Earthquake Report: ¡Oaxaca, Mexico!

There was just now an earthquake in Oaxaca, Mexico between the other large earthquakes from last 2017.09.08 (M 8.1) and 2017.09.08 (M 7.1). There has already been a M 5.8 aftershock.Here is the USGS website for today’s M 7.2 earthquake.

This M 7.2 earthquake has a depth that is close to where we think the subduction zone fault is. Currently, the hypocentral depth is 24.7 km and the depth to the slab based upon the Hayes et al. (2012) Slab 1.0 model is about 40 km. So this earthquake may be in the upper North America plate and not on the subduction zone.

UPDATE: The SSN has a reported depth of 12 km, further supporting evidence that this earthquake was in the North America plate.

This region of the subduction zone dips at a very shallow angle (flat and almost horizontal).

There was also a sequence of earthquakes offshore of Guatemala in June, which could possibly be related to the M 8.1 earthquake. Here is my earthquake report for the Guatemala earthquake.

The poster also shows the seismicity associated with the M 7.6 earthquake along the Swan fault (southern boundary of the Cayman trough). Here is my earthquake report for the Guatemala earthquake.

Today’s earthquake happened in a region that had a slightly elevated static coulomb stress following the 2017.09.08 M 8.1 earthquake as calculated by Shinji Toda from Temblor.

UPDATE: I misinterpreted the stress change results as noticed by Eric Fielding:

Below is my interpretive poster for this earthquake


I plot the seismicity from the past year, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.5.

Note the difference in aftershock patterns between the shallower M 8.2 earthquake and the deeper M 7.1 earthquake. (hint: the deep M 8.1 did not have any aftershocks)

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.2 earthquake, in addition to some relevant recent earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • We can observe that Mexico City even shook more strongly due to the basin effects. Mexico City is locate on the map and there is a MMI IV.5 contour that surrounds the large valley. Mexico City was hit strongly by a M 8.0 earthquake in 1985 and again last September. See my report for more about the basin amplification.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • I include some inset figures.

  • In the lower left corner is a map from Mann (2007) that shows the regional tectonics. Plate boundary faults are in bold line, while lineations representing the spreading history are represented by thinner lines. I place a blue star in the general location of tonight’s M 7.2 earthquake (also in other inset maps) and green stars for the other 3 earthquakes discussed here.
  • In the upper right corner is a figure from Perez and Campos (2008; as presented here) which shows the interpreted geometry of the subducting slab in this region. The profile of the seismic array used as a basis for this interpretation (the MASE array) is denoted by the brown dashed line. This line is also shown on the figure in the lower right corner).
  • To the left of the cross section is a plot showing a 2 day solution for GPS positions in this region following the M 8.1 earthquake. Note how today’s M 7.2 earthquake (the blue star) is in an area that moved significantly during and following the M 8.1 earthquake.
  • In the lower right corner is a figure prepared by Temblor here, a company that helps people learn and prepare to be more resilient given a variety of natural hazards. This figure is the result of numerical modeling of static coulomb stress changes in the lithosphere following the 2017 M 8.1 earthquake. This basically means that regions that are red have an increased stress (an increased likelihood for an earthquake) following the earthquake, while blue represents a lower stress, or likelihood. The change in stress are very very small compared to the overall stress on any tectonic fault. This means that an earthquake may be triggered from this change in stress ONLY IF the fault is already highly strained (i.e. that the fault is about ready to generate an earthquake within a short time period, like a day, month, or year or so). The take away: the M 8.1 earthquake did not increase the stress on faults in the region of the M 7.1 (Temblor suggests the amount of increased stress near the M 7.1 is about the amount of force it takes to snap one’s fingers). However, today’s M 7.2 earthquake did have an increase in the stress on faults in the region of the M 7.1 earthquake.
  • In the upper left corner is a map that shows some historic earthquake patches along the Middle America Trench. Today’s earthquake happened near the and 1978 M 7.7 earthquake and a 1999 M 7.5 earthquake not shown on this map (but in region of the 1968 earthquake.


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

  • Relevant Interpretive Posters

    Some Relevant Discussion and Figures

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

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

    • Here is the figure from Gérault et al. (2015) that shows the slab contours.

    • (a) Geodynamic context of southwestern Mexico. Topography and bathymetry from ETOPO1 [Amante and Eakins, 2009]. A white curve outlines the Trans-Mexican Volcanic Belt (TMVB) [Ferrari et al., 2012]. The black lines show the isodepths of the Cocos slab at a 20 km interval, using seismicity up to ∼45 km depth and tomography below [Kim et al., 2012a]. These slab contours show that distinct topographic domains are associated with variations in slab geometry. The yellow vector shows the relative convergence velocity between the Cocos and North America Plate near Acapulco, holding North America fixed [DeMets et al., 2010]. The pink circles show the locations of the Meso-America Subduction Experiment (MASE) stations. (b) Moho depth (red) and upper slab limit (blue) from Kim et al. [2012a, 2013]. The dashed line shows the simplified Moho depth that we used in the numerical models. (c) Measured and smoothed topography along the MASE profile as a function of the distance from the southernmost seismic station, near Acapulco. The topography is smoothed using three passages of a rectangular sliding average of width 15 km.

    • Here are some figures from Pérez-Campos et al. (2008) that show results from the MASE seismic experiment. First is the map showing the seismic array in the tectonic context.

    • MASE seismic array. Slab isodepth contours from Pardo and Sua´rez [1995] are in blue dashed lines. The dots represent epicenters of M>4 earthquakes, reported by the Servicio Sismolo´gico Nacional (SSN; in pink) from December 2004 through June 2007 and those re-located by Pardo and Sua´rez [1995] (in green). The thick orange line represents the profile of Figures 2 and 3. The arrows indicate the beginning (dark blue) and end (light blue) of the flat segment, and the tip of the slab (red).

    • These authors used receiver functions to estimate the depth to the Cocos plate (the slab depth). Below is their figure showing their results. Receiver function analyses use an array (a linear network, or grid network, but a linear network in this case) of seismometers. “A receiver function technique is a way to model the structure of the Earth by using the information from teleseismic earthquakes recorded at a three component seismograph.” More can be found on this here and here.

    • Receiver function images. The black triangles denote the position of the stations along the profile with elevation exaggerated 10 times. The thick brown line denotes the extent of the TMVB. Seismicity (SSN: pink; Pardo and Sua´rez [1995]: green), within 50 km of the MASE profile, is shown as dots. The bottom left plot shows RFs for one teleseismic event along the flat slab portion of the slab; the bottom middle plot illustrates the corresponding model (LVM = low velocity mantle and OC = oceanic crust). Compressional-wave velocity models A, B, and C shown in the bottom right plot were determined from waveform modeling of RFs. They correspond to the structure at A, B, and C of the bottom left plot.

    • And finally, here is their model of the subducting slab. The authors also use seismic tomography to evaluate the geometry of the plates in this region. Seismic tomography is the same as a CT scan of the Earth. We can think of seismic tomography as a 3-D X-Ray of the Earth, just using seismic waves instead.

    • Composite model: tomographic and RF image showing the flat and descending segments of the slab. The key features are the flat under-plated subduction for 250 km, and the location and truncation of the slab at 500 km. The zone separating the ocean crust from the continental Moho is estimated to be less than 10 km in thickness. NA = North America, C = Cocos, LC = lower crust, LVM = low velocity mantle, OC = oceanic crust.

    Some Background Materials

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

    • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
    • Strike Slip:

      Compressional:

      Extensional:

      • Below is a video that explains seismic tomography from IRIS.
      • Here is an educational animation from IRIS that helps us learn about how different earth materials can lead to different amounts of amplification of seismic waves. Recall that Mexico City is underlain by lake sediments with varying amounts of water (groundwater) in the sediments.
      • Here is an educational video from IRIS that helps us learn about resonant frequency and how buildings can be susceptible to ground motions with particular periodicity, relative to the building size.

      Social Media

        References:

      • Benz, H.M., Dart, R.L., Villaseñor, Antonio, Hayes, G.P., Tarr, A.C., Furlong, K.P., and Rhea, Susan, 2011 a. Seismicity of the Earth 1900–2010 Mexico and vicinity: U.S. Geological Survey Open-File Report 2010–1083-F, scale 1:8,000,000.
      • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011 b. Seismicity of the Earth 1900–2010 Caribbean plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-A, scale 1:8,000,000.
      • Cruz-Atienza et al., 2016. Long Duration of Ground Motion in the Paradigmatic Valley of Mexico in Scientific Reports, v. 6, DOI: 10.1038/srep38807
      • Franco, A., C. Lasserre H. Lyon-Caen V. Kostoglodov E. Molina M. Guzman-Speziale D. Monterosso V. Robles C. Figueroa W. Amaya E. Barrier L. Chiquin S. Moran O. Flores J. Romero J. A. Santiago M. Manea V. C. Manea, 2012. Fault kinematics in northern Central America and coupling along the subduction interface of the Cocos Plate, from GPS data in Chiapas (Mexico), Guatemala and El Salvador in Geophysical Journal International., v. 189, no. 3, p. 1223-1236. DOI: https://doi.org/10.1111/j.1365-246X.2012.05390.x
      • Franco, S.I., Kostoglodov, V., Larson, K.M., Manea, V.C>, Manea, M., and Santiago, J.A., 2005. Propagation of the 2001–2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico in Earth Planets Space, v. 57., p. 973-985.
      • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
      • Garcia-Casco, A., Projenza, J.A., Iturralde-Vinent, M.A., 2011. Subduction Zones of the Caribbean: the sedimentary, magmatic, metamorphic and ore-deposit records UNESCO/iugs igcp Project 546 Subduction Zones of the Caribbean in Geologica Acta, v. 9, no., 3-4, p. 217-224
      • Gérault, M., Husson, L., Miller, M.S., and Humphreys, E.D., 2015. Flat-slab subduction, topography, and mantle dynamics in southwestern Mexico in Tectonics, v. 34, p. 1892-1909, doi:10.1002/2015TC003908.
      • Quzman-Speziale, M. and Zunia, F.R., 2015. Differences and similarities in the Cocos-North America and Cocos-Caribbean convergence, as revealed by seismic moment tensors in Journal of South American Earth Sciences, http://dx.doi.org/10.1016/j.jsames.2015.10.002
      • Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
      • Lay et al., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
      • Manea, M., and Manea, V.C., 2014. On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective in JGVR, v. 175, p. 459-471.
      • Mann, P., 2007, Overview of the tectonic history of northern Central America, in Mann, P., ed., Geologic and tectonic development of the Caribbean plate boundary in northern Central America: Geological Society of America Special Paper 428, p. 1–19, doi: 10.1130/2007.2428(01). For
      • McCann, W.R., Nishenko S.P., Sykes, L.R., and Krause, J., 1979. Seismic Gaps and Plate Tectonics” Seismic Potential for Major Boundaries in Pageoph, v. 117
      • Pérez-Campos, Z., Kim, Y., Husker, A., Davis, P.M. ,Clayton, R.W., Iglesias,k A., Pacheco, J.F., Singh, S.K., Manea, V.C., and Gurnis, M., 2008. Horizontal subduction and truncation of the Cocos Plate beneath central Mexico in GRL, v. 35, doi:10.1029/2008GL035127
      • Polltz, F.F., Stein, R.S., Sevigen, V., Burgmann, R., 2012. The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide in Nature, v. 000, doi:10.1038/nature11504
      • Symithe, S., E. Calais, J. B. de Chabalier, R. Robertson, and M. Higgins, 2015. Current block motions and strain accumulation on active faults in the Caribbean in J. Geophys. Res. Solid Earth, v. 120, p. 3748–3774, doi:10.1002/2014JB011779.

    Posted in earthquake, education, geology, pacific, plate tectonics, subduction

    Earthquake Report: 1960 Valdivia, Chile M 9.5

    In commemoration of the #EarthquakeCup, I have put together a summary for the largest instrumentally recorded earthquake, the M 9.5 1960 Valdivia, Chile Earthquake.

    The Peru-Chile trench is quite active and generated Great earthquakes (M>8) in 1985, 2010, 2014, and 2015, with some large earthquakes sprinkled in for good measure (notably a M 7.6 in the region of the 1960 earthquake on 2016.12.15).

    Below is my interpretive poster for this earthquake


    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1917-2017 with magnitudes M ≥ 7.0.

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 9.5 earthquake (Moreno et al., 2010), in addition to some relevant historic earthquakes.

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
    • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. Slab 2.0 is due out later this year!
    • I include some inset figures.

    • In the upper left corner is a plate tectonic map from Wikipedia.
    • To the right of this tectonic map I include an inset map from the USGS Seismicity History poster for this region (Rhea et al., 2010). There is one seismicity cross section with its locations plotted on the map (DD’). The USGS plot these hypocenters along this cross section and I include that below.
    • In the lower left corner, I include a time-space diagram from Moernaut et al. (2010).
    • In the lower right corner, I include a figure from Moreno et al. (2011) that shows the seismogenic coupling (the amount of the plate convergence rate that is accumulated as elastic strain, to be released during earthquakes). This shows how their model fits the GPS observations.
    • To the left of the plate locking figure is a map showing the fracture zones the form the plate boundary between the Nazca and Antarctic plates (Adriosola et al., 2005). This map also shows how the oblique convergence at the subduction zone is partitioned between the subduction zone and forearc sliver faults (strike slip faults that accommodate plate margin parallel strain).
    • In the upper right corner is a figure that shows the seismic moment (amount of energy, or force) released during the 1960 and 2010 earthquakes, along with the moment deficit (the amount of energy stored by the fault and crust) for the period following 1960 (Moreno et al., 2011).



    • Here is the same map, but showing the magnetic anomalies.



    • Here is a map that shows a comparison of the shaking intensity between the 1960 and 2010 earthquakes.


    Select Earthquake Reports for the Region

    • 2014.04.01 M 8.2 Chile
    • Here is an estimate of ground shaking intensity, with contours offshore and the fault slip region plotted at 21:00 PST:

    • 2015.09.16 M 8.3 Chile


    • This map shows the MMI contours for the 1960 and 2010 earthquakes in addition to this 2016 earthquake. This helps us visualize the spatial extent for these earthquakes with a large range of magnitudes. Recall that an M 9.5 earthquake releases about 32 times the energy that an M 8.5 earthquake releases. Note how the 1960 and 2010 earthquakes span a region between the Juan Fernandez fracture zone and where the Chile Rise intersects the trench, where the 4 fracture zones (Guamblin, Darwin, Taitao, and Tres Montes) intersect the trench.

    Some Relevant Discussion and Figures

    • Below are some figures from Moreno et al. (2011) that show estimates of locking along the plate interface in this region. I include the figure captions as blockquote.
    • The first figure shows how the region of today’s earthquake is in an area of higher locking.

    • a) Optimal distribution of locking rate in the plate interface. Predicted interseismic velocities and GPS vectors corrected by the postseismic signals are shown by green and blue arrows, respectively. b) Tradeoff curve for a broad range of the smoothing parameter (β). The optimal value for β is 0.0095 located at the inflection of the curve.

    • This second figure shows the moment released during historic earthquakes and the moment accumulated due to seismogenic locking along the megathrust.

    • a) Latitudinal distribution of the coseismic moment (Mc) released by the 1960 Valdivia (Moreno et al., 2009) (red line) and 2010 Maule (Tong et al., 2010) (blue line) earthquakes, and of accumulated deficit of moment (Md) due to interseismic locking of the plate interface 50 (orange line) and 300 (gray line) years after the 1960 earthquake, respectively. The range of errors of the Md rate is depicted by dashed lines. High rate of Md was found in the earthquake rupture boundary, where slip deficit accumulated since 1835 seems to be not completely released by the 2010 Maule earthquake. b) Schematic map showing the deformation processes that control the observed deformation in the southern Andes and the similarity between coseismic and locking patches. Blue and red contours denote the coseismic slip for the 2010 Maule (Tong et al., 2010) and 1960 Valdivia (Moreno et al., 2009) earthquakes, respectively. Patches with locking degree over 0.75 are shown by brown shaded areas. The 1960 earthquake (red star) nucleated in the segment boundary, area that appears to be highly locked at present. The 2011 Mw 7.1 aftershock (gray) may indicate that stress has been transmitted to the southern limit of the Arauco peninsula.

    • Here is the space-time diagram from Moernaut et al., 2010. I include their figure caption below in blockquote.

    • Fig.: Setting and historical earthquakes in South-Central Chile. Data derived from Barrientos (2007); Campos et al. (2002); Melnick et al.(2009)

    • Here is the cross section of the subduction zone just to the south of this Sept/Nov 2015 swarm (Melnick et al., 2006). Below I include the text from the Melnick et al. (2006) figure caption as block text.

    • (A) Seismotectonic segments, rupture zones of historical subduction earthquakes, and main tectonic features of the south-central Andean convergent margin. Earthquakes were compiled from Lomnitz (1970, 2004), Kelleher (1972), Comte et al. (1986), Cifuentes (1989), Beck et al. (1998 ), and Campos et al. (2002). Nazca plate and trench are from Bangs and Cande (1997) and Tebbens and Cande (1997). Maximum extension of glaciers is from Rabassa and Clapperton (1990). F.Z.—fracture zone. (B) Regional morphotectonic units, Quaternary faults, and location of the study area. Trench and slope have been interpreted from multibeam bathymetry and seismic-reflection profiles (Reichert et al., 2002). (C) Profile of the offshore Chile margin at ~37°S, indicated by thick stippled line on the map and based on seismic-reflection profiles SO161-24 and ENAP-017. Integrated Seismological experiment in the Southern Andes (ISSA) local network seismicity (Bohm et al., 2002) is shown by dots; focal mechanism is from Bruhn (2003). Updip limit of seismogenic coupling zone from heat-fl ow measurements (Grevemeyer et al., 2003). Basal accretion of trench sediments from sandbox models (Lohrmann, 2002; Glodny et al., 2005). Convergence parameters from Somoza (1998 ).

    • In September through November of 2015, there was a M 8.3 earthquake further to the north. Below is my interpretive poster for that earthquake and here is my report, where I discuss the relations between the 2010, 2015, and other historic earthquakes in this region. Here is my report from September.

    • Here is a space time diagram from Beck et al. (1998 ). The 2015 earthquake occurs in the region of the 1943 and 1880 earthquakes. I updated this figure to show the latitudinal extent of the 2010 and 2015 earthquakes.

    Posted in earthquake, education, geology, pacific, plate tectonics, subduction

    Earthquake Report: 1971 Sylmar, CA

    This earthquake was the second earthquake in the state of CA to lead to major changes in how people in the state handled earthquake hazards and risk and today is the 47th anniversary of this earthquake. The first important earthquake was the 1933 Long Beach Earthquake, which led to major changes in the building code (first in Long Beach, then later adopted by the entire state). These changes in the building code have continued to evolve and improve, eventually adopted globally. The 1971 M 6.7 Sylmar Earthquake (a little larger than the M 6.4 damaging earthquake sequence recently that happened in Taiwan) caused major damage to buildings and other infrastructure in southern CA (e.g a hospital was destroyed, which caused many casualties). The 1906 San Francisco Earthquake was important too, so I don’t want the SAF to feel left out. Though the 1933 Long Beach and 1971 Sylmar earthquakes seem to have led to more significant changes in how people approach earthquake hazards and risk.

    A major positive result from the Sylmar Earthquake was the Alquist Priolo Act. The AP Act created a requirement to characterize all the active faults in the state of CA and to regulate how to consider how structures could be built in relation to these active faults. More about the AP Act can be found here. After several years of no support from the state, the CA Geological Survey has recently supported work in this regard, resulting in an update of their guidelines in how to apply the AP Act in Special Publication 42.

    I put together a commemorative #EarthquakeReport interpretive poster to discuss the tectonics of the region. The San Andreas fault (SAF) system is the locus of ~75% of the Pacific-North America plate boundary motion. The SAF is in some places a mature fault with a single strand and in other places, there are multiple strands (e.g. the Elsinore, San Jacinto, and SAF in southern CA or the Maacama, Bartlett Springs, and SAF in northern CA). In southern CA, the SAF makes a bend (called the “Big Bend”) that forms a region of compression. This compression is realized in the form of thrust faults and folds, creating uplift forming the mountain ranges like the Santa Monica Mountains. Some of these thrust faults breach the ground surface and some are blind (they don’t reach the surface).

    In 1971 there was a large earthquake (M 6.7) that caused tremendous amounts of damage in southern CA. A hospital was built along one of the faults and this earthquake caused the hospital to collapse killing many people. The positive result of this earthquake is that the Alquist Priolo Act was written and passed in the state legislature. I plot the moment tensor for the 1971 earthquake (Carena and Suppe, 2002).

    Then, over 2 decades later, there was the M 6.7 Northridge Earthquake. This earthquake was very damaging. Here is a page that links to some photos of the damage. Here is the USGS website for this 1971 M 6.7 Sylmar Earthquake.

    Below is my interpretive poster for this earthquake


    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5.

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.7 earthquake, in addition to some of the significant earthquakes in southern CA.

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

    • In the upper left corner is a map of the faults in southern CA (Tucker and Dolan, 2001). Strike-slip faults (like the SAF) have arrows on either side of the fault desginating the relative motion across the fault. Thrust faults have triangle barbs showing the convergence direction (the triangles are on the side of the fault that is dipping into the Earth).
    • Below this fault map is a low-angle oblique block diagram showing the configuration of thrust faults in the region of the Big Bend. These thrust faults are forming the topography in southern CA. The 1971 and 1994 earthquakes occurred along thrust faults similar to the ones shown in this block diagram.
    • In the upper right corner is a cross section of seismicity associated with the 1971 and 1994 earthquakes (Tsutsumi and Yeats, 1994). 1971 main and aftershocks are in blue and 1994 main and aftershocks are in red. Note how both earthquakes occurred along blind thrust faults. Also note that these faults were dipping in opposite directions (1971 dips to the north (south vergent) and 1994 dips to the south (north vergent).
    • In the lower right corner is another figure showing the aftershocks from the 1971 and 1994 earthquakes (Fuis et al., 2003). This shows their seismic velocity model (with fault interpretations). The 1971 and 1994 earthquake focal mechanisms are shown.
    • In the lower left corner is an illustration that shows the Likelihood of an earthquake with M ≥ 6.7 for the next 30 years. This is based upon the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3). More about UCERF3 can be found here. I placed a blue star in the general location of the 1971 Sylmar Earthquake.


    • Here is the same map, but the MMI is plotted as contours.


    Some Relevant Discussion and Figures

    • Here is the fault map from Tucker and Dolan (2001).

    • Regional neotectonic map for metropolitan southern California showing major active faults. The Sierra Madre fault is a 75-km-long active reverse fault that extends along the northern edge of the metropolitan region. Fault locations are from Ziony and Jones (1989), Vedder et al. (1986), Dolan and Sieh (1992), Sorlien (1994), and Dolan et al. (1997, 2000b). Closed teeth denote reverse fault surface trace; open teeth on dashed lines show upper edge of blind thrust fault ramps. Strike-slip fault surface traces shown by double arrows. Star denotes location of Oak Hill paleoseismologic trench site of Bonilla (1973). CSI, Clamshell-Sawpit fault; ELATB, East Los Angeles blind thrust system; EPT, Elysian park blind thrust fault; Hol Fl, Hollywood fault; PHT, Puente Hills blind thrust fault; RMF, Red Mountain fault; SCII, Santa Cruz Island fault; SSF, Santa Susana fault; SJcF, San Jacinto fault; SJF, San Jose fault; VF, Verdugo fault; A, Altadena study site of Rubin et al. (1998); LA, Los Angeles; LB, Long Beach; LC, La Crescenta; M, Malibu; NB, Newport Beach; Ox, Oxnard; P, Pasadena; PH, Port Hueneme; S, Horsethief Canyon study site in San Dimas; V, Ventura. Dark shading denotes mountains.

    • This is a figure that is based upon Fuis et al. (2001) as redrawn by UNAVCO that shows the orientation of thrust faults in this region of southern CA. Below the block diagram is a map showing the location of their seismic experiment (LARSE = Line 1; Fuis et al., 2003).

    • Schematic block diagram showing interpreted tectonics in vicinity of LARSE line 1. Active faults are shown in orange, and moderate and large earthquakes are shown with orange stars and attached dates, magnitudes, and names. Gray half-arrows show relative motions on faults. Small white arrows show block motions in vicinities of bright reflective zones A and B (see Fig. 2A). Large white arrows show relative convergence direction of Pacific and North American plates. We interpret a master decollement ascending from bright reflective zone A at San Andreas fault, above which brittle upper crust is imbricating along thrust and reverse faults and below which lower crust is flowing toward San Andreas fault (brown arrows) and depressing Moho. Fluid injection, indicated by small lenticular blue areas, is envisioned in bright reflective zones A and B.


      Shaded relief map of Los Angeles region, southern California, showing Quaternary faults (thin black lines, dotted where buried), shotpoints (gray and orange filled circles), seismographs (gray and orange lines), air-gun bursts (dashed yellow lines), and epicenters of earthquakes .M 5.8 since 1933 (focal mechanisms with attached magnitudes: 6.7a—Northridge [Hauksson et al., 1995], 6.7b—San Fernando [Heaton, 1982], 5.9—Whittier Narrows [Hauksson et al., 1988], 5.8—Sierra Madre [Hauksson, 1994], 6.3—Long Beach [Hauksson, 1987]). Faults are labeled in red; abbreviations: HF—Hollywood fault, MCF—Malibu Coast fault, MHF—Mission Hills fault, NHF—Northridge Hills fault, RF—Raymond fault, SF—San Fernando surface breaks, SSF—Santa Susana fault, SMoF—Santa Monica fault, SMFZ—Sierra Madre fault zone, VF—Verdugo fault. NH is Newhall.

    • Here are the figures from Hauksson et al. (1995) showing the regions effected by earthquakes in southern CA.

    • (A) Significant earthquakes of M >= 4.8 that have occurred in the greater Los Angeles basin area since 1920. Aftershock zones are shaded with cross hatching, including the 1994 Northridge earthquake. Dotted areas indicate surface rupture, including the rupture of the 1857 earthquake along the San Andreas fault. (B) Lower hemisphere focal mechanisms (shaded quadrants are compressional) for significant earthquakes that have occurred since 1933 in the greater Los Angeles area.

    • Here is the seismicity cross section plot from Tsutsumi and Yeats (1999).

    • Cross section down to 20 km depth across the central San Fernando Valley, including the 1971 Sylmar and 1994 Northridge earthquake zones. See Figure 2 for location of the section and Figure 3 for stratigraphic abbreviations. Wells are identified in the Appendix. Aftershock data for the 1971 (blue) and 1994 (red) earthquakes within a 10-km-wide strip including the line of this section are provided by Jim Mori at Kyoto University. Abbreviation for faults: MHF, Mission Hills fault; NHF, Northridge Hills fault; SSF, Santa Susana fault.

    • Here is the figure from Fuis et al. (2003) showing their interpretation of seismic data from the region. These data are from a seismic experiment also plotted in the map above. The panel on the left is A and the panel on the right is B. This is their figure 3.

    • Cross section along part of line 2 with superposition of various data layers. A: Tomographic velocity model plus line drawing extracted from reflection data (see text); heavier black lines represent better-correlated or higher-amplitude phases. B: Velocity model plus relocated aftershocks of 1971 San Fernando and 1994 Northridge earthquakes (brown and blue dots, respectively); main shock focal mechanisms (far hemispheres) are red (San Fernando; Heaton, 1982) and blue (Northridge; Hauksson et al., 1995). Aftershocks are projected onto line 2 from up to 10 km east.

    • This is a smaller scale cross section from Fuis et al. (2003) showing a broader view of the faults in this region. This shows the velocity model color legend that also applies to the above figure. This is their figure 4.

    • Similar to Fig. 3, with expanded depth and distance frame. See caption for Fig. 3 for definition of red, magneta, and blue lines; orange line—interpreted San Andreas fault (SAF); yellow lines—south-dipping reflectors of Mojave Desert and northern Transverse Ranges; “K” —reflection of Cheadle et al. (1986), which is out of plane of this section. SAF is not imaged directly; interpretation is based on approximate northward termination of upper reflections (best constrained) in San Fernando reflective zone (magenta lines). (See similar interpretation for SAF on line 1—Fig. 5.) Wells shown in Mojave Desert are (s) H&K Exploration Co., (t) Meridian Oil Co. (Dibblee, 1967). For well color key, see caption for Fig. 3. Thin, dashed yellow-orange line—estimated base of Cenozoic sedimentary rocks in Mojave Desert based on velocity. Darker, multicolored region (above region of light violet) represents part of velocity model where resolution ≥ 0.4 (see color bar).

    • Here is a fascinating figure from Carena and Suppe (2002) showing the 3-dimensional configuration of the faults involved in the 1971 and 1994 earthquakes.

    • Perspective view, looking from the SE, of the modeled Northridge and San Fernando thrusts. The Northridge thrust stops at a depth of about 6 km, and its upper tip east of the lateral ramp (Fig. 4) terminates almost against the San Fernando thrust, as was suggested by Morti et al. (1993). The San Fernando thrust loser tip is at a depth o 13 km, whereas the Northridge thrust lower tip is at 32 km.

    • Here is a map view of the Carena and Suppe (2002) interpretation of these fault planes.

    • Schematic geological map showing the position of the main faults and folds, as well as the depth contours (contour interval = 1 km) of the Northridge (solid) and San Fernando (dashed) thrusts.

    • Here is a structural cross section across this region (Carena and Suppe, 2002).

    • Cross-section through the San Fernando Valley with projected aftershocks of the 1994 Northridge earthquake and of the 1971 Sylmar earthquake. The Northridge aftershocks are projected from a distance of 1 km or less on each side of the cross-section (main shock projected from 2 km W), whereas those of the Sylmar earthquake are projected from 1.5 km or less (main shock projected from 5 km ESE). The sources that we used for near-surface geology and structure are Dibblee (1991) and a seismic line (Fig. 11). The large N-S changes in Upper Tertiary stratigraphic thicknesses in this region (Dibblee, 1991, 1992a), prevent detailed stratigraphic correlation across fault blocks (this figure and Fig. 12). This face suggests that the shallow faults and possible the deeper San Fernando thrust itself, are reactivating old normal faults of the southern margin of the Ventura Basin (Yeats, et al., 1994; Huftle and Yeats, 1996; Tsutsumi and Yeats, 1999). Location of cross-section is in Fig. 13.

    • Here is a comparison of the ground shaking intensity for these two earthquakes (1971 Sylmar vs. 1994 Northridge). These earthquakes had similar magnitudes, but the 1994 earthquake had a higher MMI. The upper panels are the USGS Shakemaps, which are model based estimates of shaking intensity, based on Ground Motion Predicti0on Equations (GMPE; attenuation relations). The lower panels plot two different sets of data. The orange lines are regression lines that represent how shaking intensity diminishes (attenuates) with distance from the earthquake. These are regressions based upon these GMPE relations. More about GMPE relations can be found here. The dots are data from real observations made by people who have reported this on the USGS Did You Feel It? website for each of these earthquakes. More about the DYFI program can be found here.

    Some Background Materials

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

    • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
    • Strike Slip:

      Compressional:

      Extensional:

    Documentaries

    Social Media

    References

    • Carena, S. and Supper, J., 2002. Three-dimensional imaging of active structures using earthquake aftershocks: the Northridge thrust, California in Journal of Structural Geology, v. 24, p. 887-904.
    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • Fuis, G.S>, Ryberg, T., Godfrey, N.J>, Okaya, D.A., and Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1. p. 15-18.
    • Fuis, G.S. et al., 2003. Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II in Geology, v. 31, no. 2, p. 171-174.
    • Hauksson, E., Jones, L.M., and Hutton, K., 1995. The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects in Journal of Geophysical Research, v., 100, no. B7, p. 12235-12355.
    • Tsutsumi, H. and Yeats, R.S., 1999. Tectonic Setting of the 1971 Sylmar and 1994 Northridge Earthquakes in the San Fernando Valley, California in BSSA, v. 89, p. 1232-1249.
    • Tucker, A.Z. and Dolan, J.F., 2001. Paleoseismologic Evidence for a ~8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault, Northern Los Angeles Metropolitan Region, California in BSSA, v. 91, no. 2, p. 232-249.

    Posted in collision, earthquake, education, geology, los angeles, plate tectonics, San Andreas

    Earthquake Report: Mendocino fault

    Good Morning Humboldt County!

    I was in bed checking up on social media stuff and I checked my email. There were two emails from USGS ENS showing a M 5.0 near me. I had not felt it and when I went to the USGS websites, the event had been deleted. However, when i returned to twitter, I noticed @Allomax had tweeted about a M 5.7 on the Mendocino fault. So, I got out of bed and made some coffee (half decaf). It has been a busy January (lots of earthquakes).

    Today’s M 5.8 earthquake happened along the Mendocino fault zone, very close to the 1994 M 7.1 epicenter. Given this being an offshore location, the location uncertainty with those settings, these may have both happened in the same location.

    Today’s M 5.8 earthquake was along the western part of the Mendocino fault (MF), a right-lateral (dextral) transform plate boundary. This plate boundary connects the Gorda ridge and Juan de Fuca rise spreading centers with their counterparts in the Gulf of California, with the San Andreas strike-slip fault system. Transform plate boundaries are defined that they are strike-slip and that they connect spreading ridges. In this sense of the definition, the Mendocino fault and the San Andreas fault are part of the same system. Here is the USGS website for this earthquake.

    See the figures from Rollins and Stein (2010) below. More on earthquakes in this region can be found in Earthquake Reports listed at the bottom of this page above the appendices.

    The San Andreas fault is a right-lateral strike-slip transform plate boundary between the Pacific and North America plates. The plate boundary is composed of faults that are parallel to sub-parallel to the SAF and extend from the west coast of CA to the Wasatch fault (WF) system in central Utah (the WF runs through Salt Lake City and is expressed by the mountain range on the east side of the basin that Salt Lake City is built within).

    The three main faults in the region north of San Francisco are the SAF, the MF, and the Bartlett Springs fault (BSF). I also place a graphical depiction of the USGS moment tensor for this fault. The SAF, MF, and BSF are all right lateral strike-slip fault systems. There are no active faults mapped in the region of Sunday’s epicenter, but I interpret this earthquake to have right-lateral slip. Without more seismicity or mapped faults to suggest otherwise, this is a reasonable interpretation.

    The Cascadia subduction zone is a convergent plate boundary where the Juan de Fuca and Gorda plates (JDFP and GP, respectively) subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. The Juan de Fuca and Gorda plates are formed at the Juan de Fuca Ridge and Gorda Rise spreading centers respectively. More about the CSZ can be found here.

    There was a good sized (M 6.5) MF earthquake late in 2016 2016.12.08. I present my poster for that earthquake below. Here is my report for that earthquake. Here is the updated report.

    There was also a M 5.7 earthquake in 2017 (2017.09.22). Here is my report for that earthquake.

    Below is my interpretive poster for this earthquake


    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 5.5.

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 5.8 earthquake, in addition to the 1994 Mendocino fault earthquake.

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

    • In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004). I placed a blue star in the general location of today’s M 5.7 earthquake.
    • Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. Today’s earthquake did not occur along the CSZ, so did not produce crustal deformation like this. However, it is useful to know this when studying the CSZ.
    • In the lower left corner is a figure from Rollins and Stein (2010). In their paper they discuss how static coulomb stress changes from earthquakes may impart (or remove) stress from adjacent crust/faults. This map shows the major earthquakes that have occurred in this region, prior to their publication in 2010. I place a blue star in the general location of today’s earthquake.
    • In the upper left corner is a map showing historic focal mechanisms along the MF (Dengler et al., 1995). This figure shows how the GPS sites moved during that earthquake, showing that the CSZ megathrust fault is seismologically coupled.


    • This is the report from my DYFI submission.

    , with Cande and Kent’s [1995] timescale correction.

    • Here is the Rollins and Stein (2010) figure that is in the report above. I include their figure caption as blockquote below.

    • Coulomb stress changes imparted by our models of (a) a bilateral rupture and (b) a unilateral eastward rupture for the 1994 Mw = 7.0 Mendocino Fault Zone earthquake to the epicenters of the 1995 Mw = 6.6 southern Gorda zone earthquake (N) and the 2000 Mw = 5.9 Mendocino Fault Zone earthquake (O). Calculation depth is 5 km.

    • 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.

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

    • 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. These two aftershocks align on what may be the eastern extension of the Mendocino fault.
    • 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.
    • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
    • Strike Slip:

      Compressional:

      Extensional:

    • This figure shows what a transform plate boundary fault is. Looking down from outer space, the crust on either side of the fault moves side-by-side. When one is standing on the ground, on one side of the fault, looking across the fault as it moves… If the crust on the other side of the fault moves to the right, the fault is a “right lateral” strike slip fault. The Mendocino and San Andreas faults are right-lateral (dextral) strike-slip faults. I believe this is from Pearson Higher Ed.

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      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.
    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • 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/].

    Posted in earthquake, education

    Earthquake Report: Gulf of Alaska UPDATE #2

    I thought it would be interesting to see the seismicity with time. Perhaps this could help us learn about the fault sources associated with this earthquake sequence.

    I am not sure it worked as some issues cannot be dealt with simply with this visualization.

    For example, the locations for these earthquakes may not be resolute enough [yet] to figure out the orientation of the faults at work here. The back projection data are perhaps the strongest evidence for an east-west fault. However, we still have the contradictory sense of motion along the fracture zones at the meso scale… (as revealed in the EMAG2 magnetic anomaly data).

    As a reminder, if the M 7.9 earthquake fault is E-W oriented, it would be left-lateral. The offset magnetic anomalies show right-lateral offset across these fracture zones. This was perhaps the main reason why I thought that the main fault was not E-W, but N-S. After a day’s worth of aftershocks, the seismicity may reveal some north-south trends. But, as a drama student in 7th grade (1977), my drama teacher (Ms. Naichbor, rest in peace) asked our class to go stand up on stage. We all stood in a line and she mentioned that this is social behavior, that people tend to stand in lines (and to avoid doing this while on stage). Later, when in college, professors often commented about how people tend to seek linear trends in data (lines). I actually see 3-4 N-S trends and ~2 E-W trends in the seismicity data.

    So, that being said, here is the animation I put together. I used the USGS query tool to get earthquakes from 1/22 until now, M ≥ 1.5. I include a couple inset maps presented in my interpretive posters. The music is copyright free. The animations run through twice.

    Here is a screenshot of the 14 MB video embedded below. I encourage you to view it in full screen mode (or download it).


    • Here is the seismograph at Humboldt State University, Dept. of Geology. The seismometer is located in the basement of Founders Hall, across from the Geology Dept. office.

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    Posted in alaska, earthquake, education, geology, pacific, plate tectonics, strike-slip, subduction

    Earthquake Report: Gulf of Alaska UPDATE #1

    Well. What a firestorm of social media discusions about this earthquake. It seems that, like how we learn so much when earthquakes like this happen, the amount of interacting in public on social media has been growing earthquake by earthquake.

    I spent some time this afternoon looking at the magnetic anomalies, after taking a load of a part of an old building to the county dump (transfer station) before the rain started. Stephen Hicks found a great paper (and tweeted about it, see my original report here where I include his tweet).

    In my original report, I proposed that if the earthquake happened on the USGS fault model, then there is a problem when considering the magnetic anomaly map. The USGS fault solution is left-lateral, but the magnetic anomaly offsets appear to be right-laterally offset. Upon further review, I noticed that there are some details in this area that could be interpreted as left lateral. In my poster below, I place a white arrow along the hypothetical fault (drawn as a green dashed line). I located the line based upon offsets in the magnetic anomaly data as aligned with the USGS model.

    Then I took a look at the mag anomaly map from Naugler and Wageman (1973). These authors show the isochrons for the Gulf of Alaska (GA). The fracture zone nearest today’s M 7.9 earthquake is right-lateral (supporting my original interpretation). However, the USGS fault model appears to be oblique to this fracture zone. Perhaps today’s M 7.9 is on a conjugate fault, with a different sense of motion.

    Interesting that the USGS fault model terminates on the eastern side with the epicenter from a 1999 earthquake. This earthquake has a fault plane solution that shows oblique slip, not pure strike-slip. This could be because (1) the earthquake happened on a different fault or (2) the earthquake happened on the same fault, but the fault is changing its orientation (I favor the first hypothesis).

    Some people have been stating that the aftershocks appear to be aligned in a north-south orientation. I cannot figure out how they made this observation, but maybe I am missing something. This did make me think about instances where off fault earthquakes can be triggered, or when there are major fault systems that are not reflected in the geomorphology nor other measures of long term tectonics (like magnetic anomalies or fracture zones). A great example is the 2012 M 8.6 Wharton Basin earthquakes that ruptured in response to the 2004 Sumatra-Andaman subduction zone earthquake eight years earlier. Today’s M 7.9 earthquake is rather deep (like the 2012 earthquakes), so perhaps there are some deep faults that are not reflected by the shape of the seafloor nor reflected by the gravity data for some reason (the former seems more likely to me).

    Below is my interpretive poster for this earthquake


    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5. More about the plate boundary can be found in that report.

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.9 earthquake, in addition to some relevant historic earthquakes.

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
    • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. Slab 2.0 is due out later this year!
    • I include some inset figures.

    • In the lower right corner, I place a map from Naugler and Sageman (1973). I added relative slip vectors for the fracture zones here. I place the epicenter from today’s earthquake as a cyan star.


    • As I was rereading my report (don’t always get a chance, but good to check for typos), looking at the aftershocks, and considering the problems associated with this earthquake and its tectonic setting (i.e. right-lateral fracture zones and a left-lateral USGS fault solution), I decided to make some updates to this large scale poster. There were several aftershocks while I was making this map that made a north-south trend more apparent. So, now I am favoring the following interpretation: the M 7.9 mainshock and many aftershocks are the result of a right-lateral north-striking strike-slip fault.
    • This is the exact same thing that happened following the 2012 Wharton Basin M 8.6/8.2 earthquake sequence along the outer rise of the Sumatra-Andaman subduction zone. The M 8.6 is the largest strike-slip intraplate earthquake ever recorded on modern seismometers. I present some maps from Sumatra earthquakes below. Basically, the fracture zones in the the India-Australia plate trend north-south. So that was my initial interpretation, that these earthquakes were left-lateral earthquakes on faults associated with these fracture zones. However, this was not the case. The Wharton Basin earthquake sequence involved both fracture zone related faults, in addition to conjugate faults trending east-west. There were initially fault slip models for both interpretations.


    Some Relevant Discussion and Figures

    • This is a great map from UNAVCO. This shows the static offsets to GPS sites as a result of this M 7.9 earthquake.

    • Seismically derived static displacements (first figure, pink is p1 and blue is p2) and their difference (figure 2)(Figure/Dave Mencin, UNAVCO)

    • However, there is supporting evidence that the USGS fault is the correct fault. I just watched the back projections for this earthquake prepared by IRIS. Basically, these are animations that show where the energy was released, at what time, during this earthquake. These animations show an east-west trending energy release. I am not a seismologist, so don’t know if these back projections absolutely rule out a north-south fault.
    • Here are their two animations.
    • Here are some interpretive posters from the 2012 Sumatra Outer Rise earthquake sequence.
    • I have presented materials related to the 2004 Sumatra-Andaman subduction zone earthquake here and more here.
    • I include a map in the upper right corner that shows the historic earthquake rupture areas. There is a figure from Meng et al. (2012) that shows the details about the faults and the seismicity.

    • Here is that Meng et al. (2012) figure showing the different faults that ruptured in 2012.

    • Here is a poster that shows some earthquakes in the Andaman Sea. This is from my earthquake report from 2015.11.08.

    • This map shows the fracture zones in the India-Australia plate.

    Review Stuff from my first report.

    • Here is a map for the earthquakes of magnitude greater than or equal to M 7.0 between 1900 and 2016. This is the USGS query that I used to make this map. One may locate the USGS web pages for all the earthquakes on this map by following that link.

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    Posted in alaska, earthquake, education, geology, pacific, plate tectonics, strike-slip, subduction

    Earthquake Report: Gulf of Alaska!

    I was asleep in bed, trying to catch up to prevent myself from getting ill, when there was a large earthquake in the Gulf of Alaska (GA), offshore of Kodiak, Alaska. When I wakened, I noticed a fb message from my friend Scott Willits notifying me of an M 8.2 earthquake in Alaska, posted at 2:20 AM local time. I immediately got up to check on this and was surprised that there was not a tsunami evacuation going on. I live in the small town of Manila (population ~700), on the North Spit (a sand spit west of Arcata and Eureka, CA). I live above 10 m in elevation and do not consider myself exposed to tsunami risks, local or distant (especially given that (1) the CSZ locked zone is mostly under land here and (2) that the part of the locked zone that is not under land is in shallow water; so our local tsunami will probably be much smaller than further north, like Crescent City or Brookings). I have been involved in tsunami education and outreach for over 15 years and prepared the first tsunami hazard map for northern CA (working with Dr. Lori Dengler and the Redwood Coast Tsunami Work Group). Needless to say, I am cogent and aware about the tsunami risk here in norcal.

    SO. I soon discovered that the GA earthquake happened in the Pacific plate, far from the subduction zone and that the earthquake was a strike-slip earthquake. Both of these facts explained why the sheriff had not been at my door earlier this morning. In addition, the magnitude had been adjusted to M 7.9 (no longer a Great earthquake, just a Large earthquake; earthquake classes are defined here). However, there were some small tsunami waves observed (see below) as reported by the National Tsunami Warning Center (see social media below).

    This earthquake appears to be located along a reactivated fracture zone in the GA. There have only been a couple earthquakes in this region in the past century, one an M 6.0 to the east (though this M 6.0 was a thrust earthquake). The Gulf of Alaska shear zone is even further to the east and has a more active historic fault history (a pair of earthquakes in 1987-1988). The magnetic anomalies (formed when the Earth’s magnetic polarity flips) reflect a ~north-south oriented spreading ridge (the anomalies are oriented north-south in the region of today’s earthquake). There is a right-lateral offset of these magnetic anomalies located near the M 7.9 epicenter. Interesting that this right-lateral strike-slip fault (?) is also located at the intersection of the Gulf of Alaska shear zone and the 1988 M 7.8 earthquake (probably just a coincidence?). However, the 1988 M 7.8 earthquake fault plane solution can be interpreted for both fault planes (it is probably on the GA shear zone, but I don’t think that we can really tell).

    This is strange because the USGS fault plane is oriented east-west, leading us to interpret the fault plane solution (moment tensor or focal mechanism) as a left-lateral strike-slip earthquake. So, maybe this earthquake is a little more complicated than first presumed. The USGS fault model is constrained by seismic waves, so this is probably the correct fault (east-west).

    I prepared an Earthquake Report for the 1964 Good Friday Earthquake here.

    UPDATE #1 2018.01.23 7:40 PM pacific: I have prepared an updated report here.

    Below is my interpretive poster for this earthquake


    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5. More about the plate boundary can be found in that report.

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.9 earthquake, in addition to some relevant historic earthquakes.

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
    • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. Slab 2.0 is due out later this year!
    • I include some inset figures.

    • In the upper left corner, I place a map created by Dr. Peter Haeussler, USGS, which shows the historic earthquakes along the Alaska and Aleutian subduction zones. I place the epicenter from today’s earthquake as a cyan star.
    • To the right of this map, I include first the USGS map that shows their interpretation of where the fault is (the red line) and then I include the USGS fault slip model (color = slip in meters).
    • In the upper right corner is a map from IRIS that shows seismicity with color representing depth.
    • In the lower right corner, I include a low angle oblique view of the subduction zone, showing how the Pacific plate is subducting beneath the North America plate.
    • In the lower left corner, I include a map that shows the magnetic anomalies in the GA region. I include USGS seismicity from 1918-2018 for earthquakes M ≥ 5.5.


    • UPDATE 12:45 my local time
    • The USGS updated their MMI contours to reflect their fault model. Below is my updated poster. I also added green dashed lines for the fracture zones related to today’s M 7.9 earthquake (on the magnetic anomaly inset map).


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

    • Here is an educational video from IRIS about the tectonics in Alaska.

    Some Relevant Discussion and Figures

    • Here is a map for the earthquakes of magnitude greater than or equal to M 7.0 between 1900 and 2016. This is the USGS query that I used to make this map. One may locate the USGS web pages for all the earthquakes on this map by following that link.

    • Here is a cross section showing the differences of vertical deformation between the coseismic (during the earthquake) and interseismic (between earthquakes).

    • Here is a figure recently published in the 5th International Conference of IGCP 588 by the Division of Geological and Geophysical Surveys, Dept. of Natural Resources, State of Alaska (State of Alaska, 2015). This is derived from a figure published originally by Plafker (1969). There is a cross section included that shows how the slip was distributed along upper plate faults (e.g. the Patton Bay and Middleton Island faults).

    • Here is an animation that shows earthquakes of magnitude > 6.5 for the period from 1900-2016. Above is a map showing the region and below is the animation. This is the URL for the USGS query that I used to make this animation in Google Earth.

    • Here is a link to the file for the embedded video below (5 MB mp4)
    • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
    • Strike Slip:
    • Compressional:
    • Extensional:
    • This figure shows what a transform plate boundary fault is. Looking down from outer space, the crust on either side of the fault moves side-by-side. When one is standing on the ground, on one side of the fault, looking across the fault as it moves… If the crust on the other side of the fault moves to the right, the fault is a “right lateral” strike slip fault. The Mendocino and San Andreas faults are right-lateral (dextral) strike-slip faults. I believe this is from Pearson Higher Ed.

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

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    Posted in alaska, earthquake, education, geology, pacific, plate tectonics, strike-slip, subduction, Transform

    Earthquake Report: Gulf of California

    Today we had an earthquake with magnitude M 6.3 in the Gulf of California (GOC). The GOC is formed by transtension (extension along a strike-slip fault system) along the North-America-Pacific plate boundary. Transtension happens when the plate motion across a fault is not oriented parallel to the fault. This non-optimal relation (plate motion vs fault direction) can generally happen as a result of (1) a bending fault or (2) due to stepped offsets of the fault. The dextral (right-lateral) strike-slip faults here make “right-steps” and pull apart basins form in these locations.

    The strike-slip faults are offset by oceanic spreading ridges. These spreading ridges are connected the East Pacific Rise to the south (and the Juan de Fuca Ridge and Gorda Rise to the north, via the San Andreas fault).

    The geology of this region is much more complicated than this, but this is a good place to start when trying to understand the tectonics here. This M 6.3 earthquake happened on the southern boundary of the Guamas Basin, one of these pull apart basins.

    So far there has been a single aftershock (M 4.5).

    This M 6.3 earthquake appears to be pretty typical of this part of the Gulf. There have been several M 6 earthquakes in the last century. There have been a couple M 7 earthquakes, to the north.

    Below is my interpretive poster for this earthquake

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5.

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.3 earthquakes, in addition to some relevant historic earthquakes.

    I labeled the pull-apart basins in cyan and the faults in light orange. CdBF – C. de Ballenas fault; GF – Guaymas fault; CF – Carmen fault; FF – Farallon fault; PF – Pescadero fault; AF – Alcaron fault; AR – Alcaron Ridge (from Aragón-Arreola, M. and Martín-Barajas, A., 2007).

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

      I include some inset figures.

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    • This is a nice simple figure, from the University of Sydney here, showing the terminology of strike slip faulting. It may help with the following figures.
    • Here is a fault block diagram showing how strike-slip step overs can create localized compression (positive flower) or extension (negative flower). More on strike-slip tectonics (and the source of this image) here.

    • I also put together an animation of seismicity from 1065 – 2015. First, here is a map that shows the spatial extent of this animation.

    • Here is the animation link (2 MB mp4 file) if you cannot view the embedded video below. Note how the animation begins in 1965, but has the recent seismicity plotted for reference.
    • This is an animation from Tanya Atwater. Click on this link to take you to yt (if the embedded video below does not work).
    • Here is an animation from IRIS. This link takes you to yt (if you cannot view the embedded version below). Here is a link to download the 21 MB mp4 vile file.

    Update

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

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

    References:

    Posted in earthquake, education, geology, mexico, plate tectonics, strike-slip

    Earthquake Report: Peru Update #1

    Well, I missed looking further into a key update paper and used figures from an older paper on my interpretive poster yesterday. Thanks to Stéphane Baize for pointing this out! Turns out, after their new analyses, the M 7.1 earthquake was in a region of higher seismogenic coupling, rather than low coupling (as was presented in my first poster).

    Also, Dr. Robin Lacassin noticed (as did I) the paucity of aftershocks from yesterday’s M 7.1. This was also the case for the carbon copy 2013 M 7.1 earthquake (there was 1 M 4.6 aftershock in the weeks following the M 7.1 earthquake on 2013.09.25; there were a dozen M 1-2 earthquakes in Nov. and Dec. of 2013, but I am not sure how related they are to the M 7.1 then). I present a poster below with this in mind. I also include below a comparison of the MMI modeled estimates. The 2013 seems to have possibly generated more widespread intensities, even though that was a deeper earthquake.

    Below is my interpretive poster for this earthquake

    I plot USGS seismicity from 2013.09.24 through 2014.01.26 (about 3 months), in addition to the 2018.01.14 M 7.1 earthquake.

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. However, we must await slab v 2.0 to get a better view of these slab contours in this region.
    • I include some inset figures.

    • In the upper right corner is an updated time-space figure (showing along-strike lengths for historic earthquakes), along with slip patches for some of these earthquakes (Villegas-Lanza et al., 2016). I place a blue star in the general location of the 2018.01.14 M 7.1 earthquake.
    • This is the updated seismic coupling figure from Villegas-Lanza et al. (2016). I place a blue star in the general location of the 2018.01.14 M 7.1 earthquake. Note how this M 7.1 earthquake is in a region of higher coupling.



    • Here is a comparison of the intensity modeling for these comparable earthquakes. I present the intensity maps on top (with the moment tensors, labled with their strike, dip, and rake data; note how they are almost identical!), the attenuation relations in the middle (how intensity decays with distance from the earthquake), and the PAGER alerts at the bottom. More can be found out about PAGER alerts here.

    • Here is the Villageas-Lanza et al. (2016) figure 1, showing their time-space diagram, along with the historic earthquake limits and patches.

    • (a) Seismotectonic setting of the South American subduction zone. The red ellipses indicate the approximate rupture areas of large subduction earthquakes (M≥ 7.5) between 1868 and 2015 [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Beck et al., 1998]. The blue ellipses indicate the locations of moderate tsunami-earthquakes [Pelayo and Wiens, 1990; Ihmle et al., 1998]. The bathymetry from GEBCO30s highlights the main tectonic structures of the subducting Nazca Plate, which are from north to south: Carnegie Ridge (CR), Grijalva Ridge (GR), Alvarado Ridge (AR), Sarmiento Ridge (SR), Virú Fracture Zone (VFZ), Mendaña Fracture Zone (MFZ), Nazca Ridge (NR), Nazca Fracture Zone (NFZ), Iquique Ridge, Juan Fernandez Ridge, Challenger Fracture Zone (CFZ), and Mocha Fracture Zone (MCFZ). The white arrow indicates the convergence of the Nazca Plate relative to the stable South America (SSA) reference frame [Kendrick et al., 2003]. The slab geometry isodepth contours are reported every 50 km (solid lines) and 10 km (dashed lines), based on the Slab1.0 model [Hayes et al., 2012]. The dashed rectangle corresponds to Figures 1b and 1c. The N.A.S. and C.A.S. labels indicate the North Andean and the Central Andes Slivers [Bird, 2003], respectively. (b) Temporal and spatial distributions of large subduction earthquakes with Mw ≥ 7.5 that occurred in Peru since the sixteenth century. The rupture extent values (in km) of historical (gray) and recent (red) megathrust earthquakes along the Peruvian margin are shown as a function of time (in years). A triangle indicates if a tsunami was associated with the event. The orange bands denote the entrance of the NR and the MFZ delimiting the northern, central, and southern Peru subduction segments. The rupture lengths were taken from its corresponding published slip models [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Pelayo and Wiens, 1990; Ihmle et al., 1998; Giovanni et al., 2002; Salichon et al., 2003; Pritchard et al., 2007; Bilek, 2010; Delouis et al., 2010; Moreno et al., 2010; Schurr et al., 2014], and for historical earthquakes, we estimated its approximated lengths using scaling law relationships [Wells and Coppersmith, 1994]. (c) A map of the rupture areas of large subduction earthquakes that occurred in the twentieth century [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Ihmle et al., 1998; Giovanni et al., 2002; Sladen et al., 2010; Chlieh et al., 2011], with their associated gCMT focal mechanisms. In northern Peru, the 1960 (Mw = 7.6) Piura earthquake and the 1996 (Mw = 7.5) Chimbote earthquake, which are shown by cyan-colored polygons, were identified as tsunami-earthquake events [Pelayo and Wiens, 1990; Ihmle et al., 1998; Bilek, 2010].

    • Here is a figure from Villegas-Lanza et al. (2016) that shows the along-strike variation in moment deficit. Moment deficit is the amount of energy absorbed into the tectonic system, from plate motions, that is stored as seismic strain to be released during earthquakes. Regions where the fault is slipping freely (aseismic), seismic moment does not accumulate, so there is no moment deficit there (e.g. along the subduction zone where the Nazca Ridge intersects the megathrust). The two panels on the right are their minimum and maximum seismogenic coupling maps (showing the end members of their models). I explain the coupling ratio (0-1, white to red in color) on my initial earthquake report.
    • The key update in this paper (an update to the Chlieh et al., 2011 results) is that these authors treated the accretionary part of the South America plate as an independent player, as a forearc sliver (sort of like a microplate)

    • (left) Along-trench variations of moment deficit rate for (middle) minimum and (right) maximum interseismic coupling models. Even though the interseismic pattern might vary significantly between models, the locations of the peaks and valleys in the rate of moment deficit are very persistent characteristics that highlight the locations of the principal asperities (peaks) and creeping barriers (valleys). The dashed ellipse contours in the middle map show the approximate rupture area of large earthquakes, as described in Figure 1. (see above, the time-space figure)

    • Here is the Chlieh et al. (2011) version of this figure for comparison.
    • This is the figure that adds moment deficit to the seismic moment plot and the coupling ratio to the slip patch map.

    • Comparison of interseismic coupling along the megathrust with ruptures of large megathrust earthquakes in central and southern Peru. (left) Interseismic coupling map for 3-plate model Short4; it indicates that where the Nazca ridge and the Nazca fracture zone subduct, the interseismic coupling is low. The largest earthquake there is the Mw8.1–8.2 earthquake of 1942. It is not clear whether the 1942 rupture propagated through the Nazca ridge or stopped south of it. High interseismic coupling patches correlate well with regions that experienced great megathrust earthquake Mw8.8 in 1868 and Mw8.6–8.8 in 1746. In the south, the presence of two wide asperities separated by a wide aseismic patch may explain partially the seismic behavior of this segment in the last centuries. Individual ruptures of these asperities would produce Mw8 events, as in 2001, but their simultaneous rupture could generate great Mw > 8.5 earthquakes as in 1604 or 1868. The along-strike coincidence of the high coupling areas (orange-red) with the region of high coseismic slip during the 2001 Arequipa and 2007 Pisco earthquakes suggests that strongly coupled patches during the interseismic period may indicate the location of future seismic asperities. (right) Moment deficit (dashed lines) since the last great earthquake of 1868, 1942 and 1746 compared with the seismic moment released during recent and historical earthquakes of Figure 8. The moment deficit is computed from the rate of moment deficit predicted by model Short4 considering a steady state interseismic process (Max) or 50% of it (Min) to account for time-variable interseismic process and transient events.

    • Here is an updated moment deficit figure for this part of the Peru-Chile trench (Villegas-Lanza et al., 2016). I include the Chlieh et al. (2011) figure for comparison.
    • Chlieh et al., 2011
    • This figure shows their results used to show how different parts of the subduction zone have higher or lower moment deficits. The Central Peru section shows that there is an unmet interseismic deficit, while the southern Peru profile shows that earthquakes have been keeping up with plate convergence here. The central Peru region is the region of the subduction zone shown in the 2 figures above this one (Arica, Peru is the southern boundary between the central and southern Peru regions of this subduction zone).

    • Cumulative deficit of moment and seismic moment released due to major subduction earthquakes since the 16th century (top) in central Peru and (bottom) in southern Peru. The cumulative deficit of moment is predicted from the rates of 3-plate models Short4 in central Peru and Short10 in southern Peru (Table 5 and Figure 4). The uncertainties of moment released by historical events lead to a minimum and maximum moment released (see Table S8 in the auxiliary material). The uncertainties on the cumulative deficit of moment allow that nonlinear interseismic and viscous processes could have released 50% of the accumulated moment deficit. The remaining fraction should reflect elastic strain available to drive future earthquakes unless it would have been totally released by anelastic deformation of the forearc.

    • Villegas-Lanza et al., 2016

    • Cumulative moment deficit corrected from large earthquakes moment released since 1746, computed using the maximum, mean, and minimum interseismic models presented in Figure 6 and Table S8.

      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., et al., 2011. Interseismic coupling and seismic potential along the Central Andes subduction zone in JGR, v. 116, B12405, doi:10.1029/2010JB008166
    • Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V., Antoine, P.-O., Salas-Gismodi, R., and Bolaños, R., 2007. How does the Nazca Ridge subduction influence the modern Amazonian foreland basin? in Geology, v. 35, no. 6, p. 515-518.
    • Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
    • 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
    • Ray., J.S., et al., 2012. Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ~30 Myr Hotspot Record in Journal of Petrology, v. 53., no. 7, p. 1417-1448.
    • Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., Furlong, K.P., 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 map sheet, scale 1:12,000,000.
    • Scire, A., Zandt, G., Beck, S., Long, M., and Wagner, L., 2017, The deforming Nazca slab in the mantle transition zone and lower mantle: Constraints from teleseismic tomography on the deeply subducted slab between 6°S and 32°S: Geosphere, v. 13, no. 3, p. 665–680, doi:10.1130/GES01436.1.
    • Villegas-Lanza, J.C., et al., 2016. Active tectonics of Peru: Heterogeneous interseismic coupling along the Nazca Megathrust, rigid motion of the Peruvian Sliver and Subandean shortening accommodation in JGR, doi: 10.1002/2016JB013080

    Posted in earthquake, education, geology, pacific, plate tectonics, subduction

    Earthquake Report: Peru

    We had a damaging and (sadly) deadly earthquake in southern Peru in the last 24 hours. This is an earthquake, with magnitude M 7.1, that is associated with the subduction zone forming the Peru-Chile trench (PCT). The Nazca plate (NP) is subducting beneath the South America plate (SAP). There are lots of geologic structures on the Nazca plate that tend to affect how the subduction zone responds during earthquakes (e.g. segmentation).

    In the region of this M 7.1 earthquake, two large structures in the NP are the Nazca Ridge and the Nazca fracture zone. The Nazca fracture zone is a (probably inactive) strike-slip fault system. The Nazca Ridge is an over-thickened region of the NP, thickened as the NP moved over a hotspot located near Salas y Gomez in the Pacific Ocean east of Easter Island (Ray et al., 2012).

    There are many papers that discuss how the ridge affects the shape of the megathrust fault here. The main take-away is that the NR is bull dozing into South America and the dip of the subduction zone is flat here. There is a figure below that shows the deviation of the subducting slab contours at the NR.

    There was an earthquake in 2013 that is almost a carbon copy of this 2018.01.14 M 7.1 earthquake. The USGS earthquake epicenters are about 20 km from each other and the USGS hypocenters are within 5 km. They also have almost identical fault plane solutions (moment tensors). Based upon the different cross sections, I am unsure whether this earthquake is in the upper or lower plate.

    UPDATE 2324

    Based upon Stéphane‘s comment (see social media update), I need to edit my twitter post. I posted that this earthquake happened in a region of low seismogenic coupling. This was based upon Chlieh et al. (2011) GPS inversion modeling. Stéphane pointed out that the Villegas Lanza et al. (2016) show this to be in a region of higher coupling. I consider the Villegas-Lanza paper to be more up-to-date, so will favor their interpretation of the coupling along this fault. This updated analysis includes more GPS rate sites, as well as a suite of additional types of data. They also model the crust with a better version of the Peruvian Forearc Sliver, which is the most significant change in how they treated fit their data *see their figure 7). These modifications changed significantly the spatial variation in seismogenic coupling along the plate margin where the M 7.1 was located.

    Below is my interpretive poster for this earthquake


    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 (and down to M ≥ 5.5 in a second poster).

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.3 earthquakes, in addition to some relevant historic earthquakes.

    UPDATE 2018.01.15

  • 2018.01.15 M 7.1 Peru Update #1
    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
    • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
    • I include (faintly) the MMI contours from most of the larger magnitude earthquakes for which there are data available. As mentioned above, these are estimates based upon numerical models using empirical relations between earthquakes and their shaking intensity. These MMI estimates are controlled by a variety of things, principally magnitude and distance to the fault. Some estimates are made using rectangular shaped fault sources (e.g. 2001 M 8.4), some from point source distances (e.g. 1966 M 8.1).
    • I outline the MMI VII contour because (1) this is the largest MMI contour for the M 7.1 earthquake and (2) this is the “very strong” shaking intensity that can cause moderate damage to buildings. These outlines are in dashed white and are labeled in yellow for the causative earthquake magnitude. The M 7.1 MMI VII contour is from a point source, so would probably be more rectangular in reality (though the earthquake is deeper).
    • I include some inset figures.

    • In the upper right corner is a section of the map from Rhea et al. (2010), which is a USGS map documenting the seismicity of the earth in this region. The cross section B-B’ is shown to the left. The cross section plots the earthquake depths along the profile shown on the map. The B-B’ profile crosses the subduction zone very close to where this earthquake happened. I place a blue star in the general location of today’s M 7.1 earthquake.
    • In the lower right corner is a map from Chlieh et al. (2011) that shows some historic earthquake slip patches. The colors represent the amount of slip on the earthquake fault. I place a blue star in the general location of today’s M 7.1 earthquake. Note how this M 7.1 earthquake is at the boundary of the 2001 M 8.4 and 1996 M 7.7 earthquakes.
    • In the upper right corner is a figure that addresses, from left to right:
      1. Historic (including pre-instrumental) earthquakes, their along-strike distances
      2. Slip patches for instrumental earthquakes; I place a blue star in the general location of today’s M 7.1 earthquake.
      3. Seismic moment for instrumental earthquakes; Seismic moment is the amount of energy released during an earthquake. The 2007, 1996, and 2001 earthquakes are part of their analyses and contain more details about the heterogeneous nature of earthquake faults.
    • In the lower left corner is another figure from Chlieh et a. (2011) that provides lots of details from their analyses.
      1. On the left is the a plot of the Seismic Moment as before, with the addition of Moment Deficit. Moment deficit is an estimate of the amount of energy stored in the subduction zone as imparted by plate convergence. Assumptions include (at least) plate motion rates and spatial variation in the amount the fault is seismogenically locked/coupled, etc. Today’s earthquake happened nearby a 1913 M 7.8 earthquake in a region of low moment deficit.
      2. On the right is a plot showing the historic earthquakes again, but with the addition of the coupling ratio. The coupling ratio is the proportion of 100% of the plate motion that is contributing to the strain on the fault. A coupling ratio of 1 (100%) means that 100% of the plate motion is being accumulated as stress on the fault. A ratio of 0 (0%) means that the fault is aseismic (it is slipping all the time). I place a blue star in the general location of today’s M 7.1 earthquake. The 1942, 1996, and today’s M 7.1 earthquakes are along the southern boundary of the NR and in a region of low coupling.



    USGS Earthquake Pages

      These are from this current sequence

    • M 7.1 – 40km SSW of Acari, Peru
      2018-01-14 09:18:45 UTC 15.776°S 74.744°W 36.3 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/us2000cjfy#executive
    • M 8.2 – near the coast of central Peru
      1940-05-24 16:33:59 UTC 11.094°S 77.487°W 45.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/iscgem901374#executive
    • M 8.1 – near the coast of central Peru
      1966-10-17 21:42:00 UTC 10.665°S 78.228°W 40.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/iscgem842581#executive
    • M 7.6 – near the coast of central Peru
      1974-10-03 14:21:29 UTC 12.265°S 77.795°W 13.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/usp0000888#executive
    • M 7.2 – near the coast of central Peru
      1974-11-09 12:59:49 UTC 12.500°S 77.786°W 6.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/usp00008qy#executive
    • M 7.7 – near the coast of central Peru
      1996-11-12 16:59:44 UTC 14.993°S 75.675°W 33.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/usp0007swp#executive
    • M 8.4 – near the coast of southern Peru
      2001-06-23 20:33:14 UTC 16.265°S 73.641°W 33.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/official20010623203314130_33#executive
    • M 7.6 – near the coast of southern Peru
      2001-07-07 09:38:43 UTC 17.543°S 72.077°W 33.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/usp000aj40#executive
    • M 8.0 – near the coast of central Peru
      2007-08-15 23:40:57 UTC 13.386°S 76.603°W 39.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/usp000fjta#executive
    • M 7.1 – 46km SSE of Acari, Peru
      2013-09-25 16:42:43 UTC 15.839°S 74.511°W 40.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/usb000jzma#executive
    • M 8.2 – 94km NW of Iquique, Chile
      2014-04-01 23:46:47 UTC 19.610°S 70.769°W 25.0 km depth
      https://earthquake.usgs.gov/earthquakes/eventpage/usc000nzvd#executive
      • Some Relevant Discussion and Figures

        • Here is an animation from IRIS that reviews the tectonics of the Peru-Chile subduction zone. For the animation, first is a screen shot and below that is the embedded video. This animation is from IRIS. Written and directed by Robert F. Butler, University of Portland. Animation and Graphics: Jenda Johnson, geologist. Consultant: Susan Beck, University or Arizona. Narration: Elayne Shapiro, University of Portland.

        • Here is a download link for the embedded video below (34 MB mp4)
        • The Rhea et al. (2016) document is excellent and can be downloaded here. The USGS prepared another cool poster that shows the seismicity for this region (though there does not seem to be a reference for this).

        • Here is a great view of the Nazca Ridge as it extends to the East Pacific Rise (Ray et al., 2012).

        • Satellite-derived bathymetry (Smith & Sandwell, 1997) of the SE Pacific Basin, showing the Nazca Ridge, Easter Seamount Chain and other features of the Nazca plate. Locations of dredge stations from which samples of this study were obtained are marked. All samples except those labeled with DM (for R.V. Dmitry Mendeleev cruise 14), and GS (for GS7202)
          were collected during the Drift expedition. The dashed line near seamount 115 roughly marks the boundary between the NR and ESC, and the inset shows an enlarged view of the elbow region connecting the two.

        • This shows the age progression along the NR.

        • Distribution of 40Ar39Ar ages of NR, ESC and Easter Island (EI) volcanic rocks vs along-chain distance from Salas y Gomez (SyG). Also shown are the data for lava fields and small seamounts west of EI. Lavas of the East Rift of the Easter Microplate (ER-EMP) are assigned an age of 0 Ma. Our data (plateau ages for all samples fromTable 2 except the total fusion age for DRFT 100-2 and isochron ages for DRFT 115-2 and 126-1) are indicated by filled circles and, for two anomalously young NR samples (DRFT 84-1 and 85-1), by hexagons. Open circles and other symbols are data from O’Connor et al. (1995). Error bars indicate 2s uncertainties on age, if larger than the size of the symbol. The inclined continuous and dashed lines represent linear regressions performed using the algorithm of York et al. (2004) on data for the ESC and NR, respectively, considering the 2s errors on age (years) only (i.e. assuming no significant error resides in the dredge-site locations). Data for the anomalously young DRFT 84 and 85 samples, EI, and seamounts and lava fields west of it are not included in the regressions. These regressions equate to plate motion speeds of 181 and 10·70·1cma1 during the formation of the NR and ESC, respectively. The dashed vertical line roughly marks the boundary between the NR and ESC.

        • These next 3 figures are from Kumar et al. (2016) and reveal the shape of the plate boundary based upon seismicity.
        • This map shows the earthquakes used in their study (color = depth, use this legend for the other map). The thin black lines show their estimate of where the slab is (the megathrust, where the Nazca plate meets the South America plate), depth in km. The NR is the grayed out polygon in the lower left part of the figure (see next map).

        • Map of first motion focal mechanisms plotted in lower hemisphere projec-tion. Mechanisms are color coded by earthquake depth and mainly show normal faulting across the study area. Solid lines are slab contours from Antonijevic et al.(2015). See Figs. S4 and S5 of the supplementary material for zoom-in map of focal mechanism for events inside the red and blue box respectively.

        • This map shows where the cross section profiles are located (Kumar et al., 2016). Today’s M 7.1 earthquake plots almost exactly at the southwestern tip of the P3 profile line.

        • Map showing locations of (a) trench-parallel (BB) and trench-perpendicular (P1, P2, P3, and P7) transects used to plot seismicity cross-sections. Red tick marks on BBrepresents distance interval of 100 km.

        • Here are the earthquake hypocenters plotted for the 4 cross sections plotted in the map above (Kumar et al., 2016). Today’s M 7.1 earthquake plots near the westernmost limit of profile P3. Given a hypocentral depth of ~40 km, this plots in the upper plate. So, perhaps this earthquake is not on the megathrust, but along the decollement. While plotted at a different scale, the same is true when looking at the seismicity cross section from Rhea et al. (2010). Of course, these are just models and could be wrong.

        • Seismicity cross-sections (P1, P2, P3, and P7) perpendicular to the trench. Earthquakes within ±35 km are projected onto each cross-section. The solid line in each cross section is the slab contour from Antonijevic et al.(2015). Red star in each trench-perpendicular cross section marks the intersection with BBcross section. See Figs. S2 and S3 of the supplementary material for the remaining set of trench-parallel and trench-perpendicular seismicity cross-sections.

        • Here are the figures from Chlieh et al. (2011).
        • This is the map showing slip patches (1 meter contours) for earthquakes as derived by inverting GPS geodetic data. Other historic slip patches that are less well constrained are shown in gray dashed polygons. Note the NR and Nazca fracture zone. The 2001 Arequipa M 8.4 (and M 7.6) earthquakes spanned this fracture zone (so did not serve as a segment boundary for that earthquake).

        • Seismotectonic setting of the Central Andes subduction zone with rupture of large (Mw > 7.5) subduction earthquakes on the Peru-Chile megathrust since 1746. The Central Andes sliver is squeezed between the Nazca plate and the South America Craton. Convergence rate of the Nazca plate relative to the South America Craton (black arrow) is computed from Kendrick et al. [2003]. Shortening across the Subandean foothills is represented with the red arrows (assumed parallel to the Nazca/South America plate convergence). Red contours are the 1000 m of the Andes topography and the 5000 m to 3000 m bathymetric contour lines. Historical ruptures are compiled from Beck and Ruff [1989] and Dorbath et al.
          [1990]. Slip distributions of the 2007 Mw = 8.0 Pisco, 1996 Mw = 7.7 Nazca, 2001 Mw = 8.4 Arequipa and 2007 Mw = 7.7 Tocopilla earthquakes were determined from joint inversions of the InSAR and GPS data (this study). These source models include coseismic and afterslip over a few weeks to a few months depending on case. Slip contours are reported each 1-m. The color scale indicates slip amplitude.

        • This shows the GPS derived rates of motion relative to South America. The South America and Nazca plates are shown with a Sliver between them (“accretionary prism”). This shows (1) the dominant tectonic signal is from east-west convergence due to the subduction zone and (2) that there is deformation within the Sliver (the GPS velocities rates lower from west to east). The red bars show the slip direction to earthquakes with magnitudes M > 6.0 (they are generally parallel to the GPS rates, but not everywhere).

        • Interseismic geodetic measurements in the Central Andes subduction zone. Horizontal velocities determined from campaign GPS measurements are shown relative to South America Craton. Inset shows unwrapped interseismic interferogram in mm/a projected in the line of sight (LOS) direction of the ERS-1/2 satellites [Chlieh et al., 2004]. The convergence of the Nazca plate relative to South America (black arrows) is mainly accommodated along the Peru-Chile megathrust (green arrows) with a fraction taken up along the subandean fold and thrust belt (red arrows). Red bars represent the slip direction of Mw > 6 Harvard CMT (http://www.seismology.harvard.edu/CMTsearch.html).

        • Here is the space-time figure on its side (making it a time-space diagram) showing earthquake rupture latitudinal limits with time, instrumental-historic slip patches, and seismic moment estimates for these earthquakes.

        • Historical and recent large megathrust earthquakes in central and southern Peru. (left) Dates, extents and magnitudes of historical megathrust earthquakes. (middle) We used these parameters and the ruptures areas to estimate the distribution of moment released by historical events of 1746 (Mw8.6– 8.8), 1868 (Mw8.8), 1940 (Mw8.0), 1942 (Mw8.2), 1966 (Mw8.0), 1974 (Mw8.0) and 1913 (Mw7.8). To improve consistency the rupture areas of the Mw8.0 1940/1996/1974 earthquakes (shown in Figure 1), were rescaled using the rupture area of the 2007 Mw8.0 Pisco earthquake as a reference. (right) The along-trench variations of the seismic moment associated to each earthquake.

        • This is the figure that adds moment deficit to the seismic moment plot and the coupling ratio to the slip patch map.

        • Comparison of interseismic coupling along the megathrust with ruptures of large megathrust earthquakes in central and southern Peru. (left) Interseismic coupling map for 3-plate model Short4; it indicates that where the Nazca ridge and the Nazca fracture zone subduct, the interseismic coupling is low. The largest earthquake there is the Mw8.1–8.2 earthquake of 1942. It is not clear whether the 1942 rupture propagated through the Nazca ridge or stopped south of it. High interseismic coupling patches correlate well with regions that experienced great megathrust earthquake Mw8.8 in 1868 and Mw8.6–8.8 in 1746. In the south, the presence of two wide asperities separated by a wide aseismic patch may
          explain partially the seismic behavior of this segment in the last centuries. Individual ruptures of these asperities would produce Mw8 events, as in 2001, but their simultaneous rupture could generate great Mw > 8.5 earthquakes as in 1604 or 1868. The along-strike coincidence of the high coupling areas (orange-red) with the region of high coseismic slip during the 2001 Arequipa and 2007 Pisco earthquakes suggests that strongly coupled patches during the interseismic period may indicate the location of future seismic asperities. (right) Moment deficit (dashed lines) since the last great earthquake of 1868, 1942 and 1746 compared with the seismic moment released during recent and historical
          earthquakes of Figure 8. The moment deficit is computed from the rate of moment deficit predicted by model Short4 considering a steady state interseismic process (Max) or 50% of it (Min) to account for time-variable interseismic process and transient events.

        • This figure shows their results used to show how different parts of the subduction zone have higher or lower moment deficits. The Central Peru section shows that there is an unmet interseismic deficit, while the southern Peru profile shows that earthquakes have been keeping up with plate convergence here. The central Peru region is the region of the subduction zone shown in the 2 figures above this one (Arica, Peru is the southern boundary between the central and southern Peru regions of this subduction zone).

        • Cumulative deficit of moment and seismic moment released due to major subduction earthquakes since the 16th century (top) in central Peru and (bottom) in southern Peru. The cumulative deficit of moment is predicted from the rates of 3-plate models Short4 in central Peru and Short10 in southern Peru (Table 5 and Figure 4). The uncertainties of moment released by historical events lead to a minimum and maximum moment released (see Table S8 in the auxiliary material). The uncertainties on the cumulative deficit of moment allow that nonlinear interseismic and viscous processes could have released 50% of the accumulated moment deficit. The remaining fraction should reflect elastic strain available to drive future earthquakes unless it would have been totally released by anelastic deformation of the forearc.

        • Below are some figures that use seismic tomography to estimate where the slab is (Scire et al., 2017).
        • This is a map showing all their profiles. The profile closest to today’s M 7.1 earthquake is profile B-B’.

        • Map showing seismic station locations (squares—broadband; inverted triangles—short period) for individual networks used in the study and topography of the central Andes. Slab contours (gray) are from the Slab1.0 global subduction zone model (Hayes et al., 2012). Earthquake data (circles) for deep earthquakes (depth >375 km) are from 1973 to 2012
          magnitude >4.0) and were obtained from the U.S. Geological Survey National Earthquake Information Center (NEIC) catalog (https:// earthquake .usgs .gov /earthquakes/). Red triangles mark the location of Holocene volcanoes (Global Volcanism Program, 2013). Plate motion vector is from Somoza and Ghidella (2012). Cross section lines (yellow) are shown for cross sections.

        • Here are profiles AA’, BB’, and CC’. I edited their figure to pull apart these three profiles (so there are some blurred areas in profiles AA and BB. I placed a blue star in the general location of the earthquake.

        • Trench-perpendicular cross sections through the tomography model. Velocity anomalies are shown in blue for fast anomalies, red for slow anomalies. Cross section locations are as shown in Figure 1. Dashed lines are the same as in Figure 6. Yellow dots are earthquake locations from the EHB catalog (Engdahl et al., 1998). Solid black line marks the top of the Nazca slab from the Slab1.0 model (Hayes et al., 2012).

        • Finally, here are two figures that present some observations about the geometry of the slab as it perturbates due to the NR.
        • Here are some seismic velocity profiles showing how the seismic velocity changes with depth. Warm color represents a lower Vs/Vp ratio (warmer, younger slab) and cooler colors represent higher ratio (colder, older slab). These profiles show how the dip of the subducting slab changes from north to south.

        • Three-dimensional model of the structure of shear-wave velocities between 2106 and2186. a–c, Shear-wave velocities and seismicity at depths of 75km (a), 105km (b) and 145km (c), and transects along the northern reinitiating steep slab (A–A9, B–B9), flat slab (C–C9) and southern steep slab (D–D9) segments. Colours indicate velocity deviations, dVs/Vs (%); contours show absolute velocities in kilometres per second (numbered). a–c, Black circles represent stations used in our study; red triangles are Holocene volcanoes; green stars are earthquakes within 20km of the depth shown; black lines refer to cross-sections shown in e–h. The grey dashed line in b and c shows the location of the trench 10Ma (ref. 8); the black dashed line (labelled ‘T’) indicates the location of the slab tear. ‘R’ refers to the resumption of steep subduction at the eastern edge of the flat slab. d, Inferred flat-slab geometry along the Nazca Ridge track, and slab tear north of the ridge. e–h, Cross-sections of slab segments shown in a–c. Black dots show earthquake locations from this study; black inverted triangles are stations; red triangles are Holocene volcanoes; orange triangle represents the location of a measurement of unusually high heat flow15. Dashed lines show the inferred top of the slab. The thick black line shows the crustal thickness.

        • This shows their model of how the NR has been subducted in the past 11 Ma (million years).

        • Proposed evolution of the Peruvian flat slab. a–f, Proposed contours of the subducted slab, assuming that the ridge remains buoyant for 10Ma after entering the trench. The approximate location of the subducted ridge is denoted by the black rectangular outline. Brown areas show areas of the continent underlain by flat slab at each time step. Triangles indicate volcanoes active during the 2 Myr following the time of the frame shown22. The location of the South American continent relative to the Nazca Ridge follows ref. 8. In a, we show the location of the projection of the mirror image of the Nazca Ridge (in yellow) that formed synchronously with the Nazca Ridge on the Pacific Plate when these plates were first created at the spreading centre following ref. 8. In e, red triangles show volcanism from 3Ma to 2 Ma, and brown triangles show volcanism from 2 Ma to 1 Ma. In f, volcanism is shown for 1Ma to 0 Ma (not including Holocene volcanism). g, Modern seismicity fromthis study (large circles) with depths.50 km, and contours as they would be if the removal of the ridge did not affect the longevity of the flat slab. h, Modern seismicity from this study and local seismicity at depth .50 km, as reported in the ISC catalogue for years 2004–2014, shown as smaller circles17. We plot our observed slab contours on the basis of our earthquake locations and the location of high-velocity anomalies in our tomographic results. Dashed lines indicate contours that are less certain, either because of a paucity of earthquakes or because they lie outside of our region of good tomographic resolution. The pink triangular shape shows the region with very limited seismicity that may indicate a slab window caused by tearing and the reinitiation of normal
          subduction.

        • This is a great visualization from Dr. Laura Wagner. This shows how the downgoing Nazca plate is shaped, based upon their modeling. Today’s M 7.1 earthquake is almost due south of Nazca, Peru labeled on the map.

        UPDATE 2324

        • Based upon Stéphane‘s comment (see social media update), I need to edit my twitter post. I posted that this earthquake happened in a region of low seismogenic coupling. This was based upon Chlieh et al. (2011) GPS inversion modeling. Stéphane pointed out that the Villegas Lanza et al. (2016) show this to be in a region of higher coupling.

          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., et al., 2011. Interseismic coupling and seismic potential along the Central Andes subduction zone in JGR, v. 116, B12405, doi:10.1029/2010JB008166
        • Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V., Antoine, P.-O., Salas-Gismodi, R., and Bolaños, R., 2007. How does the Nazca Ridge subduction influence the modern Amazonian foreland basin? in Geology, v. 35, no. 6, p. 515-518.
        • Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
        • 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
        • Ray., J.S., et al., 2012. Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ~30 Myr Hotspot Record in Journal of Petrology, v. 53., no. 7, p. 1417-1448.
        • Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., Furlong, K.P., 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 map sheet, scale 1:12,000,000.
        • Scire, A., Zandt, G., Beck, S., Long, M., and Wagner, L., 2017, The deforming Nazca slab in the mantle transition zone and lower mantle: Constraints from teleseismic tomography on the deeply subducted slab between 6°S and 32°S: Geosphere, v. 13, no. 3, p. 665–680, doi:10.1130/GES01436.1.
        • Villegas-Lanza, J.C., et al., 2016. Active tectonics of Peru: Heterogeneous interseismic coupling along the Nazca Megathrust, rigid motion of the Peruvian Sliver and Subandean shortening accommodation in JGR, doi: 10.1002/2016JB013080

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