Earthquake Report: Bayside (northern California): Update #1

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So, I put together another map with today’s earthquake in context with the historic seismicity and some other factors. Now the USGS magnitude is M = 4.7 and there is a moment tensor for this earthquake (that looks very similar to the focal mechanism, which is not always the case.). Here is my initial earthquake report here.

Below is a map showing the Northern California Earthquake Data Center (NCEDC) seismicity plotted. Today’s M 4.7 earthquake is plotted as a yellow star. This earthquake is similar to other earthquakes plotted in this region.

    Here are the data plotted on the map.

  • Northern California Earthquake Data Center Double Differenced earthquake epicenters, using the Northern California Earthquake Catalog (1984-2014). These epicenters are located by using the double difference method. Basically, earthquakes from a similar region are processed in such a way that, because they are in a similar region it is assumed that the seismic waves/rays travel through the same material (i.e. with the same seismic velocity). With this assumption, their positions can be better determined. These better positions are better relative to each other, but not in an absolute way. Here is an overview of the double difference method from Lamont Doherty. There is a software program that people use to process seismic data for this method (HypoDD).
  • These earthquake epicenters are plotted vs depth with color and magnitude with circle diameter.
  • I plot the depth to the slab in purple. These lines represent an estimate of the depth of the Cascadia subduction zone fault (McCrory et al., 2006).
  • I also plot the current USGS active fault and fold database. The offshore fault map is incomplete, but has been remapped by Dr. Chris Goldfinger and will be released by the USGS in the coming months. I cannot plot the new faults until it is officially released. These faults are in red and then I also plot the faults used by the USGS national seismic hazard map team in black.
  • On the eastern part of the map one may observe the non-volcanic tremor interpreted by the Pacific Northwest Seismic Network. These data can be downloaded by anyone. There is also a great online interface that lets one create animations. These tremor are basically small earthquakes that are not as resolvable on seismographs, so they cannot be located like regular earthquakes. Because of this, these tremor locations are only epicenters (no depth information).
  • The background data are topographic data and bathmetric data compiled by Dr. Jason Chaytor when he was working at the Active Tectonics Lab at Oregon State University.


    I also include some inset figures.

  • In the upper left corner I place a map of the Cascadia subduction zone. This map shows the Cascadia subduction zone, along with other major plate boundary faults in the region (Gorda Rise, Mendocino fault, San Andreas fault). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate). The map also shows the interpretation of faults that are part of the internally deforming Gorda plate. These faults within the Gorda plate are responsible for the large damaging earthquakes in 1980, 2005, and 2010 (others also in 2014, and 2015).
  • In the upper right corner I place a figure from Rollins and Stein (2010) that shows their interpretations for some earthquakes in this region. This was published in response to the January 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004). The 1980, 1992, 1994, 2005, and 2010 earthquakes are plotted and labeled.
  • In the lower left corner I place a figure from Chaytor et al. (2004) that shows their interpretation of the tectonics of the Gorda plate based upon high resolution bathymetric data (showing the shape of the seafloor).
  • I also include the moment tensor and a moment tensor legend. 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.

Here is my initial earthquake report map as presented in the first earthquake report here.


Here is the seismic record from Jaime Wayne’s Netquake Seismometer. Here is a link to the netquake page. The seismometer is located near Orick.


In this map below (from a Mendocino fault earthquake on 2016/01/01), 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.


References:

  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data. Geology 32, 353-356.
  • McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006. Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey
  • Nelson, A.R., Kelsey, H.M., Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone. Quaternary Research 65, 354-365.
  • Rollins, J.C., 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 115, 19 pp.

Posted in cascadia, earthquake, education, geology, gorda, HSU, humboldt, pacific, plate tectonics, strike-slip, subduction, tsunami

Earthquake Report: Bayside (northern California)

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Well, after installing a stilling basin for our new tide gage installation at Trinidad, CA, I was napping in my upstairs bedroom in Manila, CA. I was awakened by a short (2-3 second) short shaking earthquake. Turns out it was a M 4.8 earthquake east-southeast of my residence. Here is the USGS website for this earthquake. The depth is currently set at about 23 km, so it is near the megathrust, but is probably in the Gorda plate. There was an earthquake in this region last October, which had a different focal mechanism and was to the north a few kms.

#Update 1. I looked at the map at the bottom of this report. Today’s earthquake plots close to where the megathrust is estimated to be between 15 and 20 km (McCrory et al., 2006). So, I was correct that this earthquake is in the downgoing Gorda plate.

#Update 2. The map now has a moment tensor (blue) instead of a focal mechanism (orange). Now I am thinking that this could possibly be on an east-west fault since it is more aligned with the Mendocino fault. However, I am sticking with my initial interpretation as most of the earthquakes that we know about in the Gorda plate are northeast striking left-lateral strike slip faults.

    I put together this quick earthquake poster for this earthquake and have a few brief inset figures.

  • In the upper left corner I place a map of the Cascadia subduction zone. I discuss this figure below.
  • In the upper right corner I place three figures. These three maps each show a different measure of the ground shaking using the Modified Mercalli Intensity Scale. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.
      From left to right:

    1. The “Did You Feel It?” map. This is a map that shows the ground shaking based upon peoples’ online reporting.
    2. The Shake Map. This map shows a computer modeled estimate of the ground shaking.
    3. The MMI contour map.
    4. In the lower right corner I show the attenuation with distance plot. This is a plot showing how the ground motions attenuate (lessen) with distance from the earthquake. The orange line is an estimate of the intensity of ground motions based on a numerical model. This numerical model is based on a regression of hundreds of earthquakes (distance vs. magnitude/intensity). These regressions form the basis for Ground Motion Prediction Equations (GMPEs). The blue dots are the actual observations made by real people (using the DYFI form that I posted above). These model based estimates of ground shaking intensity are used, especially for larger earthquakes, to determine what damage might be expected.
    5. I placed a moment tensor / focal mechanism legend in the upper right corner of the map. 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 suspect that this is probably a left lateral strike slip earthquake based upon the focal mechanism and our knowledge of the tectonics of the Gorda plate.


      Here is the record from the seismometer located across the hallway from the HSU Dept of Geology Office. The seismograph is located in Van Matre Hall. Photo Credit Dr. Mark Hemphill-Haley.


      Here I have a summary of earthquakes for this region (including an earthquake in the Explorer plate to the north).


      I present material about the Cascadia subduction zone for the Friends of the Arcata Marsh (FOAM) held on 7/22/16 at the Arcata Marsh Interpretive Center. This page has some supporting material from this presentation, including the digital presentation file. The material in this post is also found on this page here.


      Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2004). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).


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


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

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

      This figure shows the regions that participate in this interseismic and coseismic deformation at Cascadia. Atwater et al., 2005. Black dots on the map show sites where evidence for coseismic subsidence has been found in coastal marshes, lakes, and estuaries.

      Here is a map showing a number of data sets. Seismicity is plotted versus depth (NCEDC). Tremor is plotted (Pacific Northwest Seismic Network). Vertical Deformation rates are plotted (unpublished). Slab depth contours (km) are plotted (McCrory et al., 2006). Fault locking zones are plotted (Wang et al., 2003; Burgette et al., 2009). Bob McPherson (Humboldt State University, Department of Geology) is currently working on a research paper where he will discuss how the seismicity reveals the location of the seismogenically locked fault zone.


      This map shows the various possible prehistoric earthquake rupture regions (patches) for the past 10,000 years. Goldfinger et al., 2012. These rupture scenarios have been adopted by the USGS hazards team that determines the seismic hazards for the USA.

        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.
      • Burgette, R. et al., 2009. Interseismic uplift rates for western Oregon and along-strike variation in locking on the Cascadia subduction zone in Journal of Geophysical Research, v. 114, B01408, doi:10.1029/2008JB005679
      • 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
      • Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gràcia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2012. Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper # 1661F. U.S. Geological Survey, Reston, VA, 184 pp.
      • McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006. Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey
      • Nelson, A.R., Kelsey, H.M., and Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, doi:10.1016/j.yqres.2006.02.009, p. 354-365.
      • Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
      • USGS Quaternary Fault Database: http://earthquake.usgs.gov/hazards/qfaults/
      • Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003. A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.

Posted in cascadia, geology, gorda, HSU, humboldt, plate tectonics, strike-slip, subduction, tsunami

Friends of the Arcata Marsh 2016.07.22

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Lisa Piscitello, Sandy Patton liked this post

I present material about the Cascadia subduction zone for the Friends of the Arcata Marsh (FOAM) held on 7/22/16 at the Arcata Marsh Interpretive Center. This page has some supporting material from this presentation, including the digital presentation file. The material in this post is also found on this page here.


    This is the digital presentation

  • Here is the digital presentation (50 MB pptx). A draft presentation is in place and will be updated the day of the presentation.

    This is a video of the presentation

  • Here I will post a video of the presentation.

    Here are some sources of information about the Cascadia subduction zone

  • For the 315th anniversary of the most recent full rupture CSZ earthquake I put together a summary of our state of knowledge about the CSZ and that 1700 A.D. Jan. 26 earthquake. 2015.01.26
  • The USGS (and others) put together an educational video about the CSZ. I post this video and other supporting information online here: 2015.10.08

Here is an educational video about Cascadia subduction zone earthquakes and tsunamis.

Here is a tsunami hindcast for the Jan 26, 1700 Cascadia subduction zone megathrust earthquake that may have ruptured all the way south to Humboldt Bay. This is the download link for the embedded video below (35 MB mp4).

Here is an animation showing the Holocene record of earthquakes along the Cascadia subduction zone (Goldfinger et al., 2012).

Here is an animation of a Cascadia subduction zone earthquake generated tsunami. This is the download link for the embedded video below (15 MB mp4).

Here is an animation of the numerical simulation for tsunami inundation at Cannon Beach (Ecola Creek).

This was prepared by Rob Witter (Witter, 2008 ).

  • This is the digital file of the embedded video below (52 MB mp4)
  • Posted in cascadia, earthquake, education, geology, HSU, humboldt, plate tectonics, subduction, tsunami

    USGS Menlo Park: 2016.06.29

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    I recently gave a talk for the USGS Earthquake Seminar Series at the USGS Earthquake Science Center in Menlo Park. Here is a web page where I post material from my talk, including some background material. Below I present some links that are limited to the talk.


      Digital Presentation:

    • Here is the Digital Presentation from this talk about Seismoturbidite Chronostratigraphy and Slope Stability offshore Sumatera, Indonesia: USGS Earthquake Science Center Seminar (175 MB)


    Posted in Uncategorized

    CASEIS 16 – Submarine Paleoseismology along the Lesser Antilles subduction zone

    Luke Blair, Sandy Patton liked this post

    Here is a link to the main research cruise page for the CASEIS 16 Lesser Antilles Submarine Paleoseismology cruise. Our goal is to find turbidites so that we might establish a prehistoric record of earthquakes along this subduction zone.

    Below is a summary to date of the material that I have prepared to describe our findings from this cruise. These posts are intended to be educational and informational.

      Here is a map of historic seismicity in this region from 1900-2016. Here is the kml file I used to create this map.
      I include some inset figures.

    • In the upper left corner I include a plate tectonic map from Pindell and Kennan (2009).
    • In the upper right corner I include a figure from Fueillet et al. (2011) that shows felt regions of the two largest historic earthquakes associated with the subduction zone. These earthquakes are pre-instrumental, so the estimates of size are based upon observations of intensity.
    • In the lower left corner is a figure from Hough (2013) that shows the felt reports from the Feb. 8, 1843 earthquake.

    • The Lesser Antilles subduction zone has a short record of earthquakes and there is little known about the prehistoric record of earthquakes. Given a record of a few hundred years, and that subduction zone earthquakes typically have recurrence intervals spanning this long to thousands of years, it is important to develop a prehistoric record of earthquakes spanning multiple earthquake cycles. Given that there appear to be cycles of cycles (a.k.a. supercycles, Sieh et al., 200Smilie: 8), this paleoearthquake record needs to span many many earthquake cycles. How long is long enough? Nobody knows because (1) not many fault systems have been studied like this and (2) nobody has observed subduction zones for this long (tens of thousands of years).
    • Hough (2013) presented an analysis of felt reports for the 1843 earthquake. This earthquake was probably on the subduction zone fault, but we do not really know since it happened before we had seismometers that would allow us to locate the earthquake. Hough (2013) suggests that the pre-1900 record of historical earthquakes is under-reported and magnitudes are under-estimated. Hough uses felt records to suggest that prior estimates of the magnitude for the 1843 earthquake were too small. Macroseismic effects from the earthquake were used to estimate a magnitude of M 7.5-8 (Bernard and Lambert, 198Smilie: 8). Feuillet et al., (2011) reconsiders the intensities of this event. Feuillet et al. (2011) focus on accounts from the Lesser Antilles and their estimated magnitude is M = 8.5, with an estimated rupture length of 300 km. Hough uses felt reports and compares these records with other subduction zone earthquakes. Below is a plot showing her comparisons. I include the figure caption below the figure as a blockquote.

    • Intensities versus distance. Intensity values for (red circles) the 8 February 1843 Lesser Antilles earthquake, (gray circles) the 2011 Tohoku-Oki earthquake, and (black circles) the 2010 Maule, Chile earthquake. Values are plotted versus epicentral distance, for the 1843 earthquake assumed to be 17.5 N, 60.5 W; the precise assumed epicenter is in question but does not significantly change the data points at regional distances.

    • Feuillet et al (2002) suggest that there is oblique convergence that leads to strain/slip partitioning. They place a forearc sliver fault along the volcanic arc (where the islands are). This figure below shows this shear couple across the subduction zone. I include their figure caption below the figure as a blockquote.

    • Tectonic model of the Lesser Antilles Arc. 500 m bathymetric contours are from Smith and Sandwell [1997]. Structural interpretation of accretionary prism is based on analysis of bathymetric and topographic contour patterns. Black arrows along the trench: NAM/CAR boundary-parallel slip with rates indicated [DeMets et al., 2000]. Large white arrows: NAM/CAR and SAM/CAR motion vectors from DeMets et al. [2000] and Weber et al. [2001], respectively. NAM/CAR GPS relative motion vectors measured at Aves and Sainte Croix are indicated [DeMets et al., 2000]. Black double arrows, local direction of extension deduced from fault geometry and distribution. Half black arrows, slip on oblique or strike slip faults. 1, in light gray, zone of sinistral extensional shear; 2-transition zone; 3-in dark grey, zone of dextral oblique thrusting. White large half arrows indicate sinistral and dextral motion along the trench, respectively.

    • Pichot et al. (2012) study the fracture zones as expressed by the Barracuda and Tiburon Ridges. These are somewhat related to some fracture zones that extend from the Mid Atlantic Ridge. These structures may be responsible for segmentation of the subduction zone (earthquakes or volcanism). This is one of the hypotheses that will be tested by conducting this marine paleoseismology research. The Tiburon rise does cause a major inflection of the subduction zone fault, so it is reasonable to think that this may cause segmented rupture along the megathrust. Below are three figures. First is a map that shows bathymetry and gravity data for the region between the Lesser Antilles and the Mid Atlantic Ridge (Pichot et al., 2012). The lower figure pair shows (a) these ridges as they relate to bathymetry and faults in the accretionary prism and (b) these structures as expressed as gravity anomalies (Pichot 2012). Below each figure I include their figure caption as a blockquote.

    • The Atlantic Ocean between 2°N and 22°N. A) 1×1 min-Satellite free air gravity map (V18.1 from Sandwell and Smith, 2009), with NAm, North America Plate; Car, Caribbean Plate; SAm, South America Plate; Afr, Africa Plate; MAR, Mid-Atlantic-Ridge; AR, Aves Ridge; LA, Lesser Antilles; TB, Tiburon Rise; BR, Barracuda Ridge; RT, Royal Trough; RR, Researcher Ridge; Caribbean structural geology after Pindell and Kennan (2009).B) Bathymetric map (Smith and Sandwell, 1997) with, BAP, Barbados Ridge accretionary prism; ORS, Orinoco river system; ADSF, Amazon Deep Sea Fan, DAP, Demerara abyssal plain.C) Regional map showing Atlantic Ocean and the study area in red box. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


      Fold axes (folds) and overlap (thrusts) (black line) parallel to the front of accretion. In continuation of wrinkles Barracuda and Tiburon, their axes are a sub-parallel towards the axis of wrinkles


      Wrinkles Barracuda, Tiburon and Saint Lucia’s characterized by gravimetric anomalies to the positive open. Wrinkles Barracuda and Tiburon are limited respectively to the north and south by the graves of Barracuda and Tiburon. The front of the accretion prism is represented by the black line with triangles. The free air gravity anomaly map is from Sandwell and Smith (2009).

      References:

    • Bernard, P. and Lambert, J., 1988. Subduction and seismic hazard in the northern Lesser Antilles: Revision of historical seismicity, Bull. Seismol. Soc. Am. v. 78, p. 1965–1983
    • Feuillet, N., Manighetti, I., and Tapponier, P., 2002. Arc parallel extension and localization of volcanic complexes in Guadeloupe, Lesser Antilles in Journal of Geophysical Research, v., 107, DOI:10.1029/2001JB000308
    • Feuillet, N., Beauducel, F., Tapponnier, P., 2011. Tectonic context of moderate to large historical earthquakes in the Lesser Antilles and mechanical coupling with volcanoes in Journal of Geophysical Research, v. 116, DOI:10.1029/2011JB008443.
    • Hough, S., 2013. Missing great earthquakes in Journal of Geophysical Research: Solid Earth, v. 118, p. 1098-1108.
    • Pichot, J., 2012. The Barracuda Ridge and Tiburon Rise, East of the Lesser Antilles: origin, evolution and geodynamic implications. [Ph.D. Thesis] l’Université de Bretagne Occidentale 286 pp.
    • Pichot, T., Patriar, M., Westbrook, G.K., Nalpas, T., Gutscher M.A., Roest, W.R., Deville, E., Moulin, M., Aslanian, D., and Rabineau, M., 2012. The Cenozoic tectonostratigraphic evolution of the Barracuda Ridge and Tiburon Rise, at the western end of the North America-South America plate boundary zone in Marine Geology, v. 303-306, p. 154-171
    • Pindell, J.L. and Kennan, L., 2009. Tectonic Evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference fram: an update, in James, K., Antonieta-Lorente, M., and Pindell, J.L., (eds), The geology and evolution of the region between North and South America, Geological Society of London Special Publication.

    Posted in Uncategorized

    Earthquake Report: East Pacific Rise and Middle America Trench

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    Sandy Patton liked this post

    Yesterday we had an earthquake along the Clipperton fracture zone (CFZ), a transform plate boundary that offsets the northern East Pacific Rise (EPR). I was busy grading so did not get to this until today.

      Here are the four large earthquakes shown on the poster below.

    • 2016.04.15 M 6.1
    • 2016.04.25 M 5.6
    • 2016.04.27 M 5.8
    • 2016.04.29 M 6.6

    Below is my poster for this and the earlier earthquakes.

    Siqueiros, Rivera, and Orozco were the “Tres Grandes,” 20th century political activists in Mexico who led the muralist movement. Their murals have inspired, and continue to inspire, political activists globally (as well as the Mexican muralism movement). Check out more about them here. I am fascinated that there are fracture zones named for Siqueiros and Rivera in this region.

      I include some inset figures.

    • Key et al. (2013) shows the fracture zones in this region. See the description below.
    • Mann (2007) shows the magnetic anomalies and tectonic plate boundaries in this region. I include this map below with a figure caption.

    There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.

    Based upon the location of the Clipperton fracture zone compared with the M 6.6 epicenter, I interpret this earthquake to have left-lateral slip on a strike-slip fault. The cool part about this earthquake suite is that there are examples of all types of earthquakes (extensional, compressional, and shear). The 4/15 M 6.1 appears to possibly have triggered the 4/25 & 4/27 M 5.6 and M 5.8 earthquakes. Then, two days later, we had the M 6.6 earthquake. This earthquake is too far to be affected by changes in coulomb stress, however it sure seems like it is more than a coincidence. I have not run a coulomb stress modeling analysis, but this is just based upon modeling others have done in different regions. While the earthquakes along the Middle America Trench (MAT) would have resulted in the Cocos plate extending slightly to the northeast, and that there is the Tehuantepec fracture zone that appears to bend and link the Clipperton fracture zone to the region near the MAT (so we want to link these earthquakes), there probably is no linkage.

    The red-orange-yellow lines are slab contour lines from Hayes et al. (2012). These lines are a best estimate for the depth to the subduction zone fault. These are based largely upon seismicity and there is currently an effort to update these contours to integrate other data types. The hypocentral depth for this earthquake is consistent with being along the subduction zone interface.


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

    Here is a great low angle oblique image showing the topography of the EPR and the two fracture zones in this region (Ryan et al., 2009).


    Here is the Mann (2007) map. I include their figure caption below as a blockquote.


    Present setting of Central America showing plates, Cocos crust produced at East Pacific Rise (EPR), and Cocos-Nazca spreading center (CNS), triple-junction trace (heavy dotted line), volcanoes (open triangles), Middle America Trench (MAT), and rates of relative plate motion (DeMets et al., 2000; DeMets, 2001). East Pacific Rise half spreading rates from Wilson (1996) and Barckhausen et al. (2001).

    Here is the Lay et al. (2013) map. I include their figure caption below as a blockquote.


    Location of the magnetotelluric survey across the fast spreading East Pacific Rise. Twenty-nine sea-floor magnetotelluric stations (white circles) were deployed across the ridge axis at 9 deg 30′ N, about 1,000 km southwest of Central America (inset; study area boxed in red). The Pacific and Cocos plates diverge symmetrically (black arrows) while the entire ridge system migrates to the northwest relative to a fixed hotspot reference frame 16 (grey arrow). The Clipperton and Siqueiros transform faults (TF) bound this ridge segment to the north and south. Slow seismic P-wave velocity contours (velocities, 7.6 km s-1) found by seismic tomography of the uppermost few kilometres of mantle follow the ridge crest along most of the segment, but deviate to the east near the magnetotelluric profile. The colour scale shows seafloor topography and the 100-km scale bar indicates the half-aperture of the magnetotelluric array.

    Earlier this year, in January, there was a series of earthquakes along the Rivera fracture zone. Here is my report for that time. Below is my interpretation poster.


    In September 2015, there were some earthquakes yet further to the north. Here is my Earthquake Report from that time.


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


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


    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.

    This is a link to a tectonic summary map from the USGS (Benz et al., 2011). Click on the map below to download the 20 MB pdf file.


    Posted in College Redwoods, geology, HSU, mexico, pacific, plate tectonics, strike-slip, subduction, Transform, Uncategorized

    Earthquake Report: Vanuatu!

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    Ajeet J liked this post

    We just had a large earthquake in the Vanuatu region, just south of a series of earthquakes from a couple weeks ago. Here is the USGS website for today’s M 7.0 earthquake. I include some figures and animations that I posted earlier.

      Here are my two reports from earlier in April, 2016:

    • 2016.04.06 M 6.9
    • 2016.04.06 M 6.9 update # 1

    Below is my preliminary interpretive poster. I have posted the moment tensor for this earthquake, along with some inset maps. The Cleveland et al. (2014) maps are explained below. I also include an inset showing the shaking intensity that uses the Modified Mercalli Intensity Scale (MMI). The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.

    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.


      Here is an animation that shows the seismicity for this region from 1960 – 2016 for earthquakes with magnitudes greater than or equal to 7.0.

    • I include some figures mentioned in the posters above, in addition to a plot from Cleveland et al. (2014). In the upper right corner, Cleveland et al. (2014) on the left plot a map showing earthquake epicenters for the time period listed below the plot on the right. On the right is a plot of earthquakes (diameter = magnitude) of earthquakes with latitude on the vertical axis and time on the horizontal axis. Cleveland et al (2014) discuss these short periods of seismicity that span a certain range of fault length along the New Hebrides Trench in this area. Above is a screen shot image and below is the video.

    • Here is a link to the embedded video below (6 MB mp4)
      Here are the two figures from Cleveland et al. (2014).

    • Figure 1. I include the figure caption below as a blockquote.

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

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

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

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

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

    Posted in College Redwoods, earthquake, education, geology, HSU, pacific, plate tectonics, subduction, tsunami

    Earthquake Anniversary: M 7.8 Gorkha (Nepal) Earthquake

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    An anniversary is for the Gorkha (Nepal) Earthquake from 1 year ago. I have several Earthquake Reports listed below. These include a variety of observations and comparisons with historic and prehistoric earthquakes that I compiled.

    USGS slip models: http://earthjay.com/?p=2478

    First Report: http://earthjay.com/?p=2357
    Comparison with historic EQs http://earthjay.com/?p=2361
    USGS Intensity Reports http://earthjay.com/?p=2387
    Surface Displacement and Ground Motion Models http://earthjay.com/?p=2391
    More historic comparisons http://earthjay.com/?p=2396
    Coseismic Surface Deformation Model http://earthjay.com/?p=2410
    Aftershock Report http://earthjay.com/?p=2437
    Mainshock & Aftershock Update #1 http://earthjay.com/?p=2439
    Mainshock & Aftershock Update #2 http://earthjay.com/?p=2450
    Mainshock & Aftershock Update #3 (and interview with Ian Pierce and Steven Angster) http://earthjay.com/?p=2466

    Here is a summary of the observations:

    Mw 7.8 Earthquake Finite Fault Plane Solution from the USGS.

    Mw 7.3 Earthquake Finite Fault Plane Solution.

    Here is the map that I put together. I have placed the USGS epicenters with two color schemes. The size of the yellow dots represents earthquake magnitude. The degree of redness designates the time (earlier-April = pink & later-May = red). Note how there are some pink colored epicenters in the region of the M 7.3 earthquake. These pink colored earthquakes all occurred in April. The red ones are from May. These epicenters may not be plotted with the greatest certainty, though any uncertainty is possibly shared between them. So, there relative positions are possibly good.


    Here is an updated regional map that incorporates Hough and Bilham (2008 ) and today’s seismicity. The historic and prehistoric earthquake slip patches are also shown. The three other data sets now include Bilham (2004), Bettinelli et al (2006), and Berryman et al. (2009). I provide information about how I compiled these data sets on this page.


    Here is the updated DYFI map. Note how broadly this earthquake was felt.


    Here are two visualizations of the seismic waves as they propagate through the Earth. These are records from the USArray Transportable Array. Your tax dollars at work, unless congress defunds these projects. This first video shows vertical motion as red and blue.

    This second video shows horizontal motion with magnitude and direction.

    Posted in asia, collision, earthquake, education, geology

    Earthquake Report: Ecuador Update #1

    20160416_ecuador_MMI_update_interpretation_thumb

    Here is an update to my preliminary report about the M 7.8 subduction zone earthquake in Ecuador. Here is my first report. More information about this earthquake is on that report page. Here is the USGS website for the M 7.8 earthquake.

      The updates are as follows.

    • The major update is the change in estimate for ground motions. I plot this map with the revised shakemap that uses Modified Mercalli Intensity (MMI) scale. The USGS calculated a fault plane and applied slip on that fault plane to get this new estimate of ground motions. These new estimates led to a new PAGER report, which produced revised estimates of probability of damage to people and their belongings.
    • A second update here is the singular tsunami observation.
    • A third update is that I have included a figure from Chlieh et al. (2014) on this poster. I took his rupture lengths and plotted them on the map as green lines with white balls at their termini. I have labeled these slip patches with their year and magnitude. These are also shown in the Chlieh inset figure.
    • A fourth update is that I include the revised shakemap as an inset because this shows the outline of the USGS fault slip model.
    • A fifth update is a map of historic focal mechanisms posted to twitter by Jascha Polet, a seismologist at Cal Poly Pomona.
    • Finally, I prepared an animation that shows the seismicity of this region from 1900-2016 for earthquakes with a magnitude greater than or equal to M 6.0. Here is the kml file that I used to prepare this animation.


    Here is a comparison of the attenuation relations. Version 1 is from reports that were made less than an hour after the earthquake. Version 2 (my numbers) incorporate additional reports.


      For better viewing, I include the Chlieh et al. (2014) figure below. There are other figures on the first report page. Chlieh et al. (2014) use GPS data to infer the spatial variation and degree to which the subduction zone megathrust is seismogenically coupled. They consider plate motion rates and estimate the moment (earthquake energy) deficit along this fault (how much strain that plate convergence has imparted upon the fault over time). Then they compare this moment deficit to regions of the fault that have slipped historically.

    • Moment deficit along strike and historic earthquake locations. Today’s earthquake may have occurred in the region marked “gap” in these figures.

    • (A) Along-strike variations of the annual moment deficit for all the interseismic models shown in Fig.5. (B)Maximum ISC model and (C)Minimum ISC model. (A)The blue, green and red lines correspond to the along-strike variation of the annual moment deficit rate respectively for models with smoothing coefficient λ1 =1.0, 0.25 and 0.1. (B) Smoother solution of Fig.5 ith a maximum moment deficit rate of 4.5 ×1018N m/yr. (C)Rougher solution of Fig.5 with a minimum moment deficit rate of 2.5 ×1018N m/yr. Yellow stars are the epicenters of subduction earthquakes with magnitude Mw>6.0 from the last 400 yr catalogue (Beauval et al., 2013). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

      Tsunami Observations:

    • Below are the current observations of tsunamis from this earthquake. This report is from the Pacific Tsunami Warning Center (PTWC). Note the small maximum tsunami height. This is good for those in Ecuador.

      These were posted by Jascha Polet on Twitter.

    • Here is a map that shows historic focal mechanisms with color representing depth.

    • Here is a a cross section that shows historic focal mechanisms with color representing depth.

      Here is the animation. First I provide a screen shot and then a link and the embedded video.


    • Here is a link to the video file embedded below (4 MB mp4)

    Posted in geology, pacific, plate tectonics, subduction, tsunami

    Earthquake Report: Ecuador!

    20160416_ecuador_interpretation_thumb

    Jeff Marshall, Edna Cummings liked this post

    We just had a large earthquake along the coast of Ecuador. Here is the USGS website for the M 7.8 earthquake.

    Below is my Earthquake Report Poster. I plot the USGS moment tensor for this earthquake, along with the Modified Mercalli Intensity Scale contours, and the subduction zone slab contours (Hayes et al., 2012).The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.

    There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.

    The red-orange-yellow lines are slab contour lines from Hayes et al. (2012). These lines are a best estimate for the depth to the subduction zone fault. These are based largely upon seismicity and there is currently an effort to update these contours to integrate other data types. The hypocentral depth for this earthquake is consistent with being along the subduction zone interface.

      I include a number of inset figures and maps.

    • In the upper left corner I include a clipping of the map and cross section from the USGS Open File Report for the historic seismicity of this region (Rhea et al., 2010).
    • In the upper right corner, I include a map that shows the regional tectonics as published by Gutscher et al. (1999). I also include a 3-D low angle oblique view of the structure of the donwgoing Nazca plate (Gutscher et al., 1999). These authors pose that the Carnegie Ridge exerts a control for the segmentation of the subduction zone.
    • In the lower right corner, I include a preliminary shakemap. More about shakemaps can be found here.


    This looks like it will be a damaging earthquake to people and their belongings. Below is the Rapid Assessment of an Earthquake’s Impact (PAGER) report. More on the PAGER program can be found here. An explanation of a PAGER report can be found here. PAGER reports are modeled estimates of damage. On the left is a histogram showing estimated casualties and on the right is an estimate of possible economic losses.


    Here is an explanation of the PAGER report.


    Below is the tectonic setting map from Gutscher et al. (1999). I include their figure caption as a blockquote.


    Tectonic setting of the study area showing major faults, relative plate motions according to GPS data [7] and the NUVEL-1 global kinematic model [8], magnetic anomalies [13] and active volcanoes [50]. Here and in Fig. 4, the locations of the 1906 (Mw D 8:8, very large open circle) and from south to north, the 1953, 1901, 1942, 1958 and 1979 (M  7:8, large open circles) earthquakes are shown. GG D Gulf of Guayaquil; DGM D Dolores–Guayaquil Megashear.

    Below is a low angle oblique view of the structures in the downgoing Nazca plate, from Gutscher et al. (1999). I include their figure caption as a blockquote.


    3-D view of the two-tear model for the Carnegie Ridge collision featuring: a steep ESE-dipping slab beneath central Colombia; a steep NE-dipping slab from 1ºS to 2ºS; the Peru flat slab segment south of 2ºS; a northern tear along the prolongation of the Malpelo fossil spreading center; a southern tear along the Grijalva FZ; a proposed Carnegie flat slab segment (C.F.S.) supported by the prolongation of Carnegie Ridge.

      Today’s earthquake is near two historic earthquakes with similar magnitudes. Below I plot a map showing the seismicity from 1900-2016 for earthquakes with magnitudes greater than or equal to M 6.0. Here is the USGS query that I used to make this map.

    • 1906.01.31 M 8.3 occurred ~100 km to the northeast.
    • 1942.05.14 M 7.8 occurred <50 km to the southwest.

      Here are the parts of the USGS Open File Report that are included above, as well as their legends.

    • Map

    • Map Legend

    • Cross Section

    • Cross Section Legend

    • The entire poster (55 MB pdf)

    UPDATE 1

      Here are a couple maps from Chlieh et al. (2014). I include their figure captions below. Chlieh et al. (2014) use GPS data to infer the spatial variation and degree to which the subduction zone megathrust is seismogenically coupled. They consider plate motion rates and estimate the moment (earthquake energy) deficit along this fault (how much strain that plate convergence has imparted upon the fault over time). Then they compare this moment deficit to regions of the fault that have slipped historically.

    • Tectonics and GPS motion rates.

    • Seismotectonic setting of the oceanic Nazca plate, South America Craton (SoAm) and two slivers: the North Andean Sliver (NAS) and the Inca Sliver (IS). The relative Nazca/SoAm plate convergence rate in Ecuador is about 55mm/yr (Kendrick et al., 2003). Black arrows indicate the diverging forearc slivers motions relative to stable SoAm are computed from the pole solutions of Nocquet et al.(2014). The NAS indicates a northeastward long-term rigid motion of about 8.5 ±1mm/yr. The ellipse indicates the approximate rupture of the great 1906 Mw=8.8 Colombia–Ecuador megathrust earthquake. The Carnegie Ridge intersects the trench in central Ecuador and coincides with the southern limit of the great 1906 event. Plate limits (thick red lines) are from Bird(2003). DGFZ =Dolores–Guayaquil Fault Zone; GG =Gulf of Guayaquil; GR =Grijalva Ridge; AR =Alvarado Ridge; SR =Sarmiento Ridge. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

    • GPS velocities along with historic earthquake patches.

    • Interseismic GPS velocity field in the North Andean Sliver reference frame. The relative Nazca/NAS convergence rate is 46 mm/yr. The highest GPS velocity of 26 mm/yr is found on La Plata Island that is the closest point to the trench axis. The GPS network adequately covers the rupture areas of the 1998 Mw=7.1, 1942 Mw=7.8and 1958 Mw=7.7 earthquakes but only 1/4th of the 1979 Mw=8.2 and 2/3rd of the great 1906 Mw=8.8 rupture area. The black star is the epicenter of the great 1906 event and white stars are the epicenters of the Mw>7.01942–1998 seismic sequence. Grey shaded ellipses are the high slip region of the 1942, 1958, 1979 and 1998 seismic sources (Beck and Ruff, 1984;Segovia, 2001; Swenson and Beck, 1996). Red dashed contours are the relocated aftershocks areas of the 1942, 1958 and 1979 events (Mendoza and Dewey, 1984). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

    • Moment deficit along strike and historic earthquake locations. Today’s earthquake may have occurred in the region marked “gap” in these figures.

    • (A) Along-strike variations of the annual moment deficit for all the interseismic models shown in Fig.5. (B)Maximum ISC model and (C)Minimum ISC model. (A)The blue, green and red lines correspond to the along-strike variation of the annual moment deficit rate respectively for models with smoothing coefficient λ1 =1.0, 0.25 and 0.1. (B) Smoother solution of Fig.5 ith a maximum moment deficit rate of 4.5 ×1018N m/yr. (C)Rougher solution of Fig.5 with a minimum moment deficit rate of 2.5 ×1018N m/yr. Yellow stars are the epicenters of subduction earthquakes with magnitude Mw>6.0 from the last 400 yr catalogue (Beauval et al., 2013). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

    UPDATE 2

      Tsunami Observations:

    • Below are the current observations of tsunamis from this earthquake. Note the small maximum tsunami height. This is good for those in Ecuador.

    Posted in Uncategorized