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Earthquake Report: M 6.9 Mid Atlantic Ridge

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There have not been that many large earthquakes this year. This is good for one main reason, there is a lower potential for human suffering.
Therefore, there are fewer Earthquake Reports for this year.
This morning (my time) there was a magnitude M 6.9 earthquake along the Romanche transform fault, a right-lateral strike-slip fault system that offsets the Mid Atlantic Ridge in the equatorial Atlantic Ocean. The fault is part of the Romanche fracture zone.
https://earthquake.usgs.gov/earthquakes/eventpage/us7000i53f/executive
The transform faults in this part of the Mid Atlantic Ridge plate boundary have a pattern of earthquakes that seem to max out in the lower 7 magnitudes. This may be (at least partly) due to the maximum length of these faults (?).
The Romanche fault is about 900 kilometers long. The Chain fault is about 250 km long. The St. Paul fault is about 350 km long.

    Earthquake magnitude is controlled by three things:

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

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

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

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


Looking at the interpretive poster, we can see that there have not been any temblors that approach the sizes listed in this table. The largest historic earthquake was M 7.1 (there were several).
So, we may ask ourselves one of the most common questions people ask regarding earthquakes. Was this M 6.9 a foreshock to a larger earthquake?
Obviously, we cannot yet know this. Nobody can predict the future (at least not yet).
However, based on the incredibly short historic record of earthquakes, we may answer this question: “no, probably not.” This answer is tempered by the very short seismic record. If magnitude 8 earthquakes occur, on average, every 1000 years, then our ~100 year record might be too short to “notice” one of these M 8 events.
If we continue to look at the historic record, we will see that there appear to be three instances where one of these M 6.5-7 earthquakes had a later earthquake of a similar magnitude.
When an earthquake fault slips, the crust surrounding the fault squishes and expands, deforming elastically (like in one’s underwear). These changes in shape of the crust cause earthquake fault stresses to change. These changes in stress can either increase or decrease the chance of another earthquake.
I wrote more about this type of earthquake triggering for Temblor here. Head over there to learn more about “static coulomb stress triggering.”
In the poster, I label these earthquakes as “Linked Earthquakes.” Perhaps the later of each earthquake pair (or triple) was triggered by the change in static coulomb stress.

    Here are the three sets of “Linked Earthquakes:”

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

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

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

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


    •

Some Relevant Discussion and Figures

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


    •

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


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

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


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

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


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

    References:

    Basic & General References

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

    • Specific References

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

Return to the Earthquake Reports page.


Earthquake Report M 7.0 Philippines

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

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures.

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


    •

20220727_philippines_interpretation.pdf 16 MB pdf

    References:

    Basic & General References

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

    • Specific References

Social Media: Here is my thread for this event.

Return to the Earthquake Reports page.

Earthquake Report: M 6.2 along the Great Sumatra fault

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There was a magnitude M 6.2 Gempa or Earthquake on 25 February 2022.
https://earthquake.usgs.gov/earthquakes/eventpage/us6000gzyg/executive
The plate boundary fault system that dominates the tectonics of the Island of Sumatera, Indonesia, is the complicated.
The oceanic India-Australia plate converges with the Eurasia plate to form the Sunda trench. This convergent plate boundary forms a subduction zone where the oceanic plate subducts beneath the continental plate.
Below is a low-angle oblique view cut into the Earth showing this plate configuration from the Earth Observatory Singapore.


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

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

The amount of plate convergence that is perpendicular to the plate boundary is accommodated by earthquake fault slip on the megathrust.
The amount of plate convergence that is parallel to the plate boundary is accommodated by earthquake fault slip on a different series of faults that we call sliver faults. The Great Sumatra fault is one of these [forearc] sliver faults.
Here is a figure from Lange et al. (2008) that shows how oblique plate convergence forms both a subduction zone and a forearc sliver fault system.


The M 6.2 earthquake is a strike-slip earthquake along the Great Sumatra fault, one of these forearc sliver faults.
Based on our knowledge of this fault system and the earthquake mechanism, we can easily interpret this to be a right-lateral strike-slip fault.
There are numerous historical analogies from the past century. Most of the events in the past few decades have been in the M 6-7 range, though there have been events of larger magnitude in the past centuries.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past 3 months, with diameter representing magnitude (see legend). I include earthquake epicenters from 1922-2022 with magnitudes M ≥ 6.0.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

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


    •

Some Relevant Discussion and Figures

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


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


    •

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


    •

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


    •

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


    • India-Australia plate subducts northeastwardly beneath the Sunda plate (part of Eurasia) at modern rates (GPS velocities are based on regional modeling of Bock et al, 2003 as plotted in Subarya et al., 2006). Historic earthquake ruptures (Bilham, 2005; Malik et al., 2011) are plotted in orange. 2004 earthquake and 2005 earthquake 5 meter slip contours are plotted in orange and green respectively (Chlieh et al., 2007, 2008). Bengal and Nicobar fans cover structures of the India-Australia plate in the northern part of the map. RR0705 cores are plotted as light blue. SRTM bathymetry and topography is in shaded relief and colored vs. depth/elevation (Smith and Sandwell, 1997).

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


    •

  • This is a video showing a visualization of the seismic waves transmitted from the 2004 SASZ earthquake from IRIS and others.

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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

    Earthquake Stress Triggering

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


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

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


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

    Shaking Intensity

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


      •

    Potential for Ground Failure

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

      FOS = Resisting Force / Driving Force

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


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


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


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


    Seismic Hazard and Seismic Risk

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


      •

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

      • The GEM Seismic Hazard Map:




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


        •

      • The GEM Seismic Risk Map:




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


      •

    Tsunami Hazard

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


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

      References:

      Basic & General References

    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
    • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
    • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
    • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
    • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
    • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
    • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
    • 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

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    • Bothara, J., Beetham, R.D., Brunston, D., Stannard, M., Brown, R., Hyland, C., Lewis, W., Miller, S., Sanders, R., Sulistio, Y., 2010. General observations of effects of the 30th September 2009 Padang earthquake, Indonesia. Bulletin of the New Zealand Society for Earthquake Engineering 43, 143-173.
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    • Chlieh, M., Avouac, J.P., Sieh, K., Natawidjaja, D.H., Galetzka, J., 2008. Heterogeneous coupling of the Sumatran megathrust constrained by geodetic and paleogeodetic measurements: Journal of Geophysical Research, v. 113, B05305, doi: 10.1029/2007JB004981.
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    • Hayes, G.P., Bernardino, Melissa, Dannemann, Fransiska, Smoczyk, Gregory, Briggs, Richard, Benz, H.M., Furlong, K.P., and Villaseñor, Antonio, 2013. Seismicity of the Earth 1900–2012 Sumatra and vicinity: U.S. Geological Survey Open-File Report 2010–1083-L, scale 1:6,000,000, https://pubs.usgs.gov/of/2010/1083/l/.
    • JACOB, J., DYMENT, J., YATHEESH, V. & BHATTACHARYA, G.C., 2009 – Marine magnetic anomalies in the NE Indian Ocean: the Wharton and Central Indian basins revisited. European Geosciences Union General Assembly, Vienna, 19-24/04.
    • Ji, C., D.J. Wald, and D.V. Helmberger, Source description of the 1999 Hector Mine, California earthquake; Part I: Wavelet domain inversion theory and resolution analysis, Bull. Seism. Soc. Am., Vol 92, No. 4. pp. 1192-1207, 2002.
    • Hurukawa, N., Wulandari, B.R., and Kasahara, M., 2014. Earthquake History of the Sumatran Fault, Indonesia, since 1892, Derived from Relocation of Large Earthquakes in BSSA,v. 104, no. 4, p. 1750-1762, doi: 10.1785/0120130201
    • Ishii, M., Shearer, P.M., Houston, H., Vidale, J.E., 2005. Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array. Nature 435, 933.
    • Kanamori, H., Rivera, L., Lee, W.H.K., 2010. Historical seismograms for unravelling a mysterious earthquake: The 1907 Sumatra Earthquake. Geophysical Journal International 183, 358-374.
    • Konca, A.O., Avouac, J., Sladen, A., Meltzner, A.J., Sieh, K., Fang, P., Li, Z., Galetzka, J., Genrich, J., Chlieh, M., Natawidjaja, D.H., Bock, Y., Fielding, E.J., Ji, C., Helmberger, D., 2008. Partial Rupture of a Locked Patch of the Sumatra Megathrust During the 2007 Earthquake Sequence. Nature 456, 631-635.
    • Lange, D., Cembrano, J., Rietbrock, A., Haberland, C., Dahm, T., and Bataille, K., 2008. First seismic record for intra-arc strike-slip tectonics along the Liquiñe-Ofqui fault zone at the obliquely convergent plate margin of the southern Andes in Tectonophysics, v. 455, p. 14-24
    • Maus, S., et al., 2009. EMAG2: A 2–arc min resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne, and marine magnetic measurements, Geochem. Geophys. Geosyst., 10, Q08005, doi:10.1029/2009GC002471.
    • Malik, J.N., Shishikura, M., Echigo, T., Ikeda, Y., Satake, K., Kayanne, H., Sawai, Y., Murty, C.V.R., Dikshit, D., 2011. Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India: Geology, v. 39, p. 559-562.
    • Meltzner, A.J., Sieh, K., Chiang, H., Shen, C., Suwargadi, B.W., Natawidjaja, D.H., Philobosian, B., Briggs, R.W., Galetzka, J., 2010. Coral evidence for earthquake recurrence and an A.D. 1390–1455 cluster at the south end of the 2004 Aceh–Andaman rupture. Journal of Geophysical Research 115, 1-46.
    • Meng, L., Ampuero, J.-P., Stock, J., Duputel, Z., Luo, Y., and Tsai, V.C., 2012. Earthquake in a Maze: Compressional Rupture Branching During the 2012 Mw 8.6 Sumatra Earthquake in Science, v. 337, p. 724-726.
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    Earthquake Report: M 5.7 & 6.2 Mendocino triple junction

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    I was returning from New Orleans where I was attending the American Geophysical Union Fall Meeting. There was a short layover in Denver and I had a short time to find some food, which is challenging with my dietary restrictions. I cannot recall precisely, but I got some notification from my CGS crew about a magnitude M 6.2 earthquake offshore of the Mendocino triple junction. One of these notifications was from Cindy as we both collaborate to prepare quick reports for earthquakes in California. These reports are sent upstream to management in our organization and others. I was unavailable to contribute this time.
    Needless to say, I was sad to have missed experiencing this good sized shaker for myself. This is the first earthquake of this size that I have missed (in Humboldt) since I moved here in 1991.
    I got home about 3 am the next morning and did not have energy to prepare an earthjay report. Though I started working on it the next day. However, I soon learned that this was a complicated earthquake and I decided to await additional analyses by the Berkeley Seismmo Lab and the USGS.
    Last week or so, their analyses were produced publicly and the earthquake catalog was updated. What we discovered is that there were two closely spaced (in time but not space) earthquakes, an M 5.7 and and M 6.2.
    https://earthquake.usgs.gov/earthquakes/eventpage/nc71127029/executive
    https://earthquake.usgs.gov/earthquakes/eventpage/nc73666231/executive
    It was complicated for the seismologists to work out because the seismic waves of the two events overlapped in time. i.e., the waves from the first quake were still passing through the Earth when the waves from the second quake started.
    Basically, there was initially an M 5.7 strike-slip earthquake along the Mendocino transform fault zone about 20 km (12.5 miles) offshore. About 10 or 11 seconds later, there was an M 6.2 strike-slip earthquake within the Gorda plate, below the megathrust fault.
    Here is a plot from the USGS. Each horizontal squiggly line is the seismograph record from an individual seismometer. They are plotted with the seismometer closest to the earthquake on the bottom row and the furthest seismometer on the uppermost row.
    The P wave (primary wave) is the first of four major types of seismic waves. Next comes the S (secondary) wave, then the Love waves, and finally the Raleigh waves.
    The P wave arrives at closer seismometers before it arrives at more distant seismometers. Because of this, we generally call this type of plot a travel time plot.


    In the above plot we can see how the M 6.1 P waves are arriving while the M 5.7 S waves are still being transmitted.
    The M 5.7 is clearly a right-lateral strike-slip event given the aftershock pattern and the known location and type of the Mendocino fault system (a right-lateral strike-slip fault.
    The interpretation for the type of earthquake for the M 6.2 is a little more complicated.
    Earthquake mechanisms (the “beach balls”) show two possible ways that the earthquake could have slipped. We use aftershock patterns and existing mapped faults to help us interpret which of these [nodal] fault planes is the more likely one.
    If we look at the earthquake poster below, we see that the M 6.2 earthquake is an almost pure strike-slip earthquake. The two possible fault planes are one that is oriented in the northwest direction (would be right-lateral) and one that is in the northeast direction (would be left-lateral).

      There are two reasons why I interpret the M 6.2 to be right-lateral (of course, I could be wrong).

    1. There are a series of aftershocks that appear to align along a northwest trajectory. See the 6 mechanisms for the earthquakes just north of the M 6.2. The epicenters for these M 3.8, 3.0, 3.2, 3.0, 3.3, and 3.8 earthquakes align with a northwest oriented trend.
    2. The 1992 Cape Mendocino Earthquake main events began with an M 7.2 thrust type earthquake mainshock followed by two triggered events with magnitudes of M 6.6 and M 6.5. These triggered events are strike-slip and within the Gorda plate (“intraplate” events). If we take a look at the earthquake catalog after it has been modeled using a “double differencing” technique (read more here), we would notice several northwest trends in seismicity. These trends appeared after the 1992 sequence.

    So, while most of our experience with the Gorda plate is with northeast oriented (striking) left-lateral strike-slip faults (e.g., 1980, 2010, 2014, etc.) it is possible that there are other faults, sub-parallel to the post-1992 seismicity trends, where the M 6.2 and other aftershocks were hosted. I mention these northwest trending faults in a recent Earthquake Report here.
    Something that is interesting is that the onshore events from this 20 Dec 2021 sequence are just to the north of the aftershocks from the 1992 sequence. They are at similar depths as those ’92 quakes and have similar earthquake mechanisms. As Spock would say, Fascinating.
    Dr. Anthony Lomax, famous for his work locating the hypocenter for the 1906 San Francisco Earthquake, has been developing excellent tools for seismologists ever since. He recently applied one of his new tools to locate earthquakes to the Mendocino triple junction region. I present some of his figures below.

    Below is my interpretive poster for this earthquake

    • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 3.0 in one version.
    • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
    • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
    • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

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

    • In the upper left corner is a small scale view of the Mendocino triple junction where the southern Cascadia subduction zone meets the right-lateral Mendocino and San Andreas strike-slip faults.
    • In the lower center is a plot of earthquake intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The blue and orange dots represent USGS Did You Feel It? reports, observations from real people. The green and orange lines show the plots from the USGS [empirical] models of shaking intensity.
    • To the left of the plot is a map that shows these same data. The colored areas are the average intensity reported by people (dyfi numbers). The colored lines represent the USGS modeled intensities. Both the plot and the map show that the shaking intensity gets smaller with distance from the earthquake.
    • In the left upper center is a map that shows liquefaction susceptibility. This is a model from the USGS that uses ground shaking data to estimate where there may be liquefaction. I drove around after the earthquake and could not locate any evidence for liquefaction but my search was far from comprehensive.
    • Here is the map with a month’s seismicity plotted.


      •

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


      •

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


      •

    Other Report Pages

      Shaking Intensity and Potential for Ground Failure

      • Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.

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

        FOS = Resisting Force / Driving Force

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


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


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

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


        •

    Some Relevant Discussion and Figures

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


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


      •

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


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

    The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. In my mind, these two aftershocks aligned on what may be the eastern extension of the Mendocino fault. However, looking at their locations, my mind was incorrect. These two earthquakes were not aftershocks, but were either left-lateral or right-lateral strike-slip Gorda plate earthquakes triggered by the M 7.1 thrust event.
    These two quakes appear to be aligned with the two northwest trends in seismicity and the 18 March 2020 M 5.2. The orientation of the mechanisms are not as perfectly well aligned, but there are lots of reasons for this (perhaps the faults were formed in a slightly different orientation, but have rotated slightly).
    There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.

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


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


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

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


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


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


          •

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


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

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

          • Mw = 5 Trinidad Chaytor

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

      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

    • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
    • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
    • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
    • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
    • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
    • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
    • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
    • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
    • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
    • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
    • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].
    • Wells, D.L., and Coopersmith, K.J., 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement in BSSA, v. 84, no. 4, p. 974-1002

    Return to the Earthquake Reports page.


    Tsunami Report: Hunga Tonga-Hunga Ha’apai Volcanic Eruption & Tsunami

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    I will be filling this in over the next few days and wanted to start collating social media materials for this event.
    There was a large volcanic eruption in the Tonga region. This eruption was observable from satellites and has generated a modest but observable tsunami from Australia to the United States.
    This event is still unfolding and it will take months until we have a deeper understanding of the causes for the tsunami. We know it is related to the explosive volcanic eruption from Hunga Tonga-Hunga Ha’apai, about 55 kms (35 miles) northwest of the largest island of the Kingdom of Tonga, Tongatapu.
    I will continue to fill in details. I am currently busy trying to manage our tsunami event response and am learning lots in the process. However, this delays my time available here.


    Below there are many tweets etc. and one may feel like they are scrolling forever. These tweets are loosely organized into several sections.

    1. Background Material
    2. Tsunami Notifications
    3. Tsunami Education
    4. Tsunami Observations
    5. Tsunami Modeling
    6. Volcano Eruption Observations
    7. Fascinating Observations

    Background Material


    Tsunami Notifications

    Tsunami | Volcano Education

    Tsunami Observations

    USA (CA)


    From here a resort on Tongatapu.

    Don’t do what the videographer here did. This was unsafe and they are incredibly lucky.

    Some videos on Youtube:

    Santa Cruz






    Crescent City

    Oregon

    Pacific

    Tsunami Modeling

    Volcano Eruption | Atmospheric Observations

    Fascinating | Sad Observations


    Gemini Cloudcam Gravity Waves from Earth to Sky Calculus on Vimeo.

    Tsunami Webcam Network

    Below is an interactive map that displays a network of publicly accessible webcams that could be used to observe tsunami waves.

    Earthquake Report: M 7.5 in Peru

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    In the middle of the night (my time) I got a notification from the EMSC earthquake notification service. I encourage everyone to download and use this app.
    There was an intermediate depth magnitude M 7.5 earthquake in Peru. The tectonics in this region of the world are dominated by the convergent plate boundary, a subduction zone formed by the convergence of the oceanic Nazca and continental South America plates.
    https://earthquake.usgs.gov/earthquakes/eventpage/us7000fxq2/executive
    As the Nazca plate subducts, it dips below the South America plate at different dip angles. In this region of Peru, the dip angle is shallow and we term this flat-slab subduction.
    This M 7.5 earthquake occurred in the downgoing Nazca plate, so was not a subduction zone megathrust event, but a “slab” event (for being in the Nazca slab).
    I prepared a much more extensive report for a M 8.0 earthquake in a nearby location that happened on 26 May 2019. Read more about the tectonics of this region in that report here.
    Was this M 7.5 an aftershock of the M 8.0? Probably not, based on the USGS M 8.0 slip model. However this M 7.5 could have been triggered by changes in static coulomb stress following the M 8.0.
    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.

    Below is my interpretive poster for this earthquake

    • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 with magnitudes M ≥ 3.0 in one version.
    • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
    • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
    • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

      I include some inset figures.

    • In the upper left corner is a large scale plate tectonic map showing the major plate boundary faults.
    • In the lower left center is a map showing how the Nazca slab is configured in different locations (Ramos and Folguera, 2009).
    • In the left center is a cross section showing seismicity in this region (Kirby et al., 1995). The source area for this plot is designated by a dashed yellow box on the map.
    • In the upper right corner is a pair of maps that show the landslide probability (left) and the liquefaction susceptibility (right) for this M 7.5 earthquake. I spend more time describing these types of data here. Read more about these maps here.
    • In the lower right corner I plot the USGS modeled intensity (Modified Mercalli Intensity scale, MMI) and the USGS “Did You Feel It?” observations (labeled in yellow). Above the map is a plot showing these same data plotted relative to distance from the earthquake. Read more about what these data sets are and what they represent in the report here.
    • Here is the map with 3 month’s seismicity plotted.


      •

      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

    • Antonijevic, S.K., et a;l., 2015. The role of ridges in the formation and longevity of flat slabs in Nature, v. 524, p. 212-215, doi:10.1038/nature14648
    • Bishop, B.T., Beck, S.L., Zandt, G., Wagner, L., Long, M., Knezevic Antonijevic, S., Kumar, A., and Tavera, H., 2017, Causes and consequences of flat-slab subduction in southern Peru: Geosphere, v. 13, no. 5, p. 1392–1407, doi:10.1130/GES01440.1.
    • Chlieh, M. Mothes, P.A>, Nocquet, J-M., Jarrin, P., Charvis, P., Cisneros, D., Font, Y., Color, J-Y., Villegas-Lanza, J-C., Rolandone, F., Vallée, M., Regnier, M., Sogovia, M., Martin, X., and Yepes, H., 2014. Distribution of discrete seismic asperities and aseismic slip along the Ecuadorian megathrust in Earth and Planetary Science Letters, v. 400, p. 292–301
    • Kumar, A., et al., 2016. Seismicity and state of stress in the central and southern Peruvian flat slab in EPSL, v. 441, p. 71-80. http://dx.doi.org/10.1016/j.epsl.2016.02.023
    • Rhea, S., Hayes, G., Villaseñor, A., Furlong, K.P., Tarr, A.C., and Benz, H.M., 2010. Seismicity of the earth 1900–2007, Nazca Plate and South America: U.S. Geological Survey Open-File Report 2010–1083-E, 1 sheet, scale 1:12,000,000.
    • Villegas-Lanza, J. C., M. Chlieh, O. Cavalié, H. Tavera, P. Baby, J. Chire-Chira, and J.-M. Nocquet (2016), Active tectonics of Peru: Heterogeneous interseismic coupling along the Nazca megathrust, rigid motion of the Peruvian Sliver, and Subandean shortening accommodation, J. Geophys. Res. Solid Earth, 121, 7371–7394, https://doi.org/10.1002/2016JB013080.
    • Wagner, L.S., and Okal, E.A., 2019. The Pucallpa Nest and its constraints on the geometry of the Peruvian Flat Slab in Tectonophysics, v. 762, p. 97-108, https://doi.org/10.1016/j.tecto.2019.04.021
    • Yepes,H., L. Audin, A. Alvarado, C. Beauval, J. Aguilar, Y. Font, and F. Cotton (2016), A new view for the geodynamics of Ecuador: Implication in seismogenic source definition and seismic hazard assessment, Tectonics, 35, 1249–1279, https://doi.org/10.1002/2015TC003941.

    Return to the Earthquake Reports page.


    Earthquake Report Lite: M 7.0 near Acapulco, Mexico

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    I don’t always have the time to write a proper Earthquake Report. However, I prepare interpretive posters for these events.
    Because of this, I present Earthquake Report Lite. (but it is more than just water, like the adult beverage that claims otherwise). I will try to describe the figures included in the poster, but sometimes I will simply post the poster here.
    Last afternoon (my time) there was an M 7.0 earthquake near Acapulco, Mexico. This event generated a tsunami, landslides, building damage, casualties (one fatality as I write this), and many emotions.
    https://earthquake.usgs.gov/earthquakes/eventpage/us7000f93v/executive
    I present my interpretive poster and a few figures. Read more about the tectonics of this region here, in a report for an M 7.4 earthquake in 2020.

    Below is my interpretive poster for this earthquake

    • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 with magnitudes M ≥ 7.0 in one version.
    • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
    • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
    • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

      I include some inset figures.

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


      •

    Tide Gage Data – Acapulco

    Earthquake Intensity

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


      •

      References:

      Basic & General References

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

      • Specific References

    Return to the Earthquake Reports page.

    Earthquake Report: 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.


      •

    Earthquake Aftershocks

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


      •

    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.


      •

      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 Lite: South Sandwich Islands

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    Reply

    I don’t always have the time to write a proper Earthquake Report. However, I prepare interpretive posters for these events.
    https://earthquake.usgs.gov/earthquakes/eventpage/us6000f53e/executive
    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.

    Below is my interpretive poster for this earthquake

    • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 with magnitudes M ≥ 3.0 in one version.
    • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
    • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
    • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

      I include some inset figures.

    • In the upper left corner is a small scale plate tectonic map showing the major plate boundaries.
    • Here is the map with 3 month’s (and a week’s) seismicity plotted.


      •

    Seismicity Cross Sections

    • Here is a map and cross section of the aftershocks.


      •

    Tide Gage Data

    • Here are plot of the tide gage data from nearby gages.




      •

      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.

    EarthquakeReport M 7.1 Philippines

    Posted on by
    Reply

    I don’t always have the time to write a proper Earthquake Report. However, I prepare interpretive posters for these events.
    Because of this, I present Earthquake Report Lite. (but it is more than just water, like the adult beverage that claims otherwise). I will try to describe the figures included in the poster, but sometimes I will simply post the poster here.
    https://earthquake.usgs.gov/earthquakes/eventpage/us6000f48v/executive

    Below is my interpretive poster for this earthquake

    • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 with magnitudes M ≥ 3.0 in one version.
    • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
    • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
    • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

      I include some inset figures.

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


      •

      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.