Earthquake Report: Bayside (northern California)

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.


      • 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:
      • 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.

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