Earthquake Report: Laytonville (northern CA)!

This morning there was another earthquake in northern CA between the San Andreas (SAF) and Maacama faults (MF). This region has been active for the past few years, with earthquakes in the M 3-4 range. Most recently, there was a M 3.8 really close to today’s M 4.1. These earthquakes may indicate the possibility of an unmapped fault. There have been earthquakes in 2000, 2014, and 2015 that align along strike (of a possible fault that is sub-parallel to the SAF/MF systems). Here is the USGS website for today’s M 4.1 earthquake.
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 earthquake. 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.
Below I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I use the USGS Quaternary fault and fold database for the faults. I outlined the Vizcaino Block, which many interpret to be a prehistoric subduction zone accretionary prism from a time before the San Andreas existed.
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 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. Based on the moment tensor and my knowledge of the tectonics of this region, I interpret this earthquake to have had a right lateral strike slip motion along an east-west fault.

    I include some inset figures in the poster.

  • In the upper right corner I include a map from McLaughlin et al. (2012) that shows the regional faulting.
  • In the lower right corner I include generalized fault map of northern California from Wallace (1990).
  • In the lower left corner I include a map that shows the seismicity for this region since 1960, including earthquakes with magnitudes greater than or equal to M 1.5. I have labeled some of the significant earthquake swarms, with magnitudes ranging from M 3-4. Here is the USGS search that I used to create this map.
  • To the right of the seismicity map is a figure that shows the evolution of the San Andreas fault system since 30 million years ago (Ma). This is a figure from the USGS here.
  • In the upper left corner I include the Earthquake Shaking Potential map from the state of California. This is a probabilistic seismic hazard map, basically a map that shows the likelihood that there will be shaking of a given amount over a period of time. More can be found from the California Geological Survey here. I place a yellow star in the approximate location of today’s earthquake.


  • Earlier this month (a couple days ago), there was an earthquake in this region. Below is my interpretive poster for that earthquake. Here is my Earthquake Report.

  • Earlier this year, there was an earthquake in this region, along the BSF. Below is my interpretive poster for that earthquake. Here is my Earthquake Report.

  • Last year there was an earthquake in this region. Below is my interpretive poster for that earthquake. Here is my Earthquake Report.

  • I place a map shows the configuration of faults in central (San Francisco) and northern (Point Delgada – Punta Gorda) CA (Wallace, 1990). Here is the caption for this map, that is on the lower left corner of my map. Below the citation is this map presented on its own.
  • Geologic sketch map of the northern Coast Ranges, central California, showing faults with Quaternary activity and basin deposits in northern section of the San Andreas fault system. Fault patterns are generalized, and only major faults are shown. Several Quaternary basins are fault bounded and aligned parallel to strike-slip faults, a relation most apparent along the Hayward-Rodgers Creek-Maacama fault trend.


  • About 75% of the relative plate motion is accommodated along the SAF and its synthetic sister faults in the northern CA region. The rest of the plate boundary motion is accommodated along the Eastern CA shear zone and Walker Lane, along with the Central Nevada Seismic Belt, and the Wasatch fault systems. In Northern CA, there is about 33-37 mm/yr strain accumulated on the SAF plate boundary system. About 18-25 mm/yr is on the SAF, 8-11 mm/yr on the MF, and 5-7 mm/yr on the Bartlett Springs fault system (Geist and Andrews, 2000).
  • Here is a map from McLaughlin et al. (2012) that shows the regional faulting. I include the figure caption as a blockquote below.

  • Maps showing the regional setting of the Rodgers Creek–Maacama fault system and the San Andreas fault in northern California. (A) The Maacama (MAFZ) and Rodgers Creek (RCFZ) fault zones and related faults (dark red) are compared to the San Andreas fault, former and present positions of the Mendocino Fracture Zone (MFZ; light red, offshore), and other structural features of northern California. Other faults east of the San Andreas fault that are part of the wide transform margin are collectively referred to as the East Bay fault system and include the Hayward and proto-Hayward fault zones (green) and the Calaveras (CF), Bartlett Springs, and several other faults (teal). Fold axes (dark blue) delineate features associated with compression along the northern and eastern sides of the Coast Ranges. Dashed brown line marks inferred location of the buried tip of an east-directed tectonic wedge system along the boundary between the Coast Ranges and Great Valley (Wentworth et al., 1984; Wentworth and Zoback, 1990). Dotted purple line shows the underthrust south edge of the Gorda–Juan de Fuca plate, based on gravity and aeromagnetic data (Jachens and Griscom, 1983). Late Cenozoic volcanic rocks are shown in pink; structural basins associated with strike-slip faulting and Sacramento Valley are shown in yellow. Motions of major fault blocks and plates relative to fi xed North America, from global positioning system and paleomagnetic studies (Argus and Gordon, 2001; Wells and Simpson, 2001; U.S. Geological Survey, 2010), shown with thick black arrows; circled numbers denote rate (in mm/yr). Restraining bend segment of the northern San Andreas fault is shown in orange; releasing bend segment is in light blue. Additional abbreviations: BMV—Burdell Mountain Volcanics; QSV—Quien Sabe Volcanics. (B) Simplifi ed map of color-coded faults in A, delineating the principal fault systems and zones referred to in this paper.

  • Here is the figure showing the evolution of the SAF since its inception about 29 Ma. I include the USGS figure caption below as a blockquote.

  • EVOLUTION OF THE SAN ANDREAS FAULT.
    This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendicino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendicino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendicino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system. (Image modified after original illustration by Irwin, 1990 and Stoffer, 2006.)

    • Here is a map that shows the shaking potential for earthquakes in CA. This comes from the state of California here.
    • Earthquake shaking hazards are calculated by projecting earthquake rates based on earthquake history and fault slip rates, the same data used for calculating earthquake probabilities. New fault parameters have been developed for these calculations and are included in the report of the Working Group on California Earthquake Probabilities. Calculations of earthquake shaking hazard for California are part of a cooperative project between USGS and CGS, and are part of the National Seismic Hazard Maps. CGS Map Sheet 48 (revised 2008) shows potential seismic shaking based on National Seismic Hazard Map calculations plus amplification of seismic shaking due to the near surface soils.


      References

    • 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/
    • 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.
    • 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. [https://pubs.er.usgs.gov/publication/pp1515].

Earthquake Report: Mendocino fault!

Yesterday there was an earthquake along the eastern extension of the Mendocino fault system. This magnitude M = 4.1 earthquake (here is the USGS website for this earthquake) is a small magnitude, but it was widely felt. I was in Manila (CA) at the time, so I am surprised that I did not feel it. I was in the bath at the time, so maybe my shampooing was too energetic?
This earthquake appears to have occurred along the Mendocino fault, 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. This earthquake appears to have occurred in a region of the Mendocino fault that ruptured in 1994. See the figures from Rollins and Stein below.
Below is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. 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 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. Based on the moment tensor and my knowledge of the tectonics of this region, I interpret this earthquake to have had a right lateral strike slip motion along an east-west fault.

    I have placed several 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)
  • 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 map from Rollins and Stein (2010), showing their interpretations of different historic earthquakes in the region. This was published in response to the January 2010 Gorda plate earthquake. Today’s earthquake is near the 1983 earthquake.
  • Above the Rollins and Stein figure are two USGS plots. The upper plot shows a map displaying the “Did You Feel It?” felt reports. The color scale is the same as for the MMI legend in the upper left corner. The lower plot shows how the shaking intensity attenuates (diminishes) with distance from the epicenter.


  • Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the 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. I did not mention the 2010 earthquake, but it most likely was just like 1980 and 2005, a left-lateral strike-slip earthquake on a northeast striking fault.

  • Here is a large scale map of the 1983 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles. Note how the aftershocks trend slightly southeast in this region. Today’s swarm does the same (and the moment tensor also shows a slightly southeast strike). Note how the interpreted fault dips slightly to the north, which is the result of north-south compression from the relative northward motion of the Pacific plate.

  • 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.

  • 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.


    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.
  • 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.

Earthquake Report: Gorda Plate!

Last night, while I was preparing an online exam for my students to take while I am at the Geological Society of America Annual Meeting in Denver Colorado, there were a couple earthquakes in the Gorda plate offshore of northern California.

    Here are the USGS web pages for the earthquakes plotted in my interpretive poster below. The two from last night are in the Gorda plate and there is also a recent earthquake associated with the Mendocino fault. Here is my report for the M 5.6 Mendocino fault earthquake.

  • 2019.09.25 M 5.0 Gorda plate
  • 2019.09.25 M 4.6 Gorda plate
  • 2016.09.03 M 5.6 Mendocino fault

Below is my interpretive poster for this earthquake. I include faults from the USGS quaternary fault and fold database. The two earthquakes do not appear to be along the same fault plane, if my interpretation is correct. This leads me to think that perhaps these earthquakes are possibly on faults antithetic to the regional structural grain.
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. Given our knowledge of the tectonics of this region, I interpret this earthquake to be a left-lateral strike-slip earthquake in the Gorda plate. I include some figures below that show evidence that supports this interpretation.


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


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


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


These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.
Mw = 5 Trinidad Chaytor
Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).


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.


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:


    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.
  • 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.
  • 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.

Earthquake Report: Mendocino fault!

Well, I felt that one. The shaking lasted about 5-7 seconds in Manila, CA (where I live). Here is the USGS website for this M = 5.6 earthquake. This earthquake appears to have occurred along the Mendocino fault, 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. This earthquake appears to have occurred in a region of the Mendocino fault that ruptured in 1994. See the figures from Rollins and Stein below.
Below is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. 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 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. Based on the moment tensor and my knowledge of the tectonics of this region, I interpret this earthquake to have had a right lateral strike slip motion along an east-west fault.

    I have placed several inset figures.

  • In the lower left corner is a map of the Cascadia subduction zone and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004)
  • Above the CSZ map is an illustration from Atwater et al. (2005). 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.
  • In the upper right corner 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 January 2010 Gorda plate earthquake.
  • In the lower right corner is an image from an introductory textbook I found several years ago. 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 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 map from Rollins and Stein, showing their interpretations of different historic earthquakes in the 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. I did not mention the 2010 earthquake, but it most likely was just like 1980 and 2005, a left-lateral strike-slip earthquake on a northeast striking fault.


Here is a large scale map of the 1983 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles. Note how the aftershocks trend slightly southeast in this region. Today’s swarm does the same (and the moment tensor also shows a slightly southeast strike). Note how the interpreted fault dips slightly to the north, which is the result of north-south compression from the relative northward motion of the Pacific plate.


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.


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.

    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.
  • 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.

Earthquake Report: Bartlett Springs fault system, Lake Pillsbury

We just had a good sized earthquake adjacent to the mapped surface trace of the Bartlett Springs fault, southeast of Lake Pillsbury. Here is the USGS website for the M 5.1 earthquake.
Here is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. 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. See the attenuation plot versus distance below to compare the difference between this model estimate and the real observations. The fit is pretty good.
I plot the moment tensor for this earthquake and I interpret this earthquake to be a northwest striking right lateral strike-slip earthquake. This is based upon the mapped faults in the region and their sense of motion, which is synthetic to the San Andreas fault system to the west. I used this kml to make this map.
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). There was a recent earthquake between the San Andreas and Maacama faults in August of 2016. The three main faults in the region north of San Francisco are the SAF, the MF, and the Bartlett Springs fault (BSF). The SAF, MF, and BSF are all right lateral strike-slip fault systems. Without more seismicity or mapped faults to suggest otherwise, this is a reasonable interpretation.

    I include some insets in the map below:

  • In the upper right corner, I place a map shows the configuration of faults in central (San Francisco) and northern (Point Delgada – Punta Gorda) CA (Wallace, 1990).
  • To the left of that, 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.
  • In the lower right corner I include one version of the Did You Feel It? map. This is the result of people submitting their observations on the USGS website.
  • In the lower left corner, I include a map that shows the major fault systems in this region (McLaughlin et al., 2012). In their 2012 publication, McLaughlin et al. present their interpretations for the evolution of the Rogers Creek-Maacama right-lateral fault system, the SAF plate boundary fault system to the west of the Bartlett Springs fault system.



This map shows today’s seismicity as red and orange circles. Faults are mapped as red lines and compare with the map above to note that from left to right (west to east) are the SAF, MF, and BSF. Fort Bragg is located in the northwest part of the map and Sacramento is in the southeast part of the map.


This plot shows how the ground motions decay with distance from the earthquake. The blue dots show the survey results and the orange line shows how a computer simulation based attenuation model suggests that the ground motions would reduce with distance from the earthquake. The orange dots show the median data for each data bin. Note how well the observations fit the attenuation (i.e. Ground Motion Prediction Equation) models.


This is my interpretive map for the earthquake from August 8, 2015. Compare the MMI contours with the interpretive map for today’s M 5.1 earthquake. These maps are at a similar scale.


I created an animation that shows the seismicity of this region from 1960 through Sunday, 2015.08.30. While this animation was made for a different earthquake (from 2015), it still reveals where the historic seismicity is located. Note how there is little recent historic seismicity in the region of today’s M 5.1earthquake. This is the USGS query that I used to create the animation, in the form of a kml file. Here is a static map that shows all earthquakes for this region.


Here is the animation. Either download the file here or click on the animation below.


Here is the (Wallace, 1990) map. I include the figure caption as a blockquote below. Below the citation is this map presented on its own.

Geologic sketch map of the northern Coast Ranges, central California, showing faults with Quaternary activity and basin deposits in northern section of the San Andreas fault system. Fault patterns are generalized, and only major faults are shown. Several Quaternary basins are fault bounded and aligned parallel to strike-slip faults, a relation most apparent along the Hayward-Rodgers Creek-Maacama fault trend.

Here is a map that shows some slip rates for the faults that make up the Pacific-North America plate boundary system (Wallace, 1990). I include the figure caption as a blockquote below. Below the citation is this map presented on its own.



Sketch map of western and southwestern California, showing selected major faults exhibiting evidence of Quaternary activity in the San Andreas fault system. Average Quaternary slip rates are based on measured values from published sources and, where measurements are incomplete or unavailable, on interpreted geologic and geomorphic evidence. Slip components on northwest-trending faults are predominantly horizontal, right lateral, and consistent with North American-Pacific plate motion Group of east-west-trending faults between lat 34°00′ and 35°30′ N. exhibit a more complex slip pattern, including chiefly reverse faulting but also some strike slip. Areas above 600-m elevation (colored) exhibit evidence of late Cenozoic uplift and enclose most areas of Quaternary uplift, but elevation pattern reveals no such major local uplifts as those near Ventura (approx 1 cm/yr) and along the Newport-Inglewood fault zone (NI). Faults dotted where concealed by water.

About 75% of the relative plate motion is accommodated along the SAF and its synthetic sister faults in the northern CA region. The rest of the plate boundary motion is accommodated along the Eastern CA shear zone and Walker Lane, along with the Central Nevada Seismic Belt, and the Wasatch fault systems. In Northern CA, there is about 33-37 mm/yr strain accumulated on the SAF plate boundary system. About 18-25 mm/yr is on the SAF, 8-11 mm/yr on the MF, and 5-7 mm/yr on the Bartlett Springs fault system (Geist and Andrews, 2000).

    Here is a map from McLaughlin et al. (2012) that shows the regional faulting. I include the figure caption as a blockquote below.

    Maps showing the regional setting of the Rodgers Creek–Maacama fault system and the San Andreas fault in northern California. (A) The Maacama (MAFZ) and Rodgers Creek (RCFZ) fault zones and related faults (dark red) are compared to the San Andreas fault, former and present positions of the Mendocino Fracture Zone (MFZ; light red, offshore), and other structural features of northern California. Other faults east of the San Andreas fault that are part of the wide transform margin are collectively referred to as the East Bay fault system and include the Hayward and proto-Hayward fault zones (green) and the Calaveras (CF), Bartlett Springs, and several other faults (teal). Fold axes (dark blue) delineate features associated with compression along the northern and eastern sides of the Coast Ranges. Dashed brown line marks inferred location of the buried tip of an east-directed tectonic wedge system along the boundary between the Coast Ranges and Great Valley (Wentworth et al., 1984; Wentworth and Zoback, 1990). Dotted purple line shows the underthrust south edge of the Gorda–Juan de Fuca plate, based on gravity and aeromagnetic data (Jachens and Griscom, 1983). Late Cenozoic volcanic rocks are shown in pink; structural basins associated with strike-slip faulting and Sacramento Valley are shown in yellow. Motions of major fault blocks and plates relative to fi xed North America, from global positioning system and paleomagnetic studies (Argus and Gordon, 2001; Wells and Simpson, 2001; U.S. Geological Survey, 2010), shown with thick black arrows; circled numbers denote rate (in mm/yr). Restraining bend segment of the northern San Andreas fault is shown in orange; releasing bend segment is in light blue. Additional abbreviations: BMV—Burdell Mountain Volcanics; QSV—Quien Sabe Volcanics. (B) Simplifi ed map of color-coded faults in A, delineating the principal fault systems and zones referred to in this paper.

    Here is a map that shows the shaking potential for earthquakes in CA. This comes from the state of California here. I include the figure caption as a blockquote below.

    Earthquake shaking hazards are calculated by projecting earthquake rates based on earthquake history and fault slip rates, the same data used for calculating earthquake probabilities. New fault parameters have been developed for these calculations and are included in the report of the Working Group on California Earthquake Probabilities. Calculations of earthquake shaking hazard for California are part of a cooperative project between USGS and CGS, and are part of the National Seismic Hazard Maps. CGS Map Sheet 48 (revised 2008) shows potential seismic shaking based on National Seismic Hazard Map calculations plus amplification of seismic shaking due to the near surface soils.

      References

    • 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.
    • 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.
    • 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/].

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

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.

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.

        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.

Friends of the Arcata Marsh 2016.07.22

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)
  • Cascadia subduction zone: M 9.0 tsunami simulation video

    NOAA Center for Tsunami Research just released an animation that shows a numerical simulation of what a tsunami may appear like when the next Cascadia subduction zone earthquake occurs. I present a summary about the CSZ tectonics on a 316th year commemoration page here. I include a yt link and embedded video and an mp4 embedded video and download link.
    Here is a screenshot from the video:

    Earthquake Report: Gorda!

    There was just a pair of earthquakes near the Gorda rise. There was first an earthquake of magnitude M = 5.0, followed by a M = 4.9. The 5.0 appears to be within the GP, but the 4.9 seems to be at the eastern boundary of the ridge. These two earthquakes do not pose a tsunami risk for northern California (or elsewhere). Nor do they likely have an affect upon the likelihood of a Cascadia subduction zone earthquake.
    **UPDATE: 2016.01.29 20:00 PST
    The USGS has prepared a moment tensor for the M 5.0 earthquake. I have updated the map. It is clear now that this is an extensional (normal) earthquake. I include an updated map below the original map.
    /end update
    **UPDATE: 2023.01.29
    The two events on this updated poster show how the faults near the spreading ridge are normal faults.

    Below we see some maps showing how these normal faults, as the plate rotates in a clockwise fashion, also rotate. During this process, they are reactivated as left-lateral strike-slip faults.

    Check out some of the earthquake reports for these strike-slip Gorda plate earthquakes listed at the bottom of this page. The 2014 M 6.8 event is a great example.

      Here are the USGS web pages for these two earthquakes

    • 2016.01.30 M 5.0
    • 2016.01.30 M 4.9

    Below is an interpretive map showing these two earthquakes as red circles, as well as other seismicity for the past week. I include an inset map of the Cascadia subduction zone (after Chaytor et al., 2004; Nelson et al., 2004), an inset map from Chaytor et al. (2004), and from Rollins and Stein (2010). I explain these inset maps below.
    I include labels for the plate boundary fault systems in the region and include generic focal mechanisms for these fault systems. 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 on the locations of these earthquakes, along with our knowledge of the local tectonics, I interpret the M 5.0 to possibly be a northeast striking left-lateral strike-slip fault earthquake. The M 4.9 could either be like that, or be an extensional earthquake (like the 2014.03.13 earthquake listed below). I summarized the regional tectonics of the Cascadia subduction zone recently here. The M 5.0 earthquake is similar to the earthquake “P” on the Rollins and Stein (2010) map.


    **UPDATE: 2016.01.29 20:00 PST
    Here is the updated map. Note the extensional moment tensor. This is aligned nicely with the ridges from the Gorda rise (which are visible in the large scale map at the bottom of this page.


    /end update

    There was an earthquake along the Gorda rise in 2014, much to the north of these two earthquakes. Here is my earthquake report for that 2014.03.13 M 5.2 earthquake.

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


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


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


    Here is a large scale map of the region for these two earthquakes. This is taken from Google Earth. The two largest orange circles (color represents depth < 33 km) are the M 5.0 and M 4.9, with diameter representing magnitude. The M 5.0 is clearly in the region where there are north-northeast striking faults.

    I have Earthquake Reports for other CSZ related earthquakes here:

    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.
    • 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.