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1992.04.25 M 7.1 Petrolia

Category : Uncategorized

The 25 April 1992 M 7.1 earthquake was a wake up call for many, like all large magnitude earthquakes are.

    Here is the USGS website for these three large earthquakes.

  • 1992-04-25 18:06:05 UTC 40.335°N 124.229°W 9.9 km depth M 7.2
  • 1992-04-26 07:41:40 UTC 40.433°N 124.566°W 18.8 km depth M 6.5
  • 1992-04-26 11:18:25 UTC 40.383°N 124.555°W 21.7 km depth M 6.6

Below is my interpretive poster for this earthquake.

I plot the seismicity for a week beginning April 25, 1992, with color representing depth and diameter representing magnitude (see legend)..

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

  • In the upper left 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)
  • 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.
  • In the upper 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. To the right of this map are two panels. The upper panel shows the location and orientation of the fault plane used by Rollins and Stein (2010) to model potential changes in coulomb stress following the 1992 M 7.2 earthquake. The Lower panel shows the results from this modeling.
  • In the lower right corner is the map from Stein et al. (1993). This map shows an estimate of coseismic vertical ground motion induced by the 1992 earthquake sequence.
  • In the upper right corner is a series of USGS shakemaps. These plot intensity using the MMI scale.
  • Below the shakemaps is the “Did You Feel It?” map and attenuation relation plot.

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

  • This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes.

  • 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.
  • Following the earthquake, there was lots of work done by local geologists, along with help from those visiting from out of the area. One of the projects included the measurement and modeling of the ground deformation related to the earthquake. The measurements consistend of a first order survey of benchmarks, along with Global Positioning System measurements at GPS monuments. The results from these analyses were presented in a U.S. Geological Survey Open-File Report 93-383 (Stein et al., 1993). Below is a map that shows a modeled estimate of the surface deformation associated with this earthquake.

  • Here is a figure from Oppenheimer et al. (1993) that shows the shaking intensity from this earthquake sequence. Below is a colorized version.

  • Simplified tectonic map in the vicinity of the Cape Mendocino earthquake sequence. Stars, epicenters of three largest earthquakes; contours, Modified Mercalli intensities (values, Roman numerals) of main shock; open circles, strong motion instrument sites (adjacent numbers give peak horizontal accelerations in g). Abbreviations FT Fortuna; F Ferndale; RD, Rio Dell; S, Scotia; P, Petrolia; H, Honeydew; MF, Mendocino fault; CSZ, seaward edge of Cascadia subduction zone; and SAF, San Andreas fault.

  • This map shows an alternate model of earthquake ground deformation (Oppenheimer et al, 1993).

  • Observed and predicted coseismic displacements for the Cape Mendocino main shock (epicenter located at star).

  • This is a figure that shows the tsunami recorded by tide gages in California, Hawaii, and Oregon (Oppenheimer et al., 1993)

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

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

  • This figure shows the fault plane and aftershocks used in their analysis of the 1992 earthquake sequence.

  • Source models for earthquakes 25 April 1992, Mw = 6.9, open circles are from Waldhauser and Schaff ’s [2008] earthquake locations for 25 April 1992 (1806 UTC) to 26 April 1992 (0741 UTC)

  • This figure shows the change in coulomb stress imparted by the M 7.1 earthquake onto different faults: (a) the CSZ and (b) the faults that were triggered to generate the two main aftershocks.

  • (a) Coulomb stress changes imparted by the 1992 Mw = 6.9 Cape Mendocino earthquake (J) to the Cascadia subduction zone. Calculation depth is 8 km. Open circles are Waldhauser and Schaff [2008] earthquake locations for 25 April 1992 to 2 May 1992, 0–15 km depth. Seismicity data were cut off at 15 km depth to prevent interference from aftershocks of K and L. Cross section A‐A′ includes seismicity between 40.24°N and 40.36°N. Cross section B‐B′ includes seismicity between 40.36°N and 40.48°N. (b) Coulomb stress changes imparted by the 1992 Mw = 6.9 earthquake (J) to Mw = 6.5 and Mw = 6.6 shocks the next day (K and L). Stress change is resolved on the average of the orientations of K and L (strike 127°/dip 90°/rake 180°). Calculation depth is 21.5 km. (c) Calculated Coulomb stress changes imparted by M ≥ 5.9 shocks in 1983, 1987, and 1992 (C, E, and J) to the epicenters of K and L. The series of three colored numbers represent stress changes imparted by C, E, and J, respectively.

  • Here is a plot of the seismograms from the NCEDC.

    Here is the USGS website for all the earthquakes in this region from 1917-2017 with M ≥ 6.5.

  • 1922.01.31 13:17 M 7.3
  • 1923.01.22 09:04 M 6.9
  • 1934-07-06 22:48 M 6.7
  • 1941-02-09 09:44 M 6.8
  • 1949-03-24 20:56 M 6.5
  • 1954-11-25 11:16 M 6.8
  • 1954-12-21 19:56 M 6.6
  • 1980-11-08 10:27 M 7.2
  • 1984-09-10 03:14 M 6.7
  • 1984-09-10 03:14 M 6.6
  • 1991-07-13 02:50 M 6.9
  • 1991-08-17 22:17 M 7.0
  • 1992-04-25 18:06 M 7.2
  • 1992-04-26 07:41 M 6.5
  • 1992-04-26 11:18 M 6.6
  • 1994-09-01 15:15 M 7.0
  • 1995-02-19 04:03 M 6.6
  • 2005-06-15 02:50 M 7.2
  • 2005-06-17 06:21 M 6.6
  • 2010-01-10 00:27 M 6.5
  • 2014-03-10 05:18 M 6.8
  • 2016-12-08 14:49 M 6.5
  • This is the map used in the animation below. Earthquake epicenters are plotted (some with USGS moment tensors) for this region from 1917-2017 with M ≥ 6.5. I labeled the plates and shaded their general location in different colors.
  • I include some inset maps.
    • In the upper right corner is a map of the Cascadia subduction zone (Chaytor et al., 2004; Nelson et al., 2004).
    • In the upper left corner is a map from Rollins and Stein (2010). They plot epicenters and fault lines involved in earthquakes between 1976 and 2010.

  • 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:
  • Here is a primer that helps people learn how to interpret focal mechanisms and moment tensors. Moment tensors are calculated differently from focal mechanisms, but the interpretation of their graphical solution is similar. This is from the USGS.

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


  • 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.
  • 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 a. 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., 2000, Upper plate contraction north of the migrating Mendocino triple junction, northern California: Implications for partitioning of strain: Tectonics, v. 19, p. 11441160.
  • 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.
  • Oppenheimer, D., Beroza, G., Carver, G., Dengler, L., Eaton, J., Gee, L., Gonzalez, F., Jayko, A., Ki., W.H., Lisowski, M., Magee, M., Marshall, G., Murray, M., McPherson, R., Romanowicz, B., Satake, K., Simpson, R., Somerille, P., Stein, R., and Valentine, D., The Cape Mendocino, California, Earthquakes of April, 1992: Subduction at the Triple Junction in Science, v. 261, no. 5120, p. 433-438.
  • Patton, J. R., Goldfinger, C., Morey, A. E., Romsos, C., Black, B., Djadjadihardja, Y., and Udrekh, 2013. Seismoturbidite record as preserved at core sites at the Cascadia and Sumatra–Andaman subduction zones, Nat. Hazards Earth Syst. Sci., 13, 833-867, doi:10.5194/nhess-13-833-2013, 2013.
  • 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.
  • 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.
  • Stein, R.S., Marshall, G.A., Murray, M.H., Balazs, E., Carver, G.A., Dunklin, T.A>, McLaughlin, R.J., Cyr, K., and Jayko, A., 1993. Permanent Ground Movement Associate with the 1992 M=7 Cape Mendocino, California, Earthquake: Implications for Damage to Infrastructure and Hazards to navigation, U.S. Geological Survey Open-File Report 93-383.
  • 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.

25 Year Remembrance Event

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The M 7.2 Cape Mendocino earthquake of April 25, 1992 and its M 6.5 and 6.6 aftershocks caused over $60 million in losses and was the only Humboldt County earthquake to gain a FEMA major disaster declaration.

On April 29th, the Redwood Coast Tsunami Work Group and the City of Eureka invites the public to a free program recounting what happened in 1992, the causes and geologic effects and how these earthquakes changed perceptions of the Cascadia subduction zone and the US West Coast tsunami hazard.

The program will include short presentations suitable for a general audience, photographs and video footage, preparedness information and an opportunity to share your experiences.

Read more on the Event Page here.

Connect with people on our social media page here.