Category Archives: caribbean

Earthquake Report: M 7.2 in Haiti

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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.
On 14 August ’21 there was a magnitude M 7.2 oblique strike-slip earthquake in Haiti. This earthquake was along the Enriquillo-Plantain Garden fault zone, which also ruptured in 2010. Here is my report for the 2010 Haiti earthquake (see more about the tectonics of this region of the world).
https://earthquake.usgs.gov/earthquakes/eventpage/us6000f65h/executive

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 with magnitudes M ≥ 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 lower right corner is a small scale plate tectonic map.
  • Above that map is a plot showing the USGS finite fault slip model. This shows the location of the fault and color represents how much the fault slipped during the earthquake.
  • In the upper right corner is a map that compares the USGS earthquake intensity models (the contoured lines) with the USGS Did You Feel It? observations from real people.
  • In the lower center is a map that shows the aftershocks from the M 7.2 earthquake and from the 2010 M 7.0 earthquake.
  • In the upper left are two maps that show models of earthquake triggered landslides and earthquake induced liquefaction for this M 7.2 event. Read more about these models here.
  • Here is the map with 3 month’s seismicity plotted.


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Earthquake Aftershocks

  • Below a map showing the aftershocks from the 2021 M 7.2 and 2010 M 7.0 Haiti earthquakes.


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Potential for Ground Failure

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

    FOS = Resisting Force / Driving Force

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


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


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

  • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides.
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.
  • Below are maps showing a comparison between the USGS modeled earthquake triggered landslides and liquefaction potential with the Centre Nationale De Information Géospatiale (CNIGS) probabilistic models of ground failure.


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    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: Cayman Islands

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Contrary to what some people spread around on the internets (some of them major earthquake experts), strike-slip earthquakes can and do generate tsunami (just like this one). More on this below.
I am in Portland, Oregon this week, attending the Winter National Tsunami Hazard Mitigation Program Meeting. While one of our workshops, several of us got an alert about a M 7.3 earthquake offshore of Cuba and Jamaica. My colleagues from Puerto Rico were immediately interested to learn more about this. We noticed that nothing was being posted to tsunami.gov.
https://earthquake.usgs.gov/earthquakes/eventpage/us60007idc/executive
The location is familiar with me as I have written reports for earthquakes in this region over the past couple of years. This earthquake happened along a strike-slip plate boundary fault. Thus, the chance of a large tsunami is low. However, strike-slip earthquakes DO generate tsunami, albeit smaller than those created by subduction zone earthquakes. In addition, earthquakes can trigger submarine landslides, which can also serve to cause tsunami (these can be very large, but generally impact the area near the landslide, like the 1998 Papua New Guinea tsunami.).
In a few minutes, the earthquake magnitude was updated to M 7.7. This is quite common, as seismological data are analyzed with greater detail after the initial automatic magnitude calculation.
A few minutes later, the USGS moment tensor (earthquake mechanism) was posted online, confirming that it was a strike-slip earthquake.
The IOC tide gage network was not working, so I could not check for tsunami observations until later. However, the Pacific Tsunami Warning Center sent out an email to the International Tsunami Bulletin Board (email list restricted to tsunami scientists) with arrival times. There was a suggestion that tsunami waves up to 1 meter may arrive along the coast in the region.
At lunch time, I went to my hotel room to put together an interpretive poster (thanks boss!) to send out on social media. By that time, a small tsunami wave had been observed at the tide gage on the west coast of Cayman Island. The PTWC sent out their final email, stating a 0.4 foot tsunami was recorded there. I went to the IOC website and the gage data were quite noisy, but it matched the PTWC email. Here is a link to the George Town Tide Gage.
Just as I was about to tweet the poster, the USGS earthquake fault slip model was published online (so I added that to the poster).
After lunch, as the workshop continued, there was a M 6.1 earthquake. I noticed it was west of the slip model. I had considered an alternate hypothesis (that the M 6.1 was triggered, not an aftershock), but now think that this is just part of the M 7.7 slip patch. Looking at the back projection data from IRIS, it suggests that this earthquake initiated in the east and propagated to the west. It makes sense to me that the fault reached a zone where the fault slip slowed down, until it reached the patch that slipped during the M 6.1. (simplifying this for this report)
Was this Cayman Islands Earthquake Sequence related to the ongoing Puerto Rico Earthquake Sequence? Probably not. They are simply too far from each other.
There are two types of earthquake triggering: static and dynamic. Triggering happens when an earthquake on one fault changes the stress on a different fault, causing that other fault to slip during an earthquake. These stress changes are small, so the “receiver” fault needs to be at a state of stress that is high enough that it would be almost ready to slip before the “source” earthquake.
Dynamic triggering happens when seismic waves from the source earthquake travel through the Earth, triggering an earthquake on the receiver fault. These changes in stress may take a while before the triggered earthquake happens, but generally, we think that this would happen while these waves are traveling through the area.
Static triggering happens when a source earthquake changes the stress in the crust surrounding the source earthquake. This change typically lasts months to years and won’t extend beyond two fault lengths of the source earthquake. So, If the source quake had a rupture length of 50 km, static triggering probably would not happen more than 100km from the source quake. This is just a rule of thumb… BUT the M 7.7 is very far from Puerto Rico, so is probably unrelated to the Puerto Rico Sequence.

Earthquake Description

This M 7.7 earthquake happened along the Oriente fault, which is the Septentrional fault further to the east. This fault is one of the boundaries between the North America plate to the north and the Caribbean plate to the south in a region called the Greater Antilles.
Further to the east, this plate boundary changes into a subduction zone along the Lesser Antilles. This subduction zone is the source of a great amount of research. There is some evidence that the megathrust subduction zone fault is not locked, so it is slipping and not capable of generating Great (M>8) earthquakes. However, I was on a team of French geologists aboard the Pourquoi Pas? in 2016. We were coring the deep sea to investigate the sedimentary record of Great earthquakes. Based on our analysis, it appears that the fault is capable of producing these large earthquakes, but the average time between earthquakes (the recurrence interval) is on he order of several millenia.
To the west of the M 7.7 earthquake, there is an oceanic spreading ridge where crust is created, forming the Cayman Trough. As the boundary steps to the south, the relative plate motion is focused on another left-lateral strike-slip fault, the Swan Island fault. This fault extends further to the west into Central America and turns into the Motagua Polochic fault system (there are actually multiple faults hypothesized to be the active part of this plate boundary here). I discuss this more in an Earthquake Report here.

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 ≥ 6.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • 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.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

  • In the lower right corner is a map from Pindell and Kennan (2009) that shows the major plate tectonic faults in the Caribbean. I place a yellow star in the general location of today’s M 7.7 earthquake.
  • In the upper right corner is a map from Symithe et al. (2015). I have used this figure in many of my reports because it is so awesome!!! This map includes the faults of the region, but also includes earthquake mechanisms (e.g. focal mechanisms).
  • To the left of the Symithe map is the USGS model for the earthquake fault that slipped during this earthquake. The color represents the amount of slip. I placed a red line with circles at the end on the map where this fault model is located. Don’t forget, this is just a model (but it matches the data that the USGS uses to constrain the model).
  • In the upper left corner is a map that shows a comparison of the USGS model for shaking intensity (using the Modified Mercalli Intensity (MMI) scale) and the USGS “Did You Feel It?” observations.
    • The shaking intensity model is based on the results of analyzing thousands of earthquakes and using these earthquakes to develop a relation between earthquake size (e.g. magnitude) and how strongly it shakes based on the distance to the earthquake.
    • The “DYFI” observations are based on the results of surveys that people submit to the USGS website. The questions people answer are about their observations of the earthquake. Here are some basic facts about this DYFI program and here is some scientific background behind the DYFI program.
  • To the right of the intensity map is a plot of the data from the map. The vertical axis represents intensity (MMI) and the horizontal axis represents the distance from the earthquake. The solid lines represent the model results from the USGS. These are the models that the USGS used to create the color on the map to the left. The DYFI observations are the blue dots (the brown dots show averages ofthe blue dots). There is a decent match, but it is far from perfect.
  • Here is the map with 3 month’s seismicity plotted.


    •

  • Here is the tsunami observation posted by the PTWC. A few years ago, it was conventional wisdom (at least, in my mind) that strike-slip earthquakes were not a producer of tsunami. In the past few years, however, most every large strike-slip submarine earthquake has generated a tsunami. We need to break this old way of viewing this.
  • The main difference for tsunami from strike-slip earthquakes is that they are smaller than from subduction or thrust faults. BUT, even a tsunami with a size of about 2-3 meters can cause millions of dollars of damage. These are still dangerous events, even though they are not as dangerous as larger tsunami.


  • As we can see from the plot below, it will take someone more skilled than I to understand the tsunami waves observed here. However, even I can see that there was a change in water surface elevation at about the right time given the distance to the earthquake from the Cayman Islands.


  • UPDATE: 2020.01.29 – This morning I saw a tweet from Christoph Gruetzner and I realized that I had only reviewed the tide gage data from nearest the quake. Below is a plot from a site in Mexico which clearly shows a tsunami wave train. This is a better record that the one above from Cayman Island. The Cayman gage is located on the western side of the island, not optimal to record waves sourced from the east (why it is so noisy). This Puerto Morelos gage (below) is a much better record, albeit still a small wave that is in a location with significant background wave “noise.”


  • Here is a figure that includes a map showing the location of these two tide gages. I will update this later, gotta get to the meeting today.


    •

Some Relevant Discussion and Figures

  • Here is the tectonic map from Symithe et al. (2015). I include their figure caption below in blockquote.


    • Seismotectonic setting of the Caribbean region. Black lines show the major active plate boundary faults. Colored circles are precisely relocated seismicity [1960–2008, Engdahl et al., 1998] color coded as a function of depth. Earthquake focal mechanism are from the Global CMT Catalog (1976–2014) [Ekstrom et al., 2012], thrust focal mechanisms are shown in blue, others in red. H = Haiti, DR = Dominican Republic, MCS = mid-Cayman spreading center, WP = Windward Passage, EPGF = Enriquillo Plaintain Garden fault.

  • Here is the tectonic map from Garcia-Casco et al. (2011). I include their figure caption below in blockquote.


    • Plate tectonic configuration of the Caribbean region showing the location of the study cases presented in this issue (numbers refer to papers, arranged as in the issue), and other important geological features of the region (compiled from several sources).

  • Here is the Benz et al. (2011) Seismicity of the Earth poster for this region.


    •

  • Here is the map from Mann et a. (1991). Note how today’s earthquake is in an area that may have overlapping faults of different types.


    • A. Tectonic map of Cayman trough region showing strike-slip faults (heavy lines), oceanic crust (gray) in Cayman trough, and magnetic anomaly identifications (numbered bars) (after Rosencrantz et a., 1988). Arrows show relative displacement directions. Fault zones: OFZ – Oriente; DFZ- Dunvale; EPGFZ – Enriquillo-Plantain Garden; WFZ – Walton; SIFZ – Swan Islands; MFZ – Motagua. Bl. Late Miocene reconstruction of Cayman trough. C. Early Miocene reconstruction.

  • Here is the large scale map from ten Brink et al. (2002) showing the bathymetry surrounding the Mid-Cayman Rise.


    • Bathymetry of central Cayman Trough adapted from Jacobs et al. (1989). Contour interval: 250 m. Dotted line: location of gravity transect.

  • Here is the USGS Tectonic Summary for this 2018.01.10 M 7.6 earthquake. A more comprehensive review can be found here.
    • The January 10, 2018, M 7.6 Great Swan Island, Honduras earthquake occurred as the result of strike slip faulting in the shallow crust near the boundary between the North America and Caribbean plates. Early focal mechanism solutions indicate that rupture occurred on a steeply dipping structure striking either west-northwest (right-lateral), or west-southwest (left-lateral). At the location of this earthquake, the North America plate moves to the west-southwest with respect to the Caribbean plate at a rate of approximately 19 mm/yr. Local to the January 10, 2018 earthquake, this motion is predominantly accommodated along the Swan Islands transform fault, a left-lateral structure. The location, depth and focal mechanism solution of today’s earthquake are consistent with rupture occurring along this plate boundary structure, or on a nearby and closely related fault.
    • While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip-faulting events of the size of the January 10, 2018, earthquake are typically about 140×20 km (length x width).
    • Nine other earthquakes of M 6 or larger have occurred within 400 km of the January 10, 2018 event over the preceding century. Previous strong earthquakes along the North America-Caribbean plate boundary in this region include the destructive M 7.5 Guatemala earthquake of February 4, 1976, which resulted in more than 23,000 fatalities. The 1976 earthquake occurred on the Motagua fault, a segment of the plate boundary that lies in southern Guatemala, about 650 km west-southwest of the hypocenter of the January 10, 2018, event. In May 2009, a M 7.3 earthquake occurred along the Swan Island transform fault approximately 300 km west of the January 10, 2018 event. The 2009 earthquake (which was much closer to land than the 2018 event) resulted in 7 fatalities, 40 injuries and 130 buildings being damaged or destroyed.

    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

  • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011. 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
  • 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
  • Mann, P., Tyburski, S.A., and Rosencratz, E., 1991. Neogene development of the Swan Islands restraining-bend complex, Caribbean Sea in Geology, v. 19, p. 823-826.
  • 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.
  • Ten Brink, U.S., Coleman, D.F., and Dillon, W.P., 2002. The nature of the crust under Cayman Trough from gravity in Marine and Petroleum Geology, v. 119, p. 971-987.

Return to the Earthquake Reports page.


Earthquake Report: Puerto Rico!

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Welcome to the next decade of the 21st century. We may look back a decade to review the second most deadly earthquake in the 21st century, from the magnitude M 7.0 Haiti Earthquake on 12 Jan 2010. I put together an overview of this event sequence here.
Since late December, southwestern Puerto Rico has seen a sequence of smaller (M3-5) earthquakes, culminating with the 29 Dec 2019 M 5 which later turned out to be a foreshock (there was also a M 4.7 that was a foreshock to the M5). Then on 6 Jan, there was a M 5.8, which was now the mainshock. Then, on the following day, there was the real mainshock, the M 6.4. Lots of other earthquakes too. The largest aftershock was the M 5.9 on 11 Jan. Below I include some comparisons for the M 6.4 and M 5.9 quakes.
Here is a plot showing the cumulative energy release from this sequence. I used the USGS NEIC earthquake catalog for events M≥0. Time is on the horizontal axis and energy release (in joules) on the vertical axis. For every earthquake, the plot steps up relative to the energy released by that quake.


These earthquakes in Puerto Rico have been deadly and damaging. Many structures there are constructed with soft stories on the ground level (the buildings are uplifted to mitigate hurricane flood hazards). Unfortunately, these soft story structures don’t perform well when subjected to earthquake shaking. Thus, there have been many structure collapses. Luckily, there have been only a few deaths. While we may all agree that having no deaths is best, there could have been more.
The M 6.4 even generated a small tsunami. This was localized and was observed clearly on only one tide gage (The Magueyes Island gage).
Here is the tsunami record, along with a map showing the location of the tide gage in southwestern Puerto Rico. These data are from a site that is my “go-to” website for looking for tsunami in tide gage data. I generally look here first.

USGS Earthquake Event Pages

The latest aftershock forecast was tweeted here. I hope people follow this link to stay up to date on these forecasts.


Here is a screenshot of the forecast updated today (12 Jan 2020). Head to the USGS site to stay up to date.

  • Speaking of aftershocks, here is a tweet that discusses what aftershocks and how we use the temporal distribution of earthquake size to distinguish between a typical foreshock-mainshock-aftershock sequence.
  • The graphic below was prepared by the Swiss Seismological Service and ETH Zurich for their discussion about these two phenomena. There is probably a continuum between these two, but there was some debate about this on the twitterverse today.


    • In so-called ‘earthquake swarms’, numerous earthquakes occur locally over an extended period without a clear sequence of foreshocks, main quakes and aftershocks. The Swiss Seismological Service (SED) registers several of earthquakes swarms every year. They are therefore nothing extraordinary. Swarms usually end after a few days or months. Only seldom does the strength and number of earthquakes increase over time or do occur single, damaging events. How an earthquake swarm develops over time is just as difficult to predict as earthquakes are in general.

    Many earthquake swarms occur in regions with complex contiguous fracture systems. The theory is that they are related to the movement of fluid gases and liquids in the Earth’s crust.

  • Now compare with this figure from Dr. Kasey Aderhold. Dr. Aderhold put this together to compare these earthquakes with the figure above. Sr. Aderhold is who shared that link on social media (in social media section below).


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UPDATE: 2020.02.02 -palindrome day!

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past 2 months, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 5.0.
  • 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.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.
  • Here is the map with 2 month’s seismicity plotted.
  • I digitized Bruna et al. (2015) fault lines. To the southeast of the M 6.4 there is mapped a northeast striking (trending) normal fault that dips to the northwest. This seemed to be the best candidate as a source for the M 6.4 earthquake. The earliest earthquakes were strike-slip oblique-normal events, so initially I thought this was a strike-slip sequence. But, as quakes kept happening, they had more extensional mechanisms.
  • To the east of the hypothetical M 6.4 source normal fault there are 2 pairs of opposing normal faults. These look typical of a transtension configuration (a strike-slip fault setting with fault geometry that includes extension parallel to the strike-slip faults). These 2 pairs of faults appear to be forming tectonic basins. The M 6.4 hypothetical source fault does not have a mapped counterpart, but the location of that hypothetical counterpart would be close to the shoreline (so could have been missed by the marine geologists who mapped the other faults further offshore).
  • Below these interpretive posters, I include an animation from Dr. Anthony Lomax below that shows a better view of this hypothetical fault geometry.


  • This is an earlier poster from 7 Jan, which has a couple inset figures.
    • In the upper left corner is a tectonic overview map from Symithe et al. (2015). I placed a blue star where the M 6.4 is located.
    • In the upper right corner is a regional-scale earthquake fault map from Bruna et al. (2015). The blue star appears again.
    • In the lower right corner I show the Bruna map with seismicity plotted. I georeferenced the Bruna map and labeled some of the faults mapped by Bruna et al. (2015).


  • This is the interpretation poster from the 29 December 2019 M 5.0 earthquake. I included the earthquake from a more zoomed out (small scale) view.
  • In the upper left corner is a general view of the faults in Puerto Rico (Piety et al., 2018). I placed a blue star in the location of the M 6.4 earthquake. There are many more faults plotted in the upper right figure from Bruna et al. (2015).
  • The M 6.4 was the most damaging earthquake in Puerto Rico since the 1918 earthquake as shown on this poster. Note how both the 2020 M 6.4 and the 1918 M 7.1 were normal type (extensional) earthquakes.


  • Here is the interpretive poster for the 2010 Haiti M 7.0 earthquake. Check out how there are more tectonic basins to the west of Puerto Rico.


  • Here is the animation from Dr. Anthony Lomax. He states that he “relocated seismicity M1.0+ using Lin & Huérfano 2011 Min 1D model & NonLinLoc-EDT with station corrections. The animation shows seismicity aligned to dip to the northwest.” This matches the hypothetical source fault mapped by Bruna et al. (2015). VERY COOL!


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Background Information

  • Here is the tectonic map from Symithe et al. (2015). Puerto Rico is in a place where the plate boundary between the North America and Caribbean plates transitions from subduction (to the east, the Lesser Antilles) to transform (to the west, the Greater Antilles). The Lesser Antilles Great (M>8) earthquake recurrence appears to be several thousand years (based on turbidite stratigraphy from our 2016 cruise). We currently don’t know how far west of the Aves Ridge that subduction zone earthquakes happen. It is possible, but the convergence is highly oblique, similar to the northern part of the 2004 Sumatra-Andaman subduction zone earthquake. Interestingly, there is a series of spreading ridges and transform faults to the east of the Sunda trench (in the Andaman Sea), just like there are the same features to the west of the Greater Antilles (e.g. the Cayman Trough).


    • Seismotectonic setting of the Caribbean region. Black lines show the major active plate boundary faults. Colored circles are precisely relocated seismicity [1960–2008, Engdahl et al., 1998] color coded as a function of depth. Earthquake focal mechanism are from the Global CMT Catalog (1976–2014) [Ekstrom et al., 2012], thrust focal mechanisms are shown in blue, others in red. H = Haiti, DR = Dominican Republic, MCS = mid-Cayman spreading center, WP = Windward Passage, EPGF = Enriquillo Plaintain Garden fault

  • This is another map showing earthquake history, fault location, and earthquake slip direction from Calais et al. (2016). Note how the relative plate motion near Puerto Rico is oriented parallel to the plate boundary (the Puerto Rico trench). This suggests that most of the plate motion would result in strike-slip earthquakes. However, the relative motion is oblique, so subduction zone earthquakes are still possibble.


    • Seismicity and kinematics of the NE Caribbean. The inset shows Caribbean and surrounding plates, red arrows show relative motions in cm/yr: a: NEIC seismicity 1974–2015 is shown with circles colored as a function of depth, stars show large (M > 7) instrumental and historical earthquakes; b: red and blue bars show earthquake slip vector directions derived from the gCMT database [www.globalcmt.org], black arrows show the present-day relative motion of the NA plate with respect to the Caribbean.

  • Here are some figures from Bruna et al. (2015). First I present their tectonic overview figure.


    • Contoured bathymetry map of the northeastern Caribbean showing a summarized tectonic setting. Isobaths based on satellite-derived bathymetry gridded at 1 arcminute intervals (Smith and Sandwell, 1997) using the free software Generic Mapping Tools (GMT; Wessel and Smith, 1998). The purple dashed rectangle marks the study area. Thick green arrows show the relative convergence motion between the North American and the Caribbean plates. GPS-derived velocities with respect to the North American plate are shown with thin red arrows, the arrow length being proportional to the displacement rate (Manaker et al., 2008). Error ellipse for each vector represents two-dimensional error, 95% confidence limit. The thick blue dashed line marks the Hispaniola-PRVI block boundary as suggested by ten Brink and Lopez-Venegas (2012). The green area shows the extension of the Muertos thrust belt (Granja Bru~na et al., 2009, 2014, this study). NOAM ¼ North American. CARIB ¼ Caribbean. EPGFZ ¼ Enriquillo-Plantain Garden fault zone. SFZ ¼ Septentrional fault zone. BF ¼ Bunce fault. SB ¼ Sombrero basin. PRVI BLOCK ¼ Puerto RicoeVirgin Islands block. VIB ¼ Virgin Islands basin. MR ¼ Mona rift. IFZ ¼ Investigator fault zone. JS ¼ Jaguey spur. SCR ¼ St. Croix rise. SCI ¼ St. Croix Island. The inset map shows GPS-derived velocities with respect to St. Croix Island (SCI), the arrow length being proportional to the displacement rate (ten Brink and Lopez-Venegas, 2012). Error ellipse for each vector represents two-dimensional error, 95% confidence limit. MI ¼ Mona Island. CI ¼ Culebra Island. STI ¼ St. Thomas Island. AI ¼ Anegada Island. SCI ¼ St. Croix Island. IFZ ¼ Investigator fault zone.

  • Here is a map that shows the major earthquake faults in Puerto Rico (Piety et al., 2018). There are many more.


    • Map of Puerto Rico showing known and possible Quaternary-active faults. Well-located faults are shown by solid lines; inferred fault locations are shown by dashed lines. The northwest end of the Great Southern Puerto Rico fault zone (GSPRFZ) likely follows the Cerro Goden fault, but an alternative location shown by Jansma et al. (2000) and Jansma and Mattioli (2005) is indicated by the dashed lines. The GSPRFZ is shown by double lines because the fault zone mapped in bedrock is up to 2 km wide. Map base is a digital elevation model (DEM) created from 30-m (∼1 arcsec) National Elevation Dataset (NED) (see Data and Resources). Bathymetric contours are from ten Brink et al. (2004).

  • This is the fault map that I used to digitize fault data in my posters above (Bruna et al., 2015). These faults were mapped using bathymetric mapping and seismic reflection analyses.


    • Regional morphotectonic interpretation. Faults picked from the seismic data and correlated along strike with the aid of swath bathymetry data. Thick orange lines mark the major onshore structures (GSPRFZ ¼ Great Southern Puerto Rico fault zone; LVF ¼ Lajas Valley fault). Thin orange lines show the faults mapped by Bawiec (1999). FC ¼ Frederickted canyon. WIFZ ¼ Western sector of the Investigator fault zone. CIFZ ¼ Central sector of the Investigator fault zone. EIFZ ¼ Eastern sector of the Investigator fault zone. PF ¼ Ponce fault. BTF ¼ Bajo Tasmanian fault. CMF ¼ Caja de Muertos fault. CF ¼ Central fault. MPC ¼ Mona passage canyon. R ¼ Recess. S ¼ Salient in the deformation front. Ss ¼ Salient in the deformation front referred in Section 4.1. JP ¼ Jungfern passage. WC ¼ Whiting canyon. VC ¼ Vieques canyon. Z ¼ Bench in the northern flank of St. Croix rise. PRSBF ¼ Puerto Rican sub-basin fault. RR ¼ Relay ramp.W¼ Canyon referred to in Section 4.4.3. Q ¼ 080-oriented fault in Section 4.4.3. T ¼ possible source of the 1867 earthquake (Barkan and ten Brink, 2010) referred in Section 5.2.

  • This plot shows the GPS observations in the Caribbean. Symithe et al. (2015) used these data to estimate the amount of seismogenic coupling (how much the faults are “locked”) in the region.


    • (top) GPS velocities used in the model shown with respect to the North American plate defined by the velocity of 25 GPS sites located in the stable interior of the plate [Calais et al., 2006]. (bottom) GPS velocities shown with respect to the Caribbean plate as defined in the best fit block model described in the text. Error ellipses are 95% confidence. Blue arrows show GPS velocities from Pérez et al. [2001] in Venezuela because of their large uncertainty and the lack of common sites with our solution, which prevents us from rigorously combining them to our solution. They are not used in the model but used to show that they are consistent with the rest of the velocity field.

  • This map shows cross sections of seismicity in the region (Symithe et al., 2015). The profile for Puerto Rico is B-B.’ Note that subduction from the north is reasonable given the seismicity, while subduction from the south is not supported by the seismicity. Recall that the absence of evidence is not evidence of absence and that the Cascadia subduction zone lacks seismicity but we have a 10,000 year record of megathrust subduction zone earthquakes there. In other words, just because there is no seismicity, that does not mean that there is no active subduction potentially leading to subduction zone type earthquakes.


    • Earthquake focal mechanisms [Ekstrom et al., 2012] and locations [Engdahl et al., 1998] along the subduction interface and cross sections showing with a thick black line the position of the Caribbean-North America plate interface used in the model. Other faults are shown with thick dashed black lines. SF = Septentrional fault, PRT = Puerto Rico trench, MT = Muertos trench, LAT = Lesser Antilles trench, NHT = Northern Hispaniola trench. White dots on the map (top) show the vertices of the triangles used to discretize the subduction interface. Grey lines on cross section show the bathymetry with significant vertical exaggeration compared to the earthquake depth scale. The area used for each cross
    section is shown by a black rectangle on the top map.

  • Here is another hypothetical view of the plate configuration from Xu et al. (2015). Note the regions of extension, one to the northwest of Puerto Rico (the Mona Rift, which also just had a large earthquake near the 1918 quake) and the Anegada Passage (AP).


    • Hypothesized model of the tectonic relationships. The PRVI sits between two subducting slabs; the dip angles of the two subducting slabs increase from east to west. The North American Plate splits in the eastern PRVI (modified after ten Brink, 2005). North arrow is black. Red arrows show the directions of movement for the PRVI and Hispaniola microplate with respect to the North American Plate. The light grey area at the centre is above 2 km bathymetry line. PRVI, Puerto Rico Virgin Islands; AP, Anegada Passage

  • Speaking of the recent quake in the Mona Rift, here is my interpretive poster for that sequence. As we saw in Xu et al. (2015), the Mona Rift is an area where the crust is stretching in an east-west direction. The 1918 M 7.1 earthquake and the 24 September 2019 M 6.0 Mona Rift earthquakes are extensional in an east-west direction. There were about 100 fatalities and there was millions of dollars of damage. The Puerto Rico Seismic Network has a review page for the 1918 earthquake.


  • Here are some plots showing GPS motion rates relative to topography and seismicity in the region (Symithe et al., 2015).
  • First, look at the profile that crosses Haiti, A-A’ (south to north, from left to right).The profile for Haiti clearly shows steps in the GPS velocity profiles. This is evidence for strike-slip faults as tectonic strain from relative plate motions is accumulated along fault boundaries, there are steps in the plate motion rates. These steps are located where the profile crosses two major strike slip faults in Haiti.
  • Next look at profile B-B’ which crosses Puerto Rico. There is no observed strike-slip strain accumulating in Puerto Rico, except there is a step in the north, far offshore of Puerto Rico. There exist several major active strike-slip faults in Puerto Rico, but they are not found in these geodetic data (PIety et al., 2018).


    • Sections across the Lesser and Greater Antilles subduction showing topography (grey line), earthquake hypocenter [Engdahl et al., 1998], velocity magnitude at the GPS sites (red circles with 95% confidence error bar), velocity predicted by the best fit model (solid red line), and velocity predicted by a forward model where we impose full coupling on the subduction interface (dashed blue line). The misfit of the data to a fully locked plate interface is apparent on the three Lesser Antilles cross sections.

  • This is a larger scale view of GPS site motion in the region from Calais et al. (2016).


    • Velocities at selected GPS sites in the NE Caribbean shown with respect to the Caribbean plate (a) and to the North American plate (b). Error ellipses are 95% confidence.

  • While this does not implicate these earthquake sequence, it helps us get a comprehensive view of the tectonics of Puerto Rico. First I show the faults used in their model, then I show the figure showing how much these authors estimate that the faults are locked.


    • Block geometry used in the models tested. Solid black lines show the block boundaries for the best fit model, thick dashed lines show other tested block boundaries. NHIS = North Hispaniola, PRVI = Puerto Rico and Virgin Islands, GONA = Gonave, HISP = Hispaniola, NLAB = North Lesser Antilles Block, SJAM = South Jamaica. CARW = Caribbean West, CARE = Caribbean East, NVEN = North Venezuela, MARA = Maracaibo, ANDE = Andes, HFBT = Hispaniola fault and thrust belt, NMF = Neiba-Matheux thrust, SJF = South Jamaica fault. Thin dashed lines are depth contours of the subduction interface used in the model, derived from the earthquake hypocenters cross sections shown in Figure 4.


    Coupling ratio estimated along the Greater-Lesser Antilles subduction interface estimated on the discretized plate interface also shown in Figure 4. Residual velocities are shown with black arrows. We omitted their error ellipses for a sake of readability. The thin dashed line indicates the boundary of the Bahamas Platform. Note the coincidence between the transition from coupled to uncoupled plate interface with the transition from Bahamas Platform collision to oceanic subduction at the Puerto Rico trench.

  • Here is another view of plate coupling for the region from Manaker et al. (2008). Apologies for the resolution as this may remind us all to provide high quality figures to the publisher of our journal articles.


    • Fault slip rates and slip rate deficit from the best-fit model. Open circles represent the surface projection of fault nodes. Heavy black lines show the model block boundaries. Vertical faults are shown to the right of each main figure. (a) Fault slip rates (mm yr−1). (b) Slip rate deficit (mm yr−1).

Tectonic Strain and Seismic Hazard

  • As the tectonic plates move relative to each other, and stuck earthquake faults resist this motion, the crust surrounding and including these faults can deform to change shape and volume. This change in shape or volume is called strain.
  • Regions of high tectonic strain are areas that are changing shape or volume more than in areas of low strain. The map below shows a Global Strain Rate Map for the region (Kreemer et al., 2014).


  • These figures show the chance of the region will experience ground shaking over a period of 50 years (the life of a building) from Mueller et al. (2010). These maps show the chance that a region will shake with a given acceleration (units are in percent g, where g = gravity; if the ground shakes with accelleration exceeding 100% g, then rocks and other things can be thrown into the air).
  • Many of us are familiar with the concept of the 100 year flood, a flood that may occur every 100 years on average. However, there could be more than one 100 year flood in a year because it is just a statistical average that can change with time. The same is true for earthquake statistics.
  • Basically, the 2% in 50 year map represents the 250 year earthquake. The 10% in 50 year map represents a 500 year earthquake.
  • Read more about the statistics used in these seismic hazard maps here.
  • The USGS National Seismic Hazard Site is here.


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Earthquake Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity, for both the M 6.4 and M 5.9 events. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored polygons (color = MMI, labeled as “dyfi x.x”).
  • In the lower center are plots showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis) for each event. The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots.
  • What do you think? Do these earthquake intensity models (from the USGS) match the observations? What do you think may control how well they do or do not fit the model? What might affect ground shaking locally or regionally?


  • Here is a video from IRIS that helps us learn about what controls the shaking intensity.

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 a map that I put together using the data available from the USGS Earthquake Event pages. More about these models can be found here.
  • The map on the left shows liquefaction susceptibility from the M 6.4 and the map on the right is for the M 5.9 earthquake. The M 6.4 event affects a much more broad region with greater intensity.
  • These models use empirical relations (earthquake data) between earthquake size, earthquake distance, and material properties of the Earth.
  • The largest assumption is that for the Earth materials. This model uses a global model for the seismic velocity in the upper 30 meters (i.e. the Vs30). This global model basically takes the topographic slope of the ground surface and converts that to Vs30. So, the model is basically based on a slope map. This is imperfect, but works moderately well at a global scale. A model based on real Earth material data would be much much better.


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Surface Deformation from Remote Sensing

  • Dr. Eric Fielding used satellite data (“Interferometric Synthetic Aperture RADAR,” or “InSAR”) to estimate how much the ground surface moved. Below is the first result where red


    •

    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

  • Bruna, J.L.G., ten Brink, U.S., Munoz-Martin, A., Carbo-Gorosabel, A., and Estrada, P.L., 2015. Shallower structure and geomorphology of the southern Puerto Rico offshore margin in Marine and Petroleum Geology, v. 67, p. 30-56, http://dx.doi.org/10.1016/j.marpetgeo.2015.04.014
  • Calais, E., Symithe, S., de Lepinay, B.B., Prepetit, C., 2016. Plate boundary segmentation in the northeastern Caribbean from geodetic measurements and Neogene geological observations in Comptes Rendus Geoscience, v. 348, p. 42-51, http://dx.doi.org/10.1016/j.crte.2015.10.007
  • Manaker, D.M., Calais, E., Freed, A.M., Ali, S.T., Przybylski, P., Mattioli, G., Jansma, O., Prepetit, C., de Chabalier, J.B., 2008. Interseismic Plate coupling and strain partitioning in the Northeastern Caribbean in GJI, v. 174, p. 889-903, doi: 10.1111/j.1365-246X.2008.03819.x
  • Piety, L.A., Redwine, J.R., Derouin, S.A., Prentice, C.S., Kelson, K.I., Klinger, R.E., and Mahan, S., 2018. Holocene Surface Ruptures on the Salinas Fault and Southeastern Great Southern Puerto Rico Fault Zone, South Coastal Plain of Puerto Rico in BSSA, v. 108, no. 2, p. 619-638, doi: 10.1785/0120170182
  • 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, J. Geophys. Res. Solid Earth, 120, 3748–3774, doi:10.1002/2014JB011779.
  • Xu, X., Keller, G.R., and Guo, X., 2015. Dip variations of the North American and North Caribbean Plates dominate the tectonic activity of Puerto Rico–Virgin Islands and adjacent areas in Geological Journal, doi: 10.1002/gj.2708

Return to the Earthquake Reports page.


Earthquake Report: 2010 Haiti M 7.0

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This is the ten year commemoration of the 2010 magnitude 7 earthquake in Haiti that caused widespread damage and casualties, triggered thousands of landslides, caused tsunami, triggered a turbidity current, and caused thousands to be internally displaced.
https://earthquake.usgs.gov/earthquakes/eventpage/usp000h60h/executive
Here I review some of the earthquake related materials from this temblor.
The M 7 earthquake happened on a strike-slip fault system that accommodates relative plate motion between the North America and Caribbean plates. There is a history and prehistory of earthquakes on this fault system.
This event was quite deadly. Here is a comparison of this earthquake relative to other earthquakes (Billham, 2010).

Deaths from earthquakes since 1900. The toll of the Haiti quake is more than twice that of any previous magnitude-7.0 event, and the fourth worst since 1900.

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 1920-2020 with magnitudes M ≥ 6.0.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

  • In the lower right corner is an inset map showing the major plate boundary faults from the Global Earthquake Model (GEM). The M 7.0 is shown as a yellow circle (as the same for the other insets).
  • In the upper left corner is a tectonic overview figure from Symithe et al. (2015) showing earthquakes colored relative to depth.
  • To the right of the Symithe et al. (2015) map is a plot showing horizontal motion based on GPS sites. The north-south profile (A-A’ in green) shows how horizontal GPS motions change as the profile crosses the two main faults. Because of these offsets, we can infer these faults are seismogenically locked and storing tectonic strain. The Enriquillo fault is accumulating about 8 mm/year of strain and the Septentrional fault is accumulating about 8 mm/year of tectonic strain. In general, if these faults rupture every 100 years, they might slip 80 mm. This is a rough approximation and there are lots of complications for such an estimate. But what is true, these faults cannot slip more than they can accumulate over time due to plate motions.
  • In the upper right corner is a map that shows the tectonic strain (deformation of the crust) due to earthquakes and interseismic ground motion (Kreemer et al., 2014).
  • To the left of the strain map are two figures from Frankel et al. (2011) that show the chance of shaking of a certain magnitude (percent gravity, or “g”) for a 50 year period (the life of a building).
  • In the lower center are 2 figures from Hayes et al. (2010) that show the USGS fault slip models.
  • Here is the map with a month’s and century’s seismicity plotted.


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  • Here is a great tectonic overview for the entire Caribbean region from Symithe et al. (2015).


    • Seismotectonic setting of the Caribbean region. Black lines show the major active plate boundary faults. Colored circles are precisely relocated seismicity [1960–2008, Engdahl et al., 1998] color coded as a function of depth. Earthquake focal mechanism are from the Global CMT Catalog (1976–2014) [Ekstrom et al., 2012], thrust focal mechanisms are shown in blue, others in red. H = Haiti, DR = Dominican Republic, MCS = mid-Cayman spreading center, WP = Windward Passage, EPGF = Enriquillo Plaintain Garden fault.

  • Here is a video from IRIS that reviews the 2010 Haiti Earthquake.

      • These figures show the chance of the region will experience ground shaking over a period of 50 years (the life of a building) from Frankel et al. (2011). These maps are based on a model that uses the seismic velocity of materials in the upper 30 meters using the topographic slope as a proxy for Earth materials. Some consider this a better estimate of shaking likelihood compared to models that consider a fixed parameter for Earth materials (e.g. bedrock of a specific range in seismic velocities).


        • Hazard maps using grid of VS30 values shown in Figure 7: (top) PGA (%g) with 10% probability of exceedance, (bottom) PGA (%g) with 2% probability of exceedance in 50 years.

      • This is an excellent tectonic overview figure from Calais et al. (2010). The upper panel shows the main tectonic faults and historic seismicity. The lower panel shows the location of some of the known historic earthquake slip patches (where the faults slipped during the earthquakes).


        • Tectonic setting of the northeastern Caribbean and Hispaniola. a, Major active plate-boundary faults (black lines), instrumental seismicity (National Earthquake Information Center database, 1974–present) and Caribbean–North America relative motion (arrow). P.R. Puerto Rico; D.R. Dominican Republic. b, Summary of the present-day tectonic setting of Hispaniola. Estimated historical rupture areas are derived from archives. 1860, 1953 and 1701 are the dates of smaller magnitude, poorly located events. Vertical strike-slip events are shown as lines; dip-slip events are shown as projected surface areas. The red arrows show geodetically inferred long-term slip rates (labelled in mmyr-1) of active faults in the region from the block model discussed here (the arrows show motion of the southern with respect to the northern block).

      • Here is a more localized view of the tectonics of Hispaniola (Fleur et al., 2015). Because the relative motion between the North America and Caribbean plates (and all the other complicated blocks, fault orientations, etc.) is oblique to the plate boundary, there are both strike-slip and thrust faults (the result of strain partitioning).


        • Tectonic setting and active faulting in Haiti. (a) Major anticlines (lines with arrows, dashed white: growing and grey: older), active thrusts (black), and strike-slip faults (EPGF and SF: in red) from this study [Mann et al., 1995; Pubellier et al., 2000; Mauffret and Leroy, 1997; Granja Bruña et al., 2014]. Blue (1): rigid Beata oceanic crust block. Dark purples: toleitic complex oceanic crust outcrops. Orange: Cul-de-Sac and Enriquillo (CSE) ramp basins; brown (2): Hispaniola volcanic arc. Black crosses: metamorphic Cretaceous basement; yellow: rigid Bahamas bank. Haiti FTB: Haiti fold and thrust belt. Grey line: trench. Double black arrows: regional compression deduced from mean orientations of folds and thrusts. (b) Active faulting in southern Haiti. Topography and bathymetry (contours each 200 m) from Global Multi-Resolution Topography (GMRT) synthesis (http://www.geomapapp.org). Faults, folds, and symbols as in Figure 1a. Simple red and black arrows: strike-slip motion. In orange: push-down troughs of Port-au-Prince Bay and Azuei and Enriquillo Lakes in the CSE ramp basin. Inset (bottom left): fault geometry and kinematics. Grey ellipse: zone with en echelon troughs in N100°E direction. Inset (top right): simplified strain ellipse in southern Haiti.

      • This shows a fantastic visualization of the tectonics of southern Hispaniola (Fleur et al., 2015). Most of the faults are thrust faults and the Equillon fault system bisects them.


        • (a) Active faulting and seismicity in the southeastern part of Haiti. Topography and bathymetry (contours each 100 m), from Advanced Spaceborne Thermal Emission and Reflection (http://asterweb.jpl.nasa.gov/) and Shuttle Radar Topography Mission 30+ (http://www2.jpl.nasa.gov/srtm/), respectively, and the 1:25000 bathymetric chart of the Hydrographic and Oceanographic Department of the French Navy (contours at 2, 5, 10, 20, 30, 50, 100, and 130m) in the Port-au-Prince Bay. Faults, folds, and symbols as in Figure 1. Red star: 2010 main shock epicenter from Mercier de Lépinay et al. [2011] with the centroid moment tensor from Harvard University (http://www.globalcmt.org); seismicity from Douilly et al. [2013], and focal mechanisms from Nettles and Hjörleifsdóttir [2010]. Location of Figure 3a is indicated. PAP, Port-au-Prince. Folds in CSE ramp basin with locations of Figures 4a and 4b are indicated: PaPT: Port-au-Prince thrust; DT: Dumay thrust; NaC: Nan Cadastre thrust (see Figure 4b); Jac: Jacquet thrust; Gan: Ganthier thrust (see figure 4a). Red and white star near DT: location of Figure 4d. (b) NNE-SSW geological cross section across the Cul-de-Sac-Enriquillo plain. Geology from www.bme.gouv.ht and Mann et al. [1991b] (supporting information Figure S5) with colors of units as in Figure 2c. Profile location shown in Figure 2a; topography as in Figure 1. No vertical exaggeration. (c) Three-dimensional block diagram showing the geology, the aftershocks [from Douilly et al., 2013], and the fault system along a N-S cross section (location in Figure 2a). The block highlighted in red is uplifting in between the LT and the EPGF.

      • These figures show the tectonic geomorphology of the area near Port-au-Prince (Fleur et al., 2015).


        • (a) Active faulting in the 2010 earthquake epicentral area. Active faults, symbols, topography, and bathymetry as in Figure 2a. Location of Figure 3b is indicated. SSW-NNE topographic profiles are shown in the inset. ΔR: fault throw at the seafloor. Vertical exaggeration (VE): 20X; α: slope of the Léogâne delta fan. (b) The Lamentin thrust in Carrefour. Topography from lidar data (contours at 5m vertical interval). Rivers in blue, with thicker traces for larger ones. Inset in the lower left corner: topographic profile BB′ along of the Lamentin fold crest (VE: 5X). Inset in the upper right corner: topographic profile AA′ perpendicular to the Lamentin thrust system (VE: 2.5X) and the most plausible geometry of the thrusts (with no vertical exaggeration). In yellow: upper Miocene limestone; in grey: Quaternary conglomerates. MT:main thrust. The width of the fold and the slope of the fan surface constrain the rooting depth of the emergent ramp to the décollement [e.g.,Meyer et al., 1998].

      • Here are some maps and photos of field evidence for active faulting in the area (Fleur et al., 2015).


        • Active folding in the Cul-de-Sac-Enriquillo ramp basin. (a) Aerial photograph of the 8 km long Ganthier Quaternary fold. (b) Lidar topography of the Nan Cadastre Quaternary thrust folding. Inset: topographic profile AA′ and possible interpretation at depth. (c) Field photograph along the eastern flank of the Bois Galette River (location in Figure 4a) showing the folded alluvial sediments of the Ganthier fold dipping ~30°N. (d) Field photograph and interpretation of the 50 ± 15° southward dipping Dumay thrusts (in red) exposed in cross section on the eastern bank of the Rivière Grise (location in Figure 2a). The fault offsets by several tens of centimeters Quaternary sediments (lacustrine and conglomerates) incised by the river.

      • This figure shows the interseismic (between earthquakes) GPS plate motion vectors (Calais et al., 2011). Each red arrow represents the direction and velocity (speed) that a GPS site is moving over the past decade or two.
      • The panel on the right shows a north-south transect of velocities relative to strike-slip (blue) and thrust (red) motion. There is clear evidence for decadal scale (“active”) strike-slip tectonic strain (deformation) across both Enriquillo and Septentrional faults. There is also compressional deformation across these fault zones, though much more compression across the Enriquillo fault (there is considerable noise in the compressional plot).


        • Interseismic GPS velocities. The GPS velocity field is determined from GPS campaigns before the 12 January 2010 earthquake. The ellipses and error bars are 95% confidence. a, Velocities with respect to the North American plate. b, Velocities with respect to the Caribbean plate. c, Velocity profile perpendicular to the plate boundary (coloured circles and one-sigma error bars) and best-fit elastic block model (solid lines). Blue D profile-perpendicular (‘strike-slip’) velocity components; orange D profile-parallel (‘shortening’) velocity components. The profile trace and width are indicated by dashed lines in a and b.

      • Here is an updated geodetic figure from Symithe and Calais (2016) showing strike-slip and thrust strain.


        • GPS velocities shown with respect to the North American plate (A) and to the Caribbean plate (B). Error ellipses are 95% confidence. (C) North–south profile including GPS sites shown with the dashed box shown on panels A and B. Velocities are projected onto directions parallel (blue) and normal (red) to the EPGF direction. MS = Massif de la Selle, CdS = Cul-de-Sac basin, MN= Matheux-Neiba range, PC= Plateau Central, PN= Plaine du Nord, EF= Enriquillo fault, SF= Septentrional fault.

      • Here is their interpretation about how this interseismic motion relates to the geologic structures (Symithe and Calais, 2016)..


        • Top and middle: comparison between the best-fit model (solid lines) and GPS observations for the strike-slip (blue) and shortening (red) components for the one– fault model, i.e. with oblique slip on the south-dipping fault. Bottom: interpretative geological cross-section using information from Saint Fleur et al. (2015). The red line indicates the model fault with its locked portion shown as solid. The surface trace of the fault in the best-fit model coincides with the northern limb of the Ganthier fold, indicated by the letter G. The gradient of GPS velocities coincides with the southern edge of the Cul-de-Sac basin, while the Matheux range appears devoid from present-day strain accumulation. D = Dumay locale where Terrier et al. (2014) report reverse faulting affecting Quaternary sediments. G = Ganthier fold (Mann et al., 1995).

      • This figure shows the coseismic displacements in the region (Calais et al., 2010). The map shows horizontal motion. The plot on the right shows these displacements in 3 directions (north-south in black; east-west in blue; up-down in red)


        • Coseismic displacements from GPS measurements. a, Map of horizontal coseismic displacements. Note the significant component of shortening, similar to the interseismic velocity field (Fig. 2). The orange arrows have been shortened by 50% to fit within the map. Displacements at stations TROU and DFRT, cited in the text, are labelled. NR Can Natural
        Resources Canada. b, Position time series at station DFRT (orange arrow labelled on a) showing four pre-earthquake measurement epochs and the post-earthquake epoch. Note the steady interseismic strain accumulation rate and the sudden coseismic displacement.

      • This figure shows the earthquake surface deformation as measured using satellite data (interferrometric RADAR). The figure also shows a slip model showing the relative amount of slip. Finally, a cross section showing the orientation of the fault that slipped. This is also from Calais et al. (2010).


        • Deformation observations and rupture model. a, Interferogram (descending track, constructed from images acquired on 9 March 2009 and 25 January 2010), GPS observed (black) and model (red) coseismic displacements. The yellow circles show aftershocks. G D Greissier, L D Léogâne, PaP D Port-au-Prince. EF D Enriquillo–Plantain Garden fault. The black rectangle shows the surface projection of the modelled rupture; the black–white dashed line is the intersection with the surface. LOS displ:D line-of-sight displacement. b, Total slip distribution from a joint inversion of InSAR and GPS data, viewed from the northwest. c, Interpretative cross-section between points A and B indicated on a. The red line shows coseismic rupture.

      • Here is a figure showing the aftershocks for the Haiti Earthquake sequence (Douilly et al., 2013). They sampled the seismicity in various transects (A, B, C, D, E, and F) and plotted these seismicity in cross sections below the map. These authors use these plots to evaluate hypothetical fault models.


        • Cross sections perpendicular to the Enriquillo fault illustrating possible fault structures. Hypocenters within the rectangular boxes are included in the corresponding cross section. The open triangles in the cross sections indicate the surface trace of the Enriquillo fault. The red line shows the main earthquake rupture on the Léogâne fault; blue lines show the Trois Baies thrust fault; green lines show south-dipping antithetic structures delineated by aftershocks possibly triggered by Coulomb stress changes following the mainshock. The black lines in the cross sections show the hypothesized location of the Enriquillo fault, which is believed to dip from 65° north (Prentice et al., 2010) to vertical.

      Earthquake Stress Triggering

      • When an earthquake fault slips, the crust surrounding the fault squishes and expands, deforming elastically (like in one’s underwear). These changes in shape of the crust cause earthquake fault stresses to change. These changes in stress can either increase or decrease the chance of another earthquake.
      • I wrote more about this type of earthquake triggering for Temblor here. Head over there to learn more about “static coulomb stress triggering.”
      • Lin et al. (2010) conducted this type of analysis for the 2010 M 7.0 Haiti Earthquake. They found that some of the faults in the region experienced an increase in fault stress (the red areas on the figure below). These changes in stress are very small, so require a fault to be at the “tipping point” for these changes in stress to cause an earthquake.
      • There has not yet been a triggered earthquake in this region. However, we don’t know much about how long these stress changes really can affect an earthquake fault (it is thought to last only a few years at most, but some suggest it may last centuries).
      • This first figure from Lin et al. (2010) shows the changes in stress on some nearby faults.


        • Coulomb stress changes imparted by the January 12, 2010, Mw=7.0 rupture resolved on surrounding faults inferred from Mann and others (2002). Thrust faults dip 45°.

      • This second figure from Lin et al. (2010) shows the regional changes in stress.


        • Coulomb stress changes imparted by the January 12, 2010, Mw=7.0 rupture to the Septentrional Fault, assuming a friction of 0.4 (a friction of 0.0 yields a similar result, with the peak stress shifted 25 km to the west). Stress changes are positive but very small. The two 1/26/10 aftershocks are the only events thus far to locate well off the source model; if they are left-lateral events on roughly E-W planes, then they would have been promoted by stress imparted by the January 12 mainshock rupture.

      • This figure shows a slip model for the earthquake (compared with coastal uplift observations) and the results of a static coulomb stress modeling.
      • In the upper panel, color represents the amount the fault slipped in centimeters.
      • In the lower panel, red areas are areas that experienced an increase in stress on a fault and blue areas experienced a stress decrease. The left map shows these stress changes imparted on south vergent (north dipping) thrust faults. The panel on the right shows north vergent (south dipping) receiver thrust faults.


        • Newstatic slipmodel for the 2010 Haiti earthquake and induced Coulomb stress changes. (a) Axonometric view from SE showing the slip distribution on two faults (EPGF and LT) determined by modeling geodetic data (GPS and interferometry) and coastal uplift values recorded by coral (see supporting information). Arrows (white for EPGF and black for LT) indicate the motion of the hanging wall with respect to the footwall. Land surfaces in grey. Red lines: active faults. Blue bars: coastal uplift measured by using corals from Hayes et al. [2010]. Red bars: uplift predicted by our model. Focal mechanisms indicated the EPGF (dark yellow) and Lamentin fault (green) geometry. (b) Coulomb stress changes induced by the slip model we determined, in map view at 7.5 km depth. Black rectangles: modeled faults. Epicentral locations of aftershocks from Douilly et al. [2013]. Insets in the upper left corners: parameters of the receiver faults used for the Coulomb stress calculation. Calculated for receiver faults having the same geometry as the strike-slip EPGF (dark yellow lines) and as the Lamentin thrust (dark green lines), respectively (Figure 5b, left and right).

      • Here is another static coulomb stress transfer model from Symithe et al. (2013). The difference between the upper and lower panels reflects the different fault friction parameter used in these two models.


        • Calculated coseismic Coulomb stress change on the regional faults of southern Haiti based on coseismic slip associated with our preferred model (Fig. 5c) and two assumptions of apparent friction. The Enriquillo fault is assumed to dip 65° to the south with a rake of 20°. The Trois Baies fault is assumed to dip 55° to the north with a rake of 70°. All other faults are assumed to dip at 60° and a rake of 90° (pure
        thrust). Major cities are noted by green circles.

      Earthquake Humanitarian Impact

      • Here is a summary figure from USAID that shows the humanitarian impact from the earthquake and other related factors. The gray arrows show the location and quantity of internally displaced persons (people who moved within Haiti following the earthquake).



        •

      • Here is a figure that is the result of some analyses of the rate at which people displaced themselves internally (Lu et al., 2012).


        • Overview of population movements. (A) Shows the geography of Haiti, with distances from PaP marked. The epicenter of the earthquake is marked by a cross. (B) Gives the proportion of individuals who traveled more than d km between day t − 1 and t. Distances are calculated by comparing the person’s current location with his or her latest observed location. In (C), we graph the change in the number of individuals in the various provinces in Haiti. (D) Gives a cumulative probability distribution of the daily travel distances d for people in PaP at the time of the earthquake. (E) Shows the cumulative probability distribution of d for people outside PaP at the time of the earthquake. Finally, (F) gives the exponent α of the power-law dependence of d—the probability of d is proportional to d−α. These are obtained by a maximum-likelihood method (33), and differ from the slopes of the lines in (D) and (E) by unity since these are the cumulative distributions.

      Earthquake Shaking Intensity

      • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
      • In the upper right corner is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots.


        •

      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 a map that I put together using the GIS data available from Harp et al. (2016).


        •

      • Here is a map from Gorum et al. (2023) that also shows the landslide distribution across the landscape.


        • Tectonic setting and landslide distribution map of the study area. (a) Area surrounding the Mw 7.0 January 2010 Haiti earthquake epicenter; beach ball shows focal mechanism (earthquake.usgs.gov). (b) Tectonic setting of the Caribbean plate boundaries. Red star and the points are locations of main shock and major aftershock distributions, respectively. (c) Topographic setting and mean local relief (white circles with±1σ whiskers) of pre- and post-earthquake landslides: alluvial plains and fans (APF), coastal cliff (CSC), deeply incised valley (DIV), dissected hilly and mountainous terrain (HDHM), round crested slopes and hills (RLH), moderately steep slopes (MR), plateau escarpments (PE), and steep faulted hills (SFH).

      • This shows a large scale comparison of landslides with different temporal origins (Gorum et al., 2013).


        • Distribution of (a) coseismic and (b) aseismic landslides along a reach of the Momance River, Haiti; black star is location of 2010 earthquake epicenter; white arrow is flow direction. Old landslides may likely be of prehistoric origin.

      • These authors considered topographic relief as a control for landslide triggering.


        • Regional distribution of co- and aseismic landslides, and re-activated slope failures. (a) Normalized spatial density of pre-earthquake aseismic landslides within 1-km radius (see text). (b) Spatial density of coseismic landslides. (c) Spatial density of re-activated landslides. (d and e) Fraction of area affected by (d) aseismic and (e) coseismic
        landslides per 0.01° latitude; circles are individual landslide locations scaled by area (see legend in panel g). Thin black dashed lines are areas affected by the landslides; thick black dashed lines are mean local relief of coseismically uplifted and subsided areas. (f and g) Histograms of (f) point density [km−2] and (g) rate [%] of re-activated landslides for 0.01° latitude bins; PaP: Port-au-Prince; PG: Petit Goave.

      • Here these authors compare uplift and subsidence measured from satellites (Gorum et al., 2013).


        • Distribution of coseismic deformation, slip, and landslide density. (a) Vertical-deformation signal from InSAR (after Hayes et al., 2010); black circles are mapped coseismic landslides; the black star is the epicenter. (b) Normalized landslide density map (cf. Fig. 4). (c) Rupture model and coseismic slip amplitudes from inversion of InSAR data, field based off-set measurements, and broadband teleseismic body-waveform data (after Hayes et al., 2010). (d) Block diagram of the Léogâne thrust and Enriquillo–Plantain Garden Fault blind rupture. Normalized landslide density superimposed on data by Mercier de Lépinay et al. (2011). Inset block diagram shows proposed fault geometry by Hayes et al., (2010) for Haiti earthquake ruptures. Thick solid lines are surface projections of each fault; PaP: Port-au-Prince.

      • Here is the conclusion figure from Gorum et al. (2013) that shows some of the controlling factors for earthquake triggered landslides.


        • Along-strike (W–E) distribution of (a) mean coseismic deformation (Hayes et al., 2010), (b) coseismic and re-activated normalized landslide density, (c) mean local relief, and (d)mean hillslope gradient in the uplifted section.N–S distribution of (e) mean coseismic deformation (Hayes et al., 2010), (f) coseismic and re-activated landslide density, (g)mean local relief, and (h) mean hillslope gradient in both uplifted and subsided parts. Inset maps show locations of the swaths. Black lines (c, d, g and h) and shadings are means and±1 σ in 60-m bins. Light and dark grey boxes delimit peaks in normalized landslide density (b), and sub-sections of differing dominant fault geometries in (e). Dashed grey lines are regional means; scale differs between panels (b and f) in coseismic and re-activated landslide density.

      • This is a take away figure putting the Haiti earthquake triggered landslides in context with other earthquakes.


        • Summary of coseismic landslide inventory data from documented reverse or thrust-fault earthquakes. Left panel shows extent of faulting recorded in historical (grey bars) and recent earthquakes (black bars; modified after McCalpin, 2009). Thick and thin black bars are lengths of surface and blind fault ruptures; estimates of surface rupture lengths (grey bars) and maximum coseismic uplift (light grey arrows) from Wells and Coppersmith (1994); lower limits from Bonilla (1988). Maximum coseismic uplift (MCU, dark grey arrows) and surface/blind ruptures: (1)Wenchuan, China, Mw 7.9 (Liu-Zeng et al., 2009); (2) Chi-Chi, Taiwan, Mw 7.6 (Chen et al., 2003); (3) Haiti Mw 7.0 (Hayes et al., 2010); (4) Iwate-Miyagi, Japan, Mw 6.9 (Ohta et al., 2008); (5) Northridge, USA, Mw 6.7 (Shen et al., 1996); and (6) Lorca, Spain, Mw 5.2 (Martinez-Diaz et al., 2012). Right panel shows hanging wall and foot-wall areas affected by coseismic landsliding, and box-and-whisker plots of local relief. Box delimits lower and upper quartiles and median; whiskers are 5th and 95th percentiles; open circles are outliers. Landslide inventory data from Gorum et al. (2011), Liao and Lee (2000), Yagi et al. (2009), Harp and Jibson (1995), and Alfaro et al. (2012); landslide lower limits are from Keefer (1984).

      Earthquake Triggered Turbidity Currents

      • Cecilia McHugh used NSF rapid response funding to collect geophysical (e.g. bathymetry, subsurface seismic profiles) and sedimentary core data in the epicentral region of the M 7.0 Haiti Earthquake. McHugh et al. (2011) discovered that the earthquake triggered turbidity currents (submarine landslides) that (A) caused suspended sediment to be found in the water column after the earthquake and (B) led to the deposition of a turbidite.
      • McHugh et al. (2011) found evidence for prior earthquake triggered turbidites in the form of sedimentary deposits. These deposits were found in sedimentary cores and in subsurface imaging (seismic reflection data).
      • Here is a sediment core that includes the 2010 seismoturbidite, as well as several previous likely seismoturbidites.


        • A: Bulk density, magnetic suscep- GC-2 tibility, 234Th (dpm/g), and photo of GC2 recovered from Canal du Sud at 1753 m. The 12 January turbidite contains 5-cm-thick basal bed of black sand and 50 cm of mud above, forming turbidite-homogenite unit. Bulk density decreases upward to nearly seawater values, and magnetic susceptibility signal is higher near base, corresponding to sand rich in magnetic minerals analyzed at 55, 113, and 143 cm (plag—plagioclase; qtz—quartz). Boxes delineate 12 January and older events.

      • Here is a seismic reflection profile from McHugh et al. (2011). The dark layers are muddy layers between the turbidites. The plot on the right shows evidence for the suspended sediment.


        • A: Semitransparent lens on Chirp profile is 12 January earthquake-generated turbidite. B: CTD (conductivity, temperature, depth) transmissometer measurements of water column obtained at 1750 m. Anomaly in beam attenuation in lower 600 m is interpreted as sediment plume that has remained in suspension since 12 January.

      Earthquake Triggered Tsunami

      • Here is a plot from Fritz et al. (2012) that shows field observations from the tsunami.


        • Tsunami flow depths and runup heights measured along coastlines in the Gulf of Gonaˆve and along Hispaniola’s south coast.

        Social Media

        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

      • Billham, R., 2010. Lessons from the Haiti Earthquake in Nature, v. 463, doi:10.1038/463878a
      • Calais, E., Mazabraud, Y., de Lepinay, B.M., Mann, P., Mattioli, G., and Jansma, P., 2002. Strain partitioning and fault slip rates in the northeastern Caribbean from GPS measurements in GRL, v. 29, no. 18, doi:10.1029/2002GL015397
      • Calais, E., Freed, A., Mattioli, G., Amerlung, F., Jonsson, S., Jansma, P., Hong, S-H., Dixon, T., Prepetit, C., and Momplaisir, R., 2010. Transpressional rupture of an unmapped fault
        during the 2010 Haiti earthquake in Nature Geoscience, http://www.nature.com/doifinder/10.1038/ngeo992
      • Douilly, R., Haase, J.S., Ellsworth, W.L., Bouin, M-P., Calais, E., Symithe, S.J., Aerbruster, J.G., de Lepinay, B.M., Deschamps, A., Mildor, S-L., Meremonte, M.E., and Hough, S.E., 2013. Crustal Structure and Fault Geometry of the 2010 Haiti Earthquake from Temporary Seismometer Deployments in BSSA, v. 103, no. 4, p. 2305-2325, doi: 10.1785/0120120303
      • Douilly, R., H. Aochi, E. Calais, and A. M. Freed, 2015. Three-dimensional dynamic rupture simulations across interacting faults: The Mw7.0, 2010, Haiti earthquake, J. Geophys. Res. Solid Earth, 120, 1108–1128, doi:10.1002/2014JB011595.
      • Frankel, A., Harmsen, S., Mueller, C., Calais, E., and Haase, J., 2011. Seismic Hazard Maps for Haiti in Earthquake Spectra, v. 27, no. 1, p. S23-S41
      • Fritz, H.M., Hillaire, J.V., Moliere, E., Wei, Y., and Mohammed, F., 2012. Twin Tsunamis Triggered by the 12 January 2010 Haiti Earthquake in Pure and Applied Geophysics, doi:10.1007/s00024-012-0479-3
      • Gorum, T., van Westen, C.J., Korup, O., van der Meijde, M., Fan, X., and van der Meer, F.D., 2013. Complex rupture mechanism and topography control symmetry of mass-wasting pattern, 2010 Haiti earthquake in Geomorphology, v. 184, p. 127-138, http://dx.doi.org/10.1016/j.geomorph.2012.11.027
      • Harp, E.L., Jibson, R.W., and Schmitt, R.G., 2016, Map of landslides triggered by the January 12, 2010, Haiti earthquake: U.S. Geological Survey Scientific Investigations Map 3353, 15 p., 1 sheet, scale 1:150,000, http://dx.doi.org/10.3133/sim3353.
      • Lin, Jian, Stein, Ross S., Sevilgen, Volkan, and Toda, Shinji, 2010. USGS-WHOI-DPRI Coulomb stress-transfer model for the January 12, 2010, MW=7.0 Haiti earthquake: U.S. Geological Survey Open-File Report 2010-1019, 7 p. http://pubs.usgs.gov/of/2010/1019/.
      • Liu, J. Y., H. Le, Y. I. Chen, C. H. Chen, L. Liu, W. Wan, Y. Z. Su, Y. Y. Sun, C. H. Lin, and M. Q. Chen, 2011. Observations and simulations of seismoionospheric GPS total electron content anomalies before the 12 January 2010 M7 Haiti earthquake, J. Geophys. Res., 116, A04302, doi:10.1029/2010JA015704.
      • Lu, X., Bengtsson, L., and olme, P., 2012. Predictability of population displacement after the 2010 Haiti earthquake in PNAS, v. 109, no. 29, https://doi.org/10.1073/pnas.1203882109
      • McHugh, C.M., Seeber, L., Braudy, N., Cormier, M-H., Davis, M.B., Diebold, J.B., Dieudonne, N., Douilly, R., R., Guilick, S.P.S., Hornbach, M.J., Johnson III,, H.E., Mishkin, K.R., Sorlien, C.C., Steckler, M.S., Symithe, S.J., and Templeton, J., 2011. Offshore sedimentary effects of the 12 January 2010 Haiti earthquake in Geology, v. 39, no. 8, p. 723-726, doi:10.1130/G31815.1
      • Saint Fleur, N., N. Feuillet, R. Grandin, E. Jacques, J. Weil-Accardo, and Y. Klinger, 2015. Seismotectonics of southern Haiti: A new faulting model for the 12 January 2010M7.0 earthquake, Geophys. Res. Lett., 42, 10,273–10,281, doi:10.1002/2015GL065505.
      • Symithe, S., E. Calais, Haase, J.S., Freed, A.M., and Douilly, R., 2013. Coseismic Slip Distribution of the 2010 M 7.0 Haiti Earthquake and Resulting Stress Changes on Regional Faults in BSSA, v. 103, np. 4, p. 2326-2343, doi: 10.1785/0120120306
      • 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, J. Geophys. Res. Solid Earth, 120, 3748–3774, doi:10.1002/2014JB011779.
      • Symithe, S. and E. Calais, 2016. Present-day Shortening in Southern Haiti from GPS measurements and implications for seismic hazard in Tectonophysics, v. 689, p. 117-124, http://dx.doi.org/10.1016/j.tecto.2016.04.034

      Return to the Earthquake Reports page.


    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.


      •

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


      •

    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.




      •

    • 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.


      •

    • 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).


      •

    • 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.

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    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


      •

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


      •

    2018.01.14 M 7.1 Peru

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


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

    2018.01.23 M 7.9 Gulf of Alaska

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

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

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


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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.


      •

    Temblor Reports:

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

    2018.10.22 M 6.8 Explorer plate

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

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


      •

    2018.10.25 M 6.8 Greece

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

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


      •

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


      •

    Temblor Reports:

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

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

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

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


      •

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


      •

    2018.11.30 M 7.0 Alaska

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

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


      •

    Temblor Reports:

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

    2018.12.05 M 7.5 New Caledonia

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

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


      •

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


      •

    2018.12.20 M 7.4 Bering Kresla

  • 2018.12.20 M 7.3 Bering Kresla UPDATE #1

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

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


      •

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


      •

    UPDATE #1

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


      •

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


      •

    2018.12.29 M 7.0 Philippines

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

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


      •

    Geologic Fundamentals

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

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


      •

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

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

      • Strike Slip:

      Compressional:

      Extensional:

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


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

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


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

    Return to the Earthquake Reports page.

    Earthquake Report: Venezuela

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    Busy week!
    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.
    GPS data support the hypothesis that the El Pilar fault is accumulating strike-slip strain, but there is a paucity of evidence that there is active convergence across the thrust fault. However, there does appear to be some small amount of contraction (Reinoza, et a.,. 2015).

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0 in one version.
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

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

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

    • In the upper left corner is a map from Pindel and Kennan (2009) that shows the plate tectonic boundaries of the Caribbean and northern South America.
    • In the lower right corner is another map that shows teh regional tectonics (Levander et al., 2006).
    • In the upper right corner is a larger scale map showing the faulting in the region surrounding today’s M 7.3 earthquake (Audenard et al., 2000).
    • Here is the map with a month’s seismicity plotted, along with USGS earthquakes M ≥ 6.0.


    Other Report Pages

    Some Relevant Discussion and Figures

    • Here is the Pindell and Kennan (2009) map.


      • Present day tectonic map of the Caribbean region.

    • Here is the Pindell and Kennan (2009) figure that shows how the large strike-slip plate boundary on teh north side of Venezuela grew from the west over time.


      • Motion histories of: North (NA) and South America (SA) relative to Indo-Atlantic hot spot (IAHS) Mu¨ller et al. (1993) reference frame (grey lines; NA wrt IAHS and SA wrt IAHS); hot spots relative to North America (dashed black line; IAHS wrt NA); Caribbean relative to North America (heaviest black line; Car wrt NA), as summarized from former relative positions of the Caribbean Trench (lighter black lines). Also shown: Cayman Trough (grey outline); Cenozoic convergence between the Americas (inset upper right; P88 ¼ Pindell et al. 1988; M99 ¼ Mu¨ller et al. 1999); seismic tomographic profile of van der Hilst (1990) (inset, lower right).

    • Here is the Audemard (2000) map showing the many faults in this region.


      •

    • This is a map from Reinoza et al. (2015) where they present their geodetic analysis (analysis of the deformation of the earth). These authors use GPS data to evaluate the potential activity of the El Pilar fault.


      • Location map of the active faults in northeastern Venezuela [Audemard et al., 2000] showing distribution of the GNSS stations: yellow squares, green circles, and red triangles are GNSS sites on which the acquisition campaigns were carried out in 2003, 2005, and 2013 respectively; the blue star corresponds to the cGNSS CUMA station of REMOS-IGVSB Network. We show the epicenter location of 1929 and 1997 events with their respective proposed ruptures (orange lines) [Audemard, 2007]. (top right) The inset box shows a schematic geodynamic map of the southeastern Caribbean [Audemard, 1999b; Audemard et al., 2000; Weber et al., 2001]. Legend: BF = Boconó Fault, EPF = El Pilar Fault, OAF = Oca Ancón fault, SMBF = Santa Marta Bucaramanga Fault, and SSF = San Sebastian Fault.

    • Here are the GPS data. The white arrows (vectors) show the observed velocities (motion rate) for the GPS sites shown on the previous map. The black arrows (vectors) show how their model results compare with the observational data.


      • Observed velocities (white arrows) with error ellipses for 66% confidence level and simulated velocities (black arrows) according to the upgrade of displacement-simulation method. All displacements are based on the South America reference frame.

    • Here are some cross sections showing the El Pilar fault, along with some of the thrust faults in the region. Section B is just to the west of where this M 7.3 earthquake happened.


      • Simplified sections across the southeastern Caribbean margin (based on maps and sections by Bellizzia et al. (1976), Stéphan et al. (1980), Campos (1981), Beck (1986), Chevalier (1987); locations in Fig. 1).

    Geologic Fundamentals

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

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


      •

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

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

      • Strike Slip:

      Compressional:

      Extensional:

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


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

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


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

      References:

    • Audemard, F.A., Machette, M.N., Cox, J.W., Dart, R.L., and Haller, K.M., 2000. Map and Database of Quaternary Faults in Venezuela and its Offshore Regions, USGS Open File Report 00-018
    • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
    • Jouanne, F., Audemard, F.A., Beckm, C., Van Welden, A., Ollarves, R., and Reinoz, C., 2011. Present-day deformation along the El Pilar Fault in eastern Venezuela: Evidence of creep along a major transform boundary in Journal of Geodynamics, v. 51., p. 398-410, doi:10.1016/j.jog.2010.11.003
    • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
    • Pindell, J.L. and Kennan, L., 2009. Tectonic evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference frame: an update in JAMES, K. H., LORENTE, M. A. & PINDELL, J. L. (eds) The Origin and Evolution of the Caribbean Plate. Geological Society, London, Special Publications, 328, 1–55.
      DOI: 10.1144/SP328.1
    • Reinoza, C., F. Jouanne, F. A. Audemard, M. Schmitz, and C. Beck (2015), Geodetic exploration of strain along the El Pilar Fault in northeastern Venezuela, J. Geophys. Res. Solid Earth, 120, 1993– 2013, doi:10.1002/2014JB011483.


    °
    ≥

    Earthquake Report: Cayman Trough!

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    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.

    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 (and down to M ≥ 4.5 in a second poster).
    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.3 earthquakes, in addition to some relevant historic earthquakes.There have been several M 6.7-M 7.5 earthquakes to the west of this fault in the last 4 decades or so.

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

    • I include some inset figures.

    • In the upper left corner is a plate tectonic map showing the major plate boundary faults in the Caribbean region. Symithe et al. (2015) plot fault plane solutions for earthquakes M ≥ 6. I place a blue star in the general location of today’s M 7.6 earthquake.
    • In the upper right corner is a different plate tectonic map from García-Casco et al. (2011). I place a blue star in the general location of today’s M 7.6 earthquake.
    • In the lower right corner is a figure from Mann et al., (1991) that shows the magnetic anomalies in the oceanic crust of the Cayman trough. The short vertical subparallel black lines are magnetic anomalies, identified from magnetic surveys with ages constrained by rocks from the seafloor. As the crust spreads from the Mid Cayman Ridge, and Earth’s magnetic field polarity flips, the changes in magnetic polarity are recorded in the crust. The crust closest to the MCR is youngest. I place a blue star in the general location of today’s M 7.6 earthquake.
    • Above the Mann et al. (1991) map is a larger scale map from ten Brink et al. (2002). This map shows the quasi detailed bathymetry in the area of the MCR. They map that both the Swan and Oriente faults terminate at the MCR. Today’s M 7.6 earthquake is to the west of this map, so there is no little blue star. :-(




    • UPDATE: 2018.01.10 9 AM pacific time. There were two observations of a small amplitude (small wave height) tsunami recorded on tide gages in the region. Below are those observations.


      •

    • Here is the tectonic map from Symithe et al. (2015). I include their figure caption below in blockquote.


      • Seismotectonic setting of the Caribbean region. Black lines show the major active plate boundary faults. Colored circles are precisely relocated seismicity [1960–2008, Engdahl et al., 1998] color coded as a function of depth. Earthquake focal mechanism are from the Global CMT Catalog (1976–2014) [Ekstrom et al., 2012], thrust focal mechanisms are shown in blue, others in red. H = Haiti, DR = Dominican Republic, MCS = mid-Cayman spreading center, WP = Windward Passage, EPGF = Enriquillo Plaintain Garden fault.

    • Here is the tectonic map from Garcia-Casco et al. (2011). I include their figure caption below in blockquote.


      • Plate tectonic configuration of the Caribbean region showing the location of the study cases presented in this issue (numbers refer to papers, arranged as in the issue), and other important geological features of the region (compiled from several sources).

    • Here is the Benz et al. (2011) Seismicity of the Earth poster for this region.


      •

    • Here is the map from Mann et a. (1991). Note how today’s earthquake is in an area that may have overlapping faults of different types.


      • A. Tectonic map of Cayman trough region showing strike-slip faults (heavy lines), oceanic crust (gray) in Cayman trough, and magnetic anomaly identifications (numbered bars) (after Rosencrantz et a., 1988). Arrows show relative displacement directions. Fault zones: OFZ – Oriente; DFZ- Dunvale; EPGFZ – Enriquillo-Plantain Garden; WFZ – Walton; SIFZ – Swan Islands; MFZ – Motagua. Bl. Late Miocene reconstruction of Cayman trough. C. Early Miocene reconstruction.

    • Here is the large scale map from ten Brink et al. (2002) showing the bathymetry surrounding the Mid-Cayman Rise.


      • Bathymetry of central Cayman Trough adapted from Jacobs et al. (1989). Contour interval: 250 m. Dotted line: location of gravity transect.

    • Here is the USGS Tectonic Summary for this 2018.01.10 M 7.6 earthquake. A more comprehensive review can be found here.
      • The January 10, 2018, M 7.6 Great Swan Island, Honduras earthquake occurred as the result of strike slip faulting in the shallow crust near the boundary between the North America and Caribbean plates. Early focal mechanism solutions indicate that rupture occurred on a steeply dipping structure striking either west-northwest (right-lateral), or west-southwest (left-lateral). At the location of this earthquake, the North America plate moves to the west-southwest with respect to the Caribbean plate at a rate of approximately 19 mm/yr. Local to the January 10, 2018 earthquake, this motion is predominantly accommodated along the Swan Islands transform fault, a left-lateral structure. The location, depth and focal mechanism solution of today’s earthquake are consistent with rupture occurring along this plate boundary structure, or on a nearby and closely related fault.
      • While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip-faulting events of the size of the January 10, 2018, earthquake are typically about 140×20 km (length x width).
      • Nine other earthquakes of M 6 or larger have occurred within 400 km of the January 10, 2018 event over the preceding century. Previous strong earthquakes along the North America-Caribbean plate boundary in this region include the destructive M 7.5 Guatemala earthquake of February 4, 1976, which resulted in more than 23,000 fatalities. The 1976 earthquake occurred on the Motagua fault, a segment of the plate boundary that lies in southern Guatemala, about 650 km west-southwest of the hypocenter of the January 10, 2018, event. In May 2009, a M 7.3 earthquake occurred along the Swan Island transform fault approximately 300 km west of the January 10, 2018 event. The 2009 earthquake (which was much closer to land than the 2018 event) resulted in 7 fatalities, 40 injuries and 130 buildings being damaged or destroyed.

    Regional Seismicity

    • There were some earthquakes associated with the Middle America Trench (MAT; a subduction zone) over the past year or so. There may be some relation between the earthquakes and the onshore structures of the Swan fault system, the Motagua-Polochic fault zone.
    • First there was a sequence of earthquakes in June near where the Motagua-Polochic fault zone splays towards the MAT. Here are my earthquake reports for these 2017.06.14 and 2017.06.22 earthquakes. The interpretive posters are below.




    • Then, in September 2017, just to the north of the June sequence, there was a M 8.2 normal fault earthquake in the downgoing Cocos plate. Here is my earthquake report for this 2017.09.08 earthquake.


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      References:

    • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011. 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
    • 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.
    • Mann, P., Tyburski, S.A., and Rosencratz, E., 1991. Neogene development of the Swan Islands restraining-bend complex, Caribbean Sea in Geology, v. 19, p. 823-826.
    • 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.
    • Ten Brink, U.S., Coleman, D.F., and Dillon, W.P., 2002. The nature of the crust under Cayman Trough from gravity in Marine and Petroleum Geology, v. 119, p. 971-987.

    Earthquake Report: Guatemala

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    There was a really cool earthquake sequence a few days ago on and offshore of Guatemala. Offshore of Guatemala in the Pacific Ocean, the Cocos plate subducts beneath the North America and Caribbean plates (NAP & CP). The transform plate boundary between the NAP and CP forms the Motagua-Polochic fault zone onshore, which bisects Guatemala.
    From late May 2017 through mid June there were several earthquakes with the largest magnitude M = 5.5. These earthquake hypocenters have depths that are deeper and shallower than the estimated depth for the subduction zone fault (Hayes et al., 2012), but many of the earthquakes simply have a default depth of 10 km. So it is difficult to say if these are all near the megathrust or are on upper plate faults (e.g. in the accretionary prism). These earthquakes have compressional fault plane solutions. Either way, they appear to have loaded some faults down-dip along the subducting slab. This may or may not be the case, but there was a deep extensional magnitude M 6.9 earthquake (with an aftershock of M = 5.1 nearby). These along dip earthquakes are probably related.

    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 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. The hypocentral depth of the M 5.5 plots this close to the location of the fault as mapped by Hayes et al. (2012).

      I include some inset figures in the poster.

    • In the upper right corner, I include a subset of figures from Benz et al. (2011). There is a map that shows USGS epicenters with dots colored by depth and magnitude represented by circle diameter. There is also plotted a cross section that is adjacent (southeast) to this earthquake sequence. Cross section B-B’ shows the earthquake hypocenters along a profile displayed on the map. Note how the subduction zone dip steepens to the northeast. On the map and the cross section, I place a blue stars in the location for the M 6.9 and the 6/14 M 5.5 earthquakes.
    • To the left of these figures is a comparison map and plot, showing the responses from real people who reported their observations during these two earthquakes. Below each map are plotted the reports from the Did You Feel It? USGS website for each earthquake. These reports are plotted as green dots with intensity on the vertical axes and distance on the horizontal axes. There are comparisons with Ground Motion Prediction Equation (attenuation relations) results (the orange model uses empirical data from central and eastern US earthquakes; the green model uses empirical data from earthquakes in California). The M 5.5 earthquake seems to fit the Central-Eastern US regression much better than the California regression. However, there are very few observations. The M 6.9 earthquake seems to fit the California regression better.
    • In the lower left corner is a map that shows the plate tectonic setting for this region of Middle America (Eric Calias). Earthquake epicenters are plotted with color representing depth and circle diameter representing magnitude. Dr. Calais shaded the Caribbean plate a little darker than the surrounding plates. Relative plate motions are plotted as white arrows. I place a blue star in the general location of the M 6.9 earthquake.
    • Above the tectonic figure, is a figure that shows how Franco et al. (2012) hypothesize that the amount that the subducting fault is locked (“coupling” or the proportion of the plate convergence rate that is stored along the fault that would eventually slip during earthquakes). Note that this earthquake sequence mostly occurred in the segment of the subduction zone that has high coupling. I place a blue star in the general location of the M 6.9 earthquake.
    • In the lower right corner I plot most all the available moment tensors for the earthquakes in this sequence. I label the Polochic and Motagua faults that delineate the Motagua-Polochic fault zone, left lateral strike-slip faults that form the boundary between the Caribbean and North America plates.


    Here are the USGS webpages for the earthquakes with moment tensors plotted above

    • Here is the USGS Did You Feel It comparison.


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    References