORECAST
      ———————–
      * TSUNAMI WAVES REACHING MORE THAN 3 METERS ABOVE THE TIDE
      LEVEL ARE POSSIBLE ALONG SOME COASTS OF
      MEXICO.
      * TSUNAMI WAVES REACHING 0.3 TO 1 METERS ABOVE THE TIDE LEVEL
      ARE POSSIBLE FOR SOME COASTS OF
      AMERICAN SAMOA… ANTARCTICA… COOK ISLANDS… ECUADOR…
      EL SALVADOR… FIJI… FRENCH POLYNESIA… GUATEMALA…
      KIRIBATI… NEW ZEALAND… SAMOA… TOKELAU… TUVALU…
      VANUATU… AND WALLIS AND FUTUNA.
      * TSUNAMI WAVES ARE FORECAST TO BE LESS THAN 0.3 METERS ABOVE
      THE TIDE LEVEL FOR THE COASTS OF
      AUSTRALIA… CHILE… CHINA… CHUUK… COLOMBIA… COSTA
      RICA… GUAM… HAWAII… HONDURAS… HOWLAND AND BAKER…
      INDONESIA… JAPAN… JARVIS ISLAND… JOHNSTON ATOLL…
      KERMADEC ISLANDS… KOSRAE… MALAYSIA… MARSHALL
      ISLANDS… MIDWAY ISLAND… NAURU… NEW CALEDONIA…
      NICARAGUA… NIUE… NORTHERN MARIANAS… NORTHWESTERN
      HAWAIIAN ISLANDS… PALAU… PALMYRA ISLAND… PANAMA…
      PAPUA NEW GUINEA… PERU… PHILIPPINES… PITCAIRN
      ISLANDS… POHNPEI… RUSSIA… SOLOMON ISLANDS…
      TAIWAN… TONGA… VIETNAM… WAKE ISLAND… AND YAP.

    • Here are the arrival time estimates. The below all come from this report.

      • ESTIMATED TIMES OF ARRIVAL
      ————————–
      * ESTIMATED TIMES OF ARRIVAL -ETA- OF THE INITIAL TSUNAMI WAVE
      FOR PLACES WITHIN THREATENED REGIONS ARE GIVEN BELOW. ACTUAL
      ARRIVAL TIMES MAY DIFFER AND THE INITIAL WAVE MAY NOT BE THE
      LARGEST. A TSUNAMI IS A SERIES OF WAVES AND THE TIME BETWEEN
      WAVES CAN BE FIVE MINUTES TO ONE HOUR.

    • Here are the tsunami observations. The below all come from this report.

      • TSUNAMI OBSERVATIONS
      ——————–
      * THE FOLLOWING ARE TSUNAMI WAVE OBSERVATIONS FROM COASTAL
      AND/OR DEEP-OCEAN SEA LEVEL GAUGES AT THE INDICATED
      LOCATIONS. THE MAXIMUM TSUNAMI HEIGHT IS MEASURED WITH
      RESPECT TO THE NORMAL TIDE LEVEL.

    • 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 Manea and Manea (2007) map showing the lithospheric contrast across the Tehuantepec Ridge, a fracture zone.


      • Generalized tectonic map of the study area. Transparent red zones show the location of active volcanic belts in México: CMVB — Central Mexican Volcanic Belt, MCVA — Modern Chiapanecan Volcanic Arc. Transparent gray area: the extinct Sierra Madre Miocenic Arc. Orange stars are the El Chichón and San Martin active volcanoes. EPR — East Pacific Rise. MAT — Middle American Trench. Right black dashed line with a question mark is the hypothetical prolongation of Polochic–Montagua fault system which represents the limit between North America (NAM) and Caribbean plates. White dashed line is the onshore prolongation of Tehuantepec Ridge. Onshore white contours represent the slab isodepths (Bravo et al., 2004; Pardo and Suarez, 1995). Arrows show convergence velocities between the Cocos and North American plates (DeMets et al., 1994). TR1 and TR2 are the cross-sections where we calculate the thermal structure across TR (Fig. 10). Cocos plate ages are from Manea et al. (2005). White line dashed squares show the location of magnetic and gravity maps (Figs. 3, 5, 6). Blue dots represent continental heat-flow measurements (mW/m2) from Ziagos et al. (1985).

    • This figure from Rebollar et al., 1999 shows focal mechanisms for some earthauakes in this area. Earthquake 6C has a similar mechanism and location as tonight’s M 8.1 earthquake.


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    • However, this is the cross section from Rebollar et al. (1999) that shows earthquake 6C to be much much shallower.


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    • In 1902, there was an M 7.8 earthquake in the same region as tonight’s M 8.1. 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.

    • Initially, I forgot to include these figures from Franco et al. (2012). These figures show various estimates of the amount of seismogenic coupling along the megathrust. I include these because the M 8.1 earthquake happened in this region of higher seismogenic coupling.


      • Inverted coupling coefficients along the MFe, MFc, MATch and MATgs, and residual velocities for best-fitting 3B model.


      Same as Fig. 7 for best-fitting 4B model, with coupling along VAF fixed to 1.


      Proposed model of faults kinematics and coupling along the Cocos slab interface, revised from Lyon-Caen et al. (2006). Numbers are velocities relative to CA plate in mmyr−1. Focal mechanisms are for crustal earthquakes (depth ≤30 km) since 1976, from CMT Harvard catalogue.

    References:

    • 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.
    • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011 b. Seismicity of the Earth 1900–2010 Caribbean plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-A, scale 1:8,000,000.
    • 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
    • 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.
    • 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
    • Symithe, S., E. Calais, J. B. de Chabalier, R. Robertson, and M. Higgins, 2015. Current block motions and strain accumulation on active faults in the Caribbean in J. Geophys. Res. Solid Earth, v. 120, p. 3748–3774, doi:10.1002/2014JB011779.