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Earthquake Report: M 6.4 Gorda plate

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Initial Narrative

Well, it has been a very busy week. I had gotten back from the American Geophysical Union Fall Meeting in Chicago late Saturday night. I had one day to hang out with my cats before I was to head down to Santa Cruz to meet with the city there to discuss installing a tide gage. Santa Cruz lacks a gage yet receives large tsunami inundations.

So, I drove down and got there about 10pm Monday evening. I was up for an hour or two and went to sleep.

At shortly after 2:30am I got a text message about a M 6.4 earthquake near Ferndale. I immediately got up and texted my colleague Cynthia Pridmore. We are tasked to prepare Earthquake Quick Reports that we (California Geological Survey, CGS) provide to the California Governor’s Office of Emergency Services (Cal OES). These reports provide technical information that helps them provide resources to local first responders during times following natural hazards impacts.

https://earthquake.usgs.gov/earthquakes/eventpage/nc73821036/executive

These reports are reviewed by the head of the Seismic Hazards Program (Tim Dawson) and by the State Geologist prior to being provided to the leadership in our organization and parent organizations. Reports for larger earthquakes and tsunami sometimes end up on the Governor’s desk.

We got our report submitted within about 45 minutes and we prepared for a long couple of days. We at CGS met at 8am to discuss our field response activities.

CGS and the U.S. Geological Survey (USGS) work closely together to document field evidence from earthquakes and tsunami. Kate Thomas (CGS) and Luke Blair (USGS) have a database ready to go within about 15 minutes after an earthquake. This database is used on mobile devices to collect observational information that include photos and other information. We use the ESRI Field Maps app for this purpose.

We decided to send CGS staff from the Eureka office out to collect information. I was to drive back to Humboldt and then join the field teams the following day.

Something that also happens following significant or damaging earthquakes is the activation of the California Earthquake Clearinghouse. Pridmore (CGS) is the chair of the EQCH and works with our partners (USGS, EERI, etc.) to decide when to activate the EQCH.

Data from these CGS/USGS field observations, along with data from other field teams, are posted onto the EQCH page for this event. Here is where those data are made available for this M 6.4 Ferndale Earthquake. The dataset of field observations are posted on that page are found by clicking on the “Resources” tab, also linked here.

When I returned to my home, the power was still out. We (CGS) had a scheduled meeting at 6pm and the EQCH meeting at 7pm. So, I went to the Eureka National Weather Service (NWS) Office on Woodley Island. They have electric power backup and satellite internet access. I work closely with the NWS and Cal OES and have been granted access to set up my workstation there during natural hazard emergencies like earthquake and tsunami. This was we can all better coordinate our actions without the burden of having power or internet outages at our residences. We are thankful for these relationships between CGS, the NWS (Ryan Aylward, Troy Nicolini) and Cal OES Eureka (Todd Becker).

So, I got up very early to work with my co-workers to continue the field investigations. There was little geological evidence from the earthquake. We identified some landslides and cracks in road fill. We did not locate any evidence for liquefaction, even though the USGS liquefaction susceptibility data suggested a high chance for that phenomena.

The Earthquake Report

This earthquake is in a tectonically complicated region of the western United States, the Mendocino triple junction. Here, three plate boundary fault systems meet (the definition of a triple junction): the San Andreas fault from the south, the Cascadia subduction zone from the north, and the Mendocino fault from the west. These plate boundary fault systems all overlap like fingers do when we fold our hands together.

The Cascadia subduction zone is a convergent (moving together) plate boundary where the Gorda and Juan de Fuca plates dive into the Earth beneath the North America plate. The fault formed here is called the megathrust subduction zone fault. Earthquakes on subduction zone faults generate the largest magnitude earthquakes of all fault types and also generate tsunami that can impact the local area and also travel across the ocean to impact places elsewhere. The most recent known Cascadia megathrust subduction zone fault earthquake was in January 1700.

The San Andreas and Mendocino fault systems are strike-slip (plates move side by side) fault systems. Many are familiar with the 1906 San Francisco Earthquake.

While the largest source of annual seismicity are intraplate Gorda plate earthquakes, the two largest contributors to seismic hazards in California are the Cascadia subduction zone (CSZ) and the San Andreas fault (SAF) systems. These sources overlap in the region of the Mendocino triple junction (MTJ) and may interact in ways we are only beginning to understand as evidenced by the 2016 M7.8 Kaikōura earthquake in New Zealand (Clark et al., 2017 Litchfield et al., 2018), which occurred along a similar subduction/transform boundary, and included co-seismic rupture of more than 20 faults.

The M 6.4 earthquake was a strike-slip earthquake within the downgoing Gorda plate (an intra plate earthquake). The earthquake started offshore and then the fault slipped to the east.

There is modest evidence that this earthquake generated focused seismic waves in the direction of fault slip (this is called directivity). In addition, the area of the lower Eel River Valley is a sedimentary basin. Sedimentary basins are known for amplifying ground shaking and trapping seismic waves, further increasing the ground shaking. The lower Eel River Valley is formed by tectonic folding caused by the northward migration of the Mendocino triple junction (read my contributions in the 2022 Pacific Cell Friends of the Pleistocene guidebook for more information about the structure of the Eel River and Van Duzen River valleys and surrounding regions.

So the seismic waves could have been trapped in the sedimentary basin formed within the Eel River Valley. However, there is an even older sedimentary basin here in which the Eel/Van Duzen river sediments are deposited within. These older sedimentary rocks have different seismic velocity properties that could also affect how seismic waves are transmitted here. There is a terrane bounding fault that separates these older rocks (Cretaceous Franciscan Formation) to the south from the younger rocks (Quaternary-Tertiary Wildcat Group) to the north.

Also, any of the large crustal fault systems (e.g., the Russ fault, the Little Salmon or Table Bluff faults, etc.) could guide seismic waves (a.k.a. act as wave guides), directing them in orientations relative to the fault systems.

My leading hypothesis is that the younger (latest Pleistocene to Holocene) river sediments that form the younger sedimentary basin and the crustal faults are both responsible for modifying the seismic wave transmission from this earthquake.

One thing people almost always ask is about whether or not there is a higher chance that there will be a Cascadia subduction zone earthquake. This is currently impossible to tell. However, we can make some estimates of how forces within the Earth might have changed after a given earthquake. There was a Gorda plate earthquake sequence in 2018 that allowed us to consider these changes in the crust to see if the megathrust was brought more close to rupture. Here is the report from that Gorda plate earthquake sequence.

I will update this report further in the future, as we collect additional information.

One last thing for now. Bob McPherson formed a research group that we call Team Gorda. Team Gorda, supported by Connie Stewart at Cal Poly Humboldt, is using recently constructed fiber cables as a seismic instrument (called distributed acoustic seismic, DAS) to learn more about the underlying tectonic structures in the region. This fiber cable acts as thousands of little seismometers. Jeff McGuire and his team just installed the interrogator in our office at the Arcata City Hall. Horst from the Berkeley Seismic Lab is also working with Bob to install seismometers along the fiber cable so that we can calibrate the DAS observations.

We ran our first DAS experiment earlier this year and plan on doing more experiments far into the future, including fiber cables that are installed from here into the Pacific Ocean (on their way to Asia).

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1922-2022 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map showing the western US and a century of seismicity.
  • In the upper right corner is a map that displays a variety of earthquake intensity information. I plot the USGS modeled intensity, the USGS Did You Feel It? observations of intensity, and the shaking magnitude using the Peak Ground Acceleration scale in units of g (gravitational acceleration). I describe this map later in the report.
  • To the left of the intensity map are two maps that show the probability (the chance of) earthquake triggered landslides and the susceptibility (the chance of) earthquake induced liquefaction. I will discuss these ground failure models later in the report.
  • In the lower right corner I include a plot of aftershocks from a three day period.
  • Here is the map with 3 month’s seismicity plotted.


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  • Here is an updated interpretive poster with 3 day’s seismicity plotted. I describe how this poster is different
  • In the lower right corner is a map from the USGS. This map shows where they interpret the location for the causative fault for this earthquake. There are also arrows (vectors) that show how Global Navigation Satellite System (GNSS, used to be called GPS) sites moved during the earthquake and how the moved using a computer simulation of the Earth that incorporate a fault that slipped like shown on the map. These arrows show the direction of motion and the amount of motion.
  • To the left of this map is the USGS finite fault model for this earthquake. The colors represent the amount that the fault slipped during the earthquake. This is the fault model that they used to estimate how the GNSS sites moved in the map to the right.
  • In the upper right corner is a map that shows the seismicty from the past week (in orange) and seismicity associated with the earthquake sequence from exactly one year before (in blue).
  • In the main part of the map I plot the earthquake mechanisms from the past century.


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    Seismicity Profile

  • I felt the M 4.1 earthquake this morning (24 Dec 2022). It was an extensional earthquake in the eastern part of the aftershock region.
  • Today I plotted the seismicity along an east-west profile.
  • I traced the Gorda plate geometry from Guo et al. (2018). This is from their profile B-B’ which is just about at 41 degrees north.
  • We can see that the mainshock (the M 6.4) and most of the aftershocks are within the Gorda plate.


  • Here is an updated plot that includes the USGS Finite Fault Model as a transparent overlay.
  • Note how most of the slip is in the North America plate.


  • Here is an updated plot that displays M 6.4 in blue and M 5.4 in green.


  • And if someone wants to learn more about what a hypocenter is, here you go >>>

    Aftershock Patterns

  • Yesterday I got to feel one of the aftershocks, an M 4.2 to the southeast of the main sequence.
  • Today I plotted all the aftershocks to date as of this morning. It appears that there were two main faults involved. One about 45 km long and another one about 25 km long.
  • I include earthquake mechanisms for all events that I could download today. I placed some mechanisms that may not be related to these 2 faults at 50% transparency.
  • This poster below includes a map (lower right corner) of the Cascadia subduction zone and the cross section showing how the crust deforms between (interseismic) and during (coseismic) earthquakes.
  • I also include a schematic showing where earthquakes might happen (upper left center). Earthquakes along the megathrust subduction zone fault are called interplate earthquakes (like the interstate highways connect between states).
  • Earthquakes within the Gorda or North America plates are called intraplate earthquakes. The M 6.4 was an intraplate earthquake within the Gorda plate. I don’t really have a good way to show intraplate strike-slip faults in this diagram (room for future work!).
  • In the upper right corner is the seismicity profile that I also show above in the report. When comparing the seismicity with the Guo et al. (2021) slab model, it appears that most of the earthquakes are within the Gorda crust. There are some above, possibly in the North America crust.


  • Here is another updated map, updated on 2 January 2023 to include the M 5.4 related earthquakes.
  • Now it appears that there are three main faults involved, at least.


  • Yesterday I was chatting with Bob McPherson as we were looking at the USGS finite fault slip model. Bob suggested that this model shows that the earthquake slipped in the Gorda and North America plates. If the slip model is correct, then Bob is correct. This is quite interesting if true.
  • UPDATE comment (4 jan ’23): I have seen other slip models that do not place M 6.4 slip above the Gorda plate. We must remember that these slip models are non unique solutions and that there is quite a bit of wiggle room for their solutions. Basically, there are knobs to turn on these models (allowing one to change parameters, such as the material properties of the Earth (e.g., the “rheology” of the crust or mantle)) and changing these parameters can change the results while still keeping a good fit to the observational data. It is not uncommon that the slip models for both nodal planes (the two possible fault planes shown on earthquake mechanisms (focal mechanisms or moment tensors)) each fit the data equally well. I have seen the fault model that was fit to the incorrect (incorrect relative to aftershocks) fault plane being chosen as the preferred slip model. So, we must remember this when we are interpreting model results like these fault slip models.
  • First lets just look at the finite fault slip model. Below we see a plot with color representing how much the fault slipped. The white star is the M 6.4 hypocenter (the 3-D location of the M 6.4). East is to the left and west is to the left (pretend you are looking at the diagram from the north side of the fault).
  • There are gray lines that represent times (10 seconds and 20 seconds after the M 6.4 mainshock) where the rupture propagation front was. So, the fault started slipping at the white star. Then, the fault moved and the outer limit of this motion radiated outwards and was at the first gray line in 10 seconds and at the second gray line at 20 seconds.
  • There are small gray arrows that show the direction and magnitude of slip motion along the fault. If we combine this plot with our knowledge that this was a left-lateral strike-slip earthquake, and we are looking to the south at the fault, we can surmise that these vectors are on the north side of the fault. Also, that the fault slipped from east of the hypocenter towards the hypocenter, and updip (shallower).


  • Yes, this would be quite interesting, if the fault broke both Gorda and North America crust. This would make our interpretation of the Mendocino triple junction even more complicated. There are not currently any faults mapped in the North America plate that align with this M 6.4 sequence.
  • It is possible that there are faults there, or that they may be blind (not reach the ground surface). If these faults are young, they may not have sufficient offset to produce deformation at the Earth’s surface.
  • We do have examples of this elsewhere, where there are crustal faults in the downgoing plate that are also in the same location but in the upper plate.
  • For example, Goldfinger et al. (1997) mapped a series of faults that cut across strike to the Cascadia subduction zone fault. Two, the Daisy Bank and Wecoma faults, are shown in their figure below. Note how these faults are mapped in the Juan de Fuca plate and propagate upwards into the accretionary prism (let’s call this the upper plate).


    • Block rotation model for the central Cascadia forearc. SeaBeam bathymetry shaded from the north. The Wecoma and Daisy Bank faults are show, with the Daisy Bank fault exposed in the foreground. Well-mapped fault traces are in solid; discontinuous traces are dashed. The arc-parallel component of oblique subduction creates a dextral share couple, which is accommodated by WNW trending left-lateral strike-slip faults. We propose that shearing of the slab due to oblique subduction is responsible for the fault involving oceanic crust. WF, Wecoma fault; DBF, Daisy bank fault; FF, Fulmar fault, “pr,” pressure ridge; “DB,” Daisy Bank; “OT?,” possible old left-lateral fault strand. Arrow heads and tails show strike-slip motion. White arrows at western end of Wecoma fault show eastward increasing slip calculated from isopach offsets.

  • Another place where I have seen this is offshore of Sumatra. When we were coring there for my Ph.D. research, we identified a strike-slip fault in the India Australia plate that propagated upwards into the accretionary prism (the “upper plate”).
  • One thing That this almost certainly requires is that the megathrust fault be seismogenically coupled in this area.
  • Basically, we need a mechanism by which, when the lower plate fault slips, that the forces are exerted to the upper plate to move in the same direction and manner as that observed in the lower plate. Having a coupled megathrust fault is one way to do this
  • And we have several examples of this in the southern CSZ. There are a number of strike-slip fault earthquakes within the Gorda plate (or along the Mendocino fault) offshore of the megathrust that generated differential motion for geodetic sites (like GNSS or GPS stations) during the earthquake.
  • Further down in the report I present the map from Dengler et al. (1994) that shows how geodetic sites in North America plate move in response to the 1994 Cape Mendocino fault right-lateral strike-slip earthquake.
  • The USGS pages for the GNSS network provide static offsets for the GNSS stations as observed for these Gorda plate earthquakes. Williams and McPherson (2006) present another example of this. Below we can see the coseismic displacements from the 2005 northeast striking left-lateral strike-slip fault earthquake.


    • Coseismic displacements from the 15-Jun-2005 M7.2 Gorda plate earthquake located (off the map) 156 km (97 miles) W (280°) from Trinidad, CA and 157 km (98 miles) WSW (251°) from Crescent City, CA. Note the similarity to the deformation pattern of the 1994 event. Continuously operating GPS stations shown here are operated and maintained through the Plate Boundary Observatory component (pboweb.unavco. org) of the National Science Foundation’s EarthScope project
    (www.earthscope.org).

  • Regardless of whether or not there is a throughgoing fault, it is clear that the megathrust fault is locked here. (either from the presence of a throughoing fault or from the static offsets at these GNSS sites.
  • Below is the USGS finite fault slip model and a comparison between the observed GNSS offsets and the offsets modeled by placing slip on the finite fault model in an elastic half space.
  • Once we have better INSAR data (presuming these data will exist), this slip model may improve.


    • Mapped Geology

  • Here is a map that shows the mapped geologic units. Some of the map is from McLaughlin et al. (2000) and some is from the California Division of Mines and Geology (CDMG, 1999) which is now the California Geological Survey.
  • There are about 30 units in each dataset, so I chose to simply use their labels from the respective databases. The CDMG labels are basically the same as the geologic unit (e.g., Franciscan is something like TKJf) while McLaughlin mapped units relative to their geomorphic expression, so units have strange labels (e.g., Franciscan is something like co1 or cm1).
  • Note that the seismicity trend from the M 6.4 does not align with the faults nor the geologic units in the North America plate. This makes the linkage between rupturing in the Gorda and the North America plates more tenuous (though still possible).


    Earlier Report Interpretive Posters

  • Here is the poster from last year’s earthquake sequence.


  • Here are two relevant interpretive posters from the 1992 Cape Mendocino Earthquake.
  • This one is an overview of the earthquake.


  • This one helps us compare the mainshock and two main triggered earthquakes.


  • Here is a poster that shows a comparison between the 1991 Honeydew and 1992 Cape Mendocino earthquakes..


    Some Relevant Discussion and Figures

    • 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.


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    • 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.

    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. In my mind, these two aftershocks aligned on what may be the eastern extension of the Mendocino fault. However, looking at their locations, my mind was incorrect. These two earthquakes were not aftershocks, but were either left-lateral or right-lateral strike-slip Gorda plate earthquakes triggered by the M 7.1 thrust event.

    These two quakes appear to be aligned with the two northwest trends in seismicity and the 18 March 2020 M 5.2. The orientation of the mechanisms are not as perfectly well aligned, but there are lots of reasons for this (perhaps the faults were formed in a slightly different orientation, but have rotated slightly).

    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.

    • 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.

    • 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).


      • 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.

    • 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.


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  • 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.


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  • 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.


    • A: Mapped faults and fault-related ridges within Gorda plate based on basement structure and surface morphology, overlain on bathymetric contours (gray lines—250 m interval). Approximate boundaries of three structural segments are also shown. Black arrows indicated approximate location of possible northwest- trending large-scale folds. B, C: uninterpreted and interpreted enlargements of center of plate showing location of interpreted second-generation strike-slip faults and features that they appear to offset. OSC—overlapping spreading center.

  • These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.

    • Mw = 5 Trinidad Chaytor

    Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.

Mendocino triple junction video

Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
  • In the upper panel is the USGS Did You Feel It reports map, showing reports as colored dots using the MMI color scale. Underlain on this map are colored areas showing the USGS modeled estimate for shaking intensity (MMI scale).
  • I also plot, in colored squares, the ground motions recorded on seismometers operated by the CGS Strong Motion Instrument Program (SMIP), run by Hamid Haddadi. Units are relative to gravitation acceleration where 1 = 1g. g is defined as the acceleration at the Earth’s surface (9.8 m/s^2). Here is the data page for this M 6.4 earthquake. The largest acceleration (1.36g) is from a seismometer attached to a bridge and seismologists think that this large acceleration is due to the bridge in some way. Here is the SMIP data page for the M 5.4 earthquake.
  • Below the upper map plot is the USGS MMI Intensity scale, which lists the level of damage for each level of intensity, along with approximate measures of how strongly the ground shakes at these intensities, showing levels in acceleration (Peak Ground Acceleration, PGA) and velocity (Peak Ground Velocity, PGV).
  • In the lower panel is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orange and purple dots. Note how well (or poorly) the reports fit the brown line (the model that represents how MMI works based on quakes in California). The increased intensity on the left of the plot (which are closer to the earthquake) are the records that show intensities higher than expected from the modeling.


  • Here is an animation from the USGS and Cal Tech that shows a simulation of seismic waves from this M 6.4 earthquake.
  • There is a link to this video from the earthquake page.

Shaking Intensity and Potential for Ground Failure

  • Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.

    • There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

    When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.


    Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.

    Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.

    An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.

    Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


    Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.


  • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a low probability for landslides. However, we have already seen photographic evidence for landslides and the lower limit for earthquake triggered landslides is magnitude M 5.5 (from Keefer 1984)
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.


Seismic Hazard and Seismic Risk

  • These are two maps from the Global Earthquake Model (GEM) project, the GEM Seismic Hazard and the GEM Seismic Risk maps from Pagani et al. (2018) and Silva et al. (2018).

  • The GEM Seismic Hazard Map:




  • The Global Earthquake Model (GEM) Global Seismic Hazard Map (version 2018.1) depicts the geographic distribution of the Peak Ground Acceleration (PGA) with a 10% probability of being exceeded in 50 years, computed for reference rock conditions (shear wave velocity, VS30, of 760-800 m/s). The map was created by collating maps computed using national and regional probabilistic seismic hazard models developed by various institutions and projects, and by GEM Foundation scientists. The OpenQuake engine, an open-source seismic hazard and risk calculation software developed principally by the GEM Foundation, was used to calculate the hazard values. A smoothing methodology was applied to homogenise hazard values along the model borders. The map is based on a database of hazard models described using the OpenQuake engine data format (NRML). Due to possible model limitations, regions portrayed with low hazard may still experience potentially damaging earthquakes.
  • Here is a view of the GEM seismic hazard map for the USA.


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  • The GEM Seismic Risk Map:




  • The Global Seismic Risk Map (v2018.1) presents the geographic distribution of average annual loss (USD) normalised by the average construction costs of the respective country (USD/m2) due to ground shaking in the residential, commercial and industrial building stock, considering contents, structural and non-structural components. The normalised metric allows a direct comparison of the risk between countries with widely different construction costs. It does not consider the effects of tsunamis, liquefaction, landslides, and fires following earthquakes. The loss estimates are from direct physical damage to buildings due to shaking, and thus damage to infrastructure or indirect losses due to business interruption are not included. The average annual losses are presented on a hexagonal grid, with a spacing of 0.30 x 0.34 decimal degrees (approximately 1,000 km2 at the equator). The average annual losses were computed using the event-based calculator of the OpenQuake engine, an open-source software for seismic hazard and risk analysis developed by the GEM Foundation. The seismic hazard, exposure and vulnerability models employed in these calculations were provided by national institutions, or developed within the scope of regional programs or bilateral collaborations.
  • Here is a view of the GEM seismic risk map for the USA. Note how the seismic risk is higher in places of larger population (like Los Angeles and San Francisco).


    •

Stress Triggering

  • When an earthquake fault slips, the crust surrounding the fault squishes and expands, deforming elastically (like in one’s underwear). These changes in shape of the crust cause earthquake fault stresses to change. These changes in stress can either increase or decrease the chance of another earthquake.
  • I wrote more about this type of earthquake triggering for Temblor here. Head over there to learn more about “static coulomb stress triggering.”
  • Rollins and Stein (2010) conducted this type of analysis for the 2010 M 6.5 Gorda Earthquake. They found that some of the faults in the region experienced an increase in fault stress (the red areas on the figure below). These changes in stress are very small, so require a fault to be at the “tipping point” for these changes in stress to cause an earthquake.
  • There was a triggered earthquake in this sequence. There was a M 5.9 event about 25 days after the mainshock, this earthquake happened in a region that saw increased stress after the M 6.5. The M 5.9 appears to have been on the same fault as the M 6.5
  • First, here is the fault model that Rollins and Stein used in their analysis of stress changes from the 2010 earthquake.


    • Source models for earthquakes S and T, 10 January 2010, M = 6.5, and 4 February 2010, Mw = 5.9.

  • Let’s take a look at some examples of analogic earthquakes to the 2010 temblor. First, here is a plot showing changes in stress following the 1980 Trinidad Earthquake (a very damaging earthquake in the region). This is the largest historic earthquake in the region at magnitude M 7.3 (other than the 1906 San Francisco Earthquake).


    • Coulomb stress changes imparted by the 1980 Mw = 7.3 earthquake (B) to a matrix of faults representing the Mendocino Fault Zone, the Cascadia subduction zone, and NE striking left‐lateral faults in the Gorda zone. The Mendocino Fault Zone is represented by right‐lateral faults whose strike rotates from 285° in the east to 270° in the west; Cascadia is represented by reverse faults striking 350° and dipping 9°; faults in the Gorda zone are represented by vertical left‐lateral faults striking 45°. The boundary between the left‐lateral “zone” and the reverse “zone” in the fault matrix is placed at the 6 km depth contour on Cascadia, approximated by extending the top edge of the Oppenheimer et al.
    [1993] model for the 1992 Cape Mendocino earthquake (J). Calculation depth is 5 km. The numbered brackets are groups of aftershocks from Hill et al. [1990].

  • Next let’s look at the stress changes following the 2005 M 7.2 earthquake.


    • Coulomb stress changes imparted by the Shao and Ji (2005) variable slip model for the 15 June 2005 Mw = 7.2 earthquake (P) to the epicenter of the 17 June 2005 Mw = 6.6 earthquake (Q). Calculation depth is 10 km.

  • Here is the figure we have all been waiting for (actually, the next one is cool too). This figure shows the changes in stress associated with the 2010 M 6.5 earthquake. Remember, these are just models.


    • Coulomb stress changes imparted by the D. Dreger (unpublished report, 2010, [no longer] available at http://seismo.berkeley.edu/∼dreger/jan10210_ff_summary.pdf) model for the January 2010 M = 6.5 shock (S) to nearby faults. East of the dashed line, stress changes are resolved on the Cascadia subduction zone, represented by a northward extension of the Oppenheimer et al. [1993] rupture plane for the 1992 Mw = 6.9 Cape Mendocino earthquake. West of the dashed line, stress changes are resolved on the NW striking nodal plane for the February 2010 Mw = 5.9 earthquake (T) at a depth of 23.6 km.

  • This is the main take-away figure from Rollins and Stein (2010). For each map, there is a source fault (in black) and receiver faults (red or blue, depending on the change in stress).
  • For example, in a, the source is a gorda plate left-lateral strike-slip fault. Parts of the Cascadia megathrust are represented on the right (triangles, labeled thrust). They also model changes in stress on the Mendocino fault (the red and blue lines at the bottom of “a”).


  • And, you thought it couldn’t get any better. Here is yet another fantastic figure showing the stress change on the Cascadia megathrust fault and on the Mendocino fault following the 2010 M 6.5 earthquake.


    •

    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Pagani, M., J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
  • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
  • Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

    • Specific 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.
  • 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.
  • Guo, H., McGuire, J., and Zhang, H., 2021. Correlation of porosity variations and rheological transitions on the southern Cascadia megathrust in Nature Geoscience, https://doi.org/10.1038/s41561-021-00740-1
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
  • McLaughlin, R.J., Ellen, S.D., Blake, M.C. Jr., Jayko, A.S., Irwin, W.P., Aalto, F.R., Carver, G.A., and Clarke, S.H. Jr., 2000. Geology of the Cape Mendocino, Eureka, Garberville, and Southwestern Part of the Hayfork 30 x 60 Minute Quadrangles and Adjacent Offshore Area, Northern California, USGS Miscellaneous Field Studies Map MF-2336, http://pubs.usgs.gov/mf/2000/2336/
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].
  • Wells, D.L., and Coopersmith, K.J., 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement in BSSA, v. 84, no. 4, p. 974-1002
  • Wells, R.E., Blakely, R.J., Wech, A.G., McCrory, P.A., Michael, A., 2017. Cascadia subduction tremor muted by crustal faults in Geology, v. 45, no. 6, p. 515–518, https://doi.org/10.1130/G38835.1
  • Williams, T.B. and McPherson, R.C., (2006). Gorda Plate Deformation Contributes to Shortening Between the Klamath Block and the On-land Portion of the Accretionary Prism to the S. Cascadia Subduction Zone. In Hemphill-Haley, M., McPherson, R., Patton, J. R., Stallman, J., Leroy, T.H., Sutherland, D., and Williams, T.B., eds. (2006) Pacific Cell Friends of the Pleistocene Field Trip Guidebook, The Triangle of Doom: Signatures of Quaternary Crustal Deformation in the Mendocino Deformation Zone (MDZ) Arcata, CA.

Return to the Earthquake Reports page.

Earthquake Report: Mendocino triple junction

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Well, it was a big mag 5 day today, two magnitude 5+ earthquakes in the western USA on faults related to the same plate boundary! Crazy, right? The same plate boundary, about 800 miles away from each other, and their coincident occurrence was in no way related to each other.
In the past 9 months it was also a big mag 5 MTJ year. There have been 3 mag 5+ earthquakes in the Mendocino triple junction (MTJ) region. The first one in June of 2019, at the time, appeared to be related to the Mendocino fault. The 9 March M 5.8 event was clearly associated with the right lateral Mendocino transform fault. The latest in this series of unrelated earthquakes is possibly associated with NW striking faults in the Gorda plate. I will discuss this below and include background about all the different faults in the region.

I was on the phone with my friend, collaborator, and business partner Thomas Harvey Leroy (the man with 4 first names: Tom, Harvey, Lee, and Roy) yesterday afternoon. We were determining the best course of action after a tenant of ours moved out leaving PG&E with an unpaid ~$9000 bill and we could not turn the power back on until the bill was paid. His son walked up to him and asked if what he had just felt was an earthquake. Because Tom was pacing back and forth, he did not feel it (as Tom likes to say, “feel the pain.”). He wishes that he had felt it.
My social media feed was immediately dominated by posts about the earthquake in Humboldt County. I put together a quick map (see below). My good friend and collaborator Bob McPherson (a seismologist who ran the Humboldt Bay Seismic Network in the late 70s and 80s) sent me several text messages about the earthquake. we texted back and forth. I initially thought it might be Mendo fault and so did he.
Then the USGS moment tensor (earthquake mechanism) came in with an orientation similar to that of Gorda plate earthquakes further to the north. These earthquakes are typically on northeast striking (trending) left-lateral strike-slip faults (see more here about types of earthquakes). So, I stated that I thought it was like those, a left-lateral strike-slip fault earthquake. So I deleted my social media posts and updated the map to show it could be either left-lateral or right-lateral (the map below shows both options), but that we thought it was in the Gorda plate, not the Mendocino fault.


Then Bomac mentioned these northwest trends in seismicity that we noticed (as a group) about 5 years ago, seismicity trends (seismolineaments is what Tom calls them) that first appeared following the 1992 Cape Mendocino Earthquake.
We don’t yet have a full explanation for these trends in seismicity, but the orientation fits a stress field from north-south compression (from the northward motion of the Pacific plate relative to the Gorda plate). This north-south compression is also the explanation for the left-lateral strike-slip fault earthquakes in the Gorda plate (Silver, 1971).

How are these 3 M5+ MTJ events related?

Well, they are not directly related to each other (i.e. none of these earthquakes caused any of the other earthquakes). The exception is that the 2019 M 5.6 may have affected the stress in the crust leading to the March M 5.2, but this is unlikely. What is even less likely that the M 5.8 was caused by the June 5.6 or caused the march 5.2.
WHy?
Well, there are two kinds of earthquake triggering.

  1. Dynamic Triggering – When seismic waves travel through the Earth, they change the stresses in the crust. IF the faults are “locked and loaded” (i.e. they are just about ready to slip in an earthquake), there may be an earthquake on the “receiver” fault. Generally, once the seismic waves are done travelling, this effect is over. Though, some suggest that this affect on the stress changes may last longer (but not much longer).
  2. Static Triggering – When an earthquake fault slips, it deforms (changes the shape) of the crust surrounding that earthquake. These changes can cause increases and decreases in the stress on faults (either increasing or decreasing the chance for an earthquake). Just like for dynamic triggering, the fault needs to be about ready to slip. The effect on fault slip changes in “static coulomb stress” generally extend a distance of about 2-3 times the fault length of the “source” fault.

Below is a figure from Wells and Coppersmith (1994) that shows the empirical relations between surface rupture length (SRL, the length of the fault that ruptures to the ground surface) and magnitude. If one knows the SRL (horizontal axis), they can estimate the magnitude (vertical axis). The left plot shows the earthquake data. The right plot shows how their formulas “predict” these data.

(a) Regression of surface rupture length on magnitude (M). Regression line shown for all-slip-type relations. Short dashed line indicates 95% confidence interval. (b) Regression lines for strike-slip, reverse, and normal-slip relations. See Table 2 for regression coefficients. Length of regression lines shows the range of data for each relation.
* note, i corrected this caption by changing the word “relationships” to “relations.”

Using these empirical relations (which are crude and may not cover earthquakes as small as this M 5.8, but they are better than nothing), the “surface rupture length” of this M 5.8 might be about 5 km. So, changes in static coulomb stress from the M 5.8 extended, at most, about 16 km (or about 10 miles). Yesterday’s M 5.2. is about 72 km away, far too distant to be statically triggered by the 5.8.
The M 5.6 might have a rupture length crudely about 3 km might affect the region up to 9 km away. The M 5.2 is ~16 km from the M 5.6, so probably too far to be affected.
However, these earthquakes are related because they are all in the same region and are responding to the same tectonic forces.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner are a map of the tectonic plates and their boundary faults (Chaytor et al., 2006; Nelson et al., 2006). To the right is a and cross section cutting into the Earth from West (left) to East (right) that shows the downgoing (subducting) Gorda plate beneath the North America plate (Plafker, 1972).
  • In the upper right corner is a map of the MTJ area. The Great Salt Lake is the large light blue bleb in the upper right. We can see the mountains to the east of SLC, the Wasatch Range. The Earthquake Intensity uses the MMI scale (the colors), read more about this here. This map represents an estimate of ground shaking from the M 5.7 based on a statistical model using the results of tens of thousands of earthquakes.
  • In the lower left corner to the right of the legend is a plot showing how these USGS models “predict” the ground shaking intensity will be relative to distance from the earthquake. These models are represented by the broan and green lines. People can fill out an online form to enter their observations and these “Did You Feel It?” observations are converted into an intensity number and these are plotted as dots in this figure.
  • There are several sources of seismicity on this map, but i tried to make it easier to interpret using color choices. I recognize this poster does not satisfy Access and Functional Needs. I will work on that.
    • The three main earthquakes are plotted in pastel yellow and orange-yellow colors.
    • Earthquakes from the past 3 months are light green.
    • The earthquakes from the past century are faint gray.
    • The earthquakes located using a double differenced locating method are colored relative to depth.
  • Look at the westernmost NW trend in seismicity. How does the depth of the earthquakes change along that transect?
  • Yes! The earthquakes deepen to the southeast. These earthquakes are revealing to us the location (e.g. depth) of the Gorda plate as it dives deeper to the east.
  • Here is the map with 3 month’s (in green) and 1 century’s (in gray, mislabeled) seismicity plotted. I also include seismicity from a catalog with events relocated using the Double Differencing method.


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I also outlined the two main northwest trends in seismicity with dashed white line polygons. The 18 March event is in the southern end of the western seismicity trend.
There is a nice northeast trend in seismicity that I also outlined. This is probably representative of one of the typical left-lateral Strike-slip Gorda plate earthquakes.

Other Report Pages

Some Relevant Discussion and Figures

  • 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.


    •

  • 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.

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. In my mind, these two aftershocks aligned on what may be the eastern extension of the Mendocino fault. However, looking at their locations, my mind was incorrect. These two earthquakes were not aftershocks, but were either left-lateral or right-lateral strike-slip Gorda plate earthquakes triggered by the M 7.1 thrust event.
These two quakes appear to be aligned with the two northwest trends in seismicity and the 18 March 2020 M 5.2. The orientation of the mechanisms are not as perfectly well aligned, but there are lots of reasons for this (perhaps the faults were formed in a slightly different orientation, but have rotated slightly).
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.

  • 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.


    • 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).


      • 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.

    • 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.


        •

      • 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.


        • A: Mapped faults and fault-related ridges within Gorda plate based on basement structure and surface morphology, overlain on bathymetric contours (gray lines—250 m interval). Approximate boundaries of three structural segments are also shown. Black arrows indicated approximate location of possible northwest- trending large-scale folds. B, C: uninterpreted and interpreted enlargements of center of plate showing location of interpreted second-generation strike-slip faults and features that they appear to offset. OSC—overlapping spreading center.

      • These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.

        • Mw = 5 Trinidad Chaytor

        Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.

    Further North

    If we move a little further north, we can take a look at the Blanco fault. This is a right-lateral strike-slip fault just like the Mendocino and San Andreas faults.
    If we turn our head at an oblique angle, we may consider the San Andreas, the Mendocino, and the Blanco faults to be all part of the same transform fault.
    Transform faults are often (or solely) defined as a strike-slip fault system that terminates at each end with a spreading ridge. These 3 systems link spreading ridges in the Gulf of California, through the Gorda Rise, to the Juan de Fuca ridge (and further).
    The Blanco fault is as, or more active than the Mendocino fault. The excellent people in Oregon who are aware of their exposure to seismic and tsunami hazards from the Cascadia subduction zone are always interested when there are earthquake notifications.
    Earthquakes on the Blanco fault are some of these events that people notice and ask about, “should I be concerned?” The answer is generally, “those earthquakes are too far away and too small to change the chance of the “Big One.” (remember the discussion about dynamic triggering above?)
    There was a recent earthquake (2018) on the Blanco fault that brought the public to question this again. My report about that earthquake spent a little space addressing these fault length >> magnitude >> triggering issues.
    As we know, the tectonics of the northeast Pacific is dominated by the Cascadia subduction zone, a convergent plate boundary, where the Explorer, Juan de Fuca, and Gorda oceanic plates dive eastward beneath the North America plate.
    These oceanic plates are created (formed, though I love writing “created” in science writing) at oceanic spreading ridges/centers.
    When oceanic spreading centers are offset laterally, a strike-slip fault forms called a transform fault. The Blanco transform fault is a right-lateral strike-slip fault (like the San Andreas fault). Thanks to Dr. Harold Tobin for pointing out why this is not a fracture zone.

    • This is the figure from Dziak et al. (2000) for us to evaluate. I include their long figure caption below.


      • (Top) Sea Beam bathymetric map of the Cascadia Depression, Blanco Ridge, and Gorda Depression, eastern Blanco Transform Fault Zone (BTFZ).Multibeam bathymetry was collected by the NOAA R/V’s Surveyor and Discoverer and the R/V Laney Chouest during 12 cruises in the 1980’s and 90’s. Bathymetry displayed using a 500 m grid interval. Numbers with arrows show look directions of three-dimensional diagrams in Figures 2 and 3. (Bottom) Structure map, interpreted from bathymetry, showing active faults and major geologic features of the region. Solid lines represent faults, dashed lines are fracture zones, and dotted lines show course of turbidite channels. When possible to estimate sense of motion on a fault, a filled circle shows the down-thrown side. Inset maps show location and generalized geologic structure of the BTFZ. Location of seismic reflection and gravity/magnetics profiles indicated by opposing brackets. D-D’ and E-E’ are the seismic reflection profiles shown in Figures 8a and 8b, and G-G’ is the gravity and magnetics profile shown in Figure 13. Submersible dive tracklines from sites 1 through 4 are highlighted in red. L1 and L2 are two lineations seen in three-dimensional bathymetry shown in Figures 2 and 3. Location of two Blanco Ridge slump scars indicated by half-rectangles, inferred direction of slump shown by arrow, and debris location (when identified) designated by an ‘S’. CD stands for Cascadia Depression, BR is Blanco Ridge, GD is Gorda Depression, and GR is Gorda Ridge. Numbers on north and south side of transform represent Juan de Fuca and Pacific plate crustal ages inferred from magnetic anomalies. Long-term plate motion rate between the Pacific and southern Juan de Fuca plates from Wilson (1989).

    When there are quakes on the BF, people always wonder if the Cascadia megathrust is affected by this… “are we at greater risk because of those BF earthquakes?”
    The main take away is that we are not at a greater risk because of these earthquakes.

    • Here is the map with a century’s seismicity plotted, for earthquakes of magnitude M ≥ 6.0 for the 29 Aug 2019 M 6.3 Blanco fault earthquake.


      •

    • The poster includes earthquake information for earthquakes with M ≥ 6.0. I prepared this for a magnitude M 6.2 Blanco fault earthquake on 22 August 2018. I place fault mechanisms for all existing USGS mechanisms from the Blanco fracture zone and I include some examples from the rest of the region. These other mechanisms show how different areas have different tectonic regimes. Earthquakes within the Gorda plate are largely responding to being deformed in a tectonic die between the surrounding stronger plates (northeast striking (oriented) left-lateral strike-slip earthquakes). I include one earthquake along the Mendocino fracture zone, a right-lateral (dextral) strike-slip earthquake from 1994. I include one of the more memorable thrust earthquakes, the 1992 Cape Mendocino earthquake. I also include an extensional earthquake from central Oregon that may represent extension (basin and range?) in the northwestern region of the basin and range.


      •

    Social Media

    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
  • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
  • Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

    • Specific 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.
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].
  • Wells, D.L., and Coopersmith, K.J., 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement in BSSA, v. 84, no. 4, p. 974-1002

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Earthquake Report: Salt Lake City

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As I was waking up this morning, I rolled over to check my social media feed and moments earlier there was a good sized shaker in Salt Lake City, Utah. I immediately thought of my good friend Jennifer G. who lives there with her children. I immediately started looking into this earthquake.
https://earthquake.usgs.gov/earthquakes/eventpage/uu60363602/executive
The second thing I thought of was Chris DuRoss, a USGS geologist I first met when he was presenting his research of the record of prehistoric earthquakes along the Wasatch fault at the Seismological Society of America (SSA) meeting that was being held in SLC that year. Gosh, that was in 2013. My, how time passes. Dr. DuRoss now works for the USGS and continues to research the seismic hazards of the intermountain west and beyond from his office in Golden, Colorado.
The third thing I thought of was all the buildings in the SLC area that are not designed to withstand the shaking from the earthquakes that we expect will occur on that fault system. About 85% of the population of the state of Utah lives within 15 miles of the Wasatch fault. This is sobering.
I quickly put together a poster for this earthquake to help people learn a little more. I have a second earthquake to interpret tonight, so I will update this report later with more background on the Wasatch fault tectonics and seismic hazard.
There is also a great resource from the University of Utah, an event page for this earthquake sequence.

Tectonic Background

The west coast of the United States and Mexico is dominated by the plate boundary between the Pacific and North America plates. Many are familiar with the big players in this system:

  • The right-lateral transform fault zone called the San Andreas fault (SAF) where the North America plate on the east moves south relative to the Pacific plate. They are both moving north-ish, but the Pacific plate is moving “North” faster than the North America plate.
  • The convergent plate boundary called the Cascadia subduction zone (CSZ) where the Gorda, Juan de Fuca, and Explorer plates are diving beneath the North America plate, forming a megathrust subduction fault system.

There are many other faults that are also part of this plate boundary system. The San Andreas fault zone “proper” accommodates about 85% of the relative plate motion. The rest of the relative plate motion (15%) is accounted for by slip on other strike-slip fault systems.
There are “sibling” faults to the SAF near the SAF (like the Hayward fault in the San Francisco Bay Area) and further away (like the Eastern California shear zone, the Owens Valley fault, and the Walker Lane fault systems).
Just like Dr. Steve Wesnousky showed us, the crust in the Walker Lane is moving around like a layer of solid wax floating around on a tray of melted wax. So, there are faults in lots of different kinds of directions, and different kinds of faults too.
The easternmost right-lateral strike slip fault is the Wasatch fault.
East of Sierra Nevada. in Nevada and western Utah, there is lots of East-West oriented extension (i.e. the Basin and Range) where the crust in western Nevada is moving west compared to the crust in Salt Lake City, Utah.
The Wasatch is also one of these extensional faults we call Normal faults.
In Salt Lake City, the Wasatch fault is oriented roughly north-south and is generally located on the eastern side of the valley, near the base of the mountains. The Crust on the western side of the fault is moving west relative to the mountains.
The fault then dips down towards the west. Because the motion is east-west, and the fault dips at an angle, the valley goes down over time relative to the mountains (thus forming the valley).
Today’s earthquake happened in the middle of the valley, where the Wasatch fault is deep beneath. The earthquake was a “normal” fault earthquake with east-west extension. So, the earthquake and aftershocks are on a fault related to the Wasatch (or we are wrong about the precise location of the fault, the earthquake, or both).
The USGS has an earthquake forecast product where the scientists at the Earthquake Center use a statistical model to estimate the possibility of earthquakes of different magnitude ranges may occur in the future over ranges of time periods after the main earthquake.
Don’t run outside during an earthquake.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the lower right corner is a map of the western USA with USGS seismicity from the past century for earthquakes M 5.5+. Note all the north-south oriented lines in Nevada and Utah. These are formed by all the normal faults from the east-west extension in the basin and range.
  • In the upper right corner is a map of the Salt Lake City (SLC) area. The Great Salt Lake is the large light blue bleb in the upper right. We can see the mountains to the east of SLC, the Wasatch Range. The Earthquake Intensity uses the MMI scale (the colors), read more about this here. This map represents an estimate of ground shaking from the M 5.7 based on a statistical model using the results of tens of thousands of earthquakes.
  • In the upper left corner is a plot showing how these USGS models “predict” the ground shaking intensity will be relative to distance from the earthquake. These models are represented by the broan and green lines. People can fill out an online form to enter their observations and these “Did You Feel It?” observations are converted into an intensity number and these are plotted as dots in this figure.
  • In the left-center is a map from DuRoss et al. (2016) that shows the Wasatch fault along the base of the Wasatch Range. Note that the fault is subdivided into different segments. We think that sometimes these different segments may rupture at different times and sometimes some of them may rupture at the same time.I placed a blue star in the location of today’s earthquake (projected onto the surface).
  • Here is the map with 1 year’s and 1 century’s seismicity plotted.


    •

  • Two great resources for information about the tectonics of Utah are here:

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the map from DuRoss et al. (2016).
  • The main fault is in red. There are additional faults, like the white lines west of Salt Lake City. These are traces of the West Valley fault zone (WVFZ). Note the next mountain range to the west (left) and that there is another north-south series of faults drawn at the base of those mountains too. This is the Oquirrh Great Salt Lake fault zone, a series of west dipping faults (just like the Wasatch fault)
  • The Wasatch fault is very long and notice how it is not continuous. One of the important things that we may want to know is if these all slip at the same time during an earthquake, or only some of them slip, or just one of them. This is one of the largest sources of uncertainty when it comes to estimating the seismic hazard of a region.
  • The authors (and others before them) subdivided the segments and these segments are labeled on this map.


    • Central segments of the WFZ (red), which have evidence of repeated Holocene surface-faulting earthquakes. Circles indicate sites with data that we reanalyzed using OxCal (abbreviations shown in Table 2); triangles indicate sites where data or documentation was inadequate for reanalysis (HC, Hobble Creek; PP, Pole Patch; WC, Water Canyon; WH, Woodland Hills). Other Quaternary faults in northern Utah (white lines) include the ECFZ, East Cache fault zone; OGSLFZ, Oquirrh Great Salt Lake fault zone; ULFF, Utah Lake faults and folds; WVFZ, West Valley fault zone. Fault traces are from Black et al. [2003]. Horizontal bars mark primary segment boundaries. Inset map shows the trace of the WFZ in northern Utah and southern Idaho.

  • This is a figure that shows what we think may be the way that these fault segments link (or not) through time (DuRoss et la., 2016).
  • The fault line map is on top (note how North is not always “up.”). The bottom chart aligns with the fault segments (along the north south distance represented by the red dashed and dotted line in the map).
  • The vertical axis is time in thousands of years ago (1950 is on the bottom and 7 thousand years ago is on top). Each blue bar represents the time that an earthquake may haven happened in the past and how those earthquakes may match the earthquake history of an adjacent segment.
  • If the blue bar on one segment matches the age range for an adjacent fault, that earthquake may have involved both segments. However due to the limitation with radiocarbon, we can never really know this.


    • Late Holocene surface-faulting earthquakes identified at trench sites along the central WFZ. Circles with labels indicate sites with data that were reanalyzed using OxCal, and unlabeled white triangles indicate sites where data or documentation was inadequate for reanalysis. Distance is measured along simplified fault trace (dash dotted line) shown in top panel. Individual earthquake-timing probability density functions (PDFs) and mean times are derived from OxCal models for the paleoseismic sites; number in brackets is event number, where one is the youngest.

  • Here is a cross section, showing us what we think may be how the faults extend beneath the ground surface. Drt, DuRoss tweeted this today.
  • The Wasatch fault begins on the right, at the base of the Wasatch Mountains and dips to the west (to the left) beneath Salt Lake City.
  • There are additional (antithetic) faults dipping to the east and these are called the West Valley fault zone. They are also normal faults formed from extension.
  • These faults are plotted in white in the above map.
  • The earthquake location is also plotted using two different information sources. According to Dr. RuRoss, these earthquakes may have happened on a previously unknown fault.


    •

Earthquake Triggered Landslides

    There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

    When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.


    Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
    Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
    An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
    Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


    Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.

  • Here is a map that I put together using the data available from the USGS Earthquake Event pages. More about these models can be found here.
  • The map shows liquefaction susceptibility from the M 5.7 earthquake.
  • These models use empirical relations (earthquake data) between earthquake size, earthquake distance, and material properties of the Earth.
  • The largest assumption is that for the Earth materials. This model uses a global model for the seismic velocity in the upper 30 meters (i.e. the Vs30). This global model basically takes the topographic slope of the ground surface and converts that to Vs30. So, the model is basically based on a slope map. This is imperfect, but works moderately well at a global scale. A model based on real Earth material data would be much much better.


    •

    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
  • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
  • Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

    • Specific References

  • DuRoss, C. B., S. F. Personius, A. J. Crone, S. S. Olig, M. D. Hylland, W. R. Lund, and D. P. Schwartz (2016), Fault segmentation: New concepts from the Wasatch Fault Zone, Utah, USA, J. Geophys. Res. Solid Earth, 121, 1131–1157, doi:10.1002/2015JB012519.

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Earthquake Report: Mendocino triple junction

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Well, I was on the road for 1.5 days (work party for the Community Village at the Oregon Country Fair). As I was driving home, there was a magnitude M 5.6 earthquake in coastal northern California.
https://earthquake.usgs.gov/earthquakes/eventpage/nc73201181/executive
I didn’t realize this until I was almost home (finally hit the sack around 4 am).
This earthquake follows a sequence of quakes further to the northwest, however their timing is merely a coincidence. Let me repeat this. The M 5.6 earthquake is not related to the sequence of earthquakes along the Blanco fracture zone.
Contrary to what people have posted on social media, there was but a single earthquake. This earthquake happened beneath the area of Petrolia, nearby the 1991 Honeydew Earthquake. More about the Honeydew Earthquake can be found here.
This region also had a good sized shaker in 1992, the Cape Mendocino Earthquake, which led to the development of the National Tsunami Hazard Mitigation Program. More about the Cape Mendocino Earthquake can be found on the 25th anniversary page here and in my earthquake report here.
The regional tectonics in coastal northern California are dominated by the Pacific-North America plate boundary. North of Cape Mendocino, this plate boundary is convergent and forms the Cascadia subduction zone (CSZ). To the south of Cape Mendocino, the plate boundary is the right-lateral (dextral) San Andreas fault (SAF). Where these 2 fault systems meet, there is another plate boundary system, the right-lateral strike-slip Mendocino fault (don’t write Mendocino fracture zone on your maps!). Where these 3 systems meet is called the Mendocino triple junction (MTJ).
The MTJ is a complicated region as these plate boundaries overlap in ways that we still do not fully understand. Geologic mapping in the mid- to late-20th century provides some basic understanding of the long term history. However, recent discoveries have proven that this early work needs to be revisited as there are many unanswered questions (and some of this early work has been demonstrated to be incorrect). Long live science!
Last night’s M 5.6 temblor happened where one strand of the MF trends onshore (another strand bends towards the south). But, it also is where the SAF trends onshore. At this point, I am associating this earthquake with the MF (so, a right-lateral strike-slip earthquake). The mechanism suggest that this is not a SAF related earthquake. However, it is oriented in a way that it could be in the Gorda plate (making it a left-lateral strike-slip earthquake). However, this quake is at the southern edge of the Gorda plate (sedge), so it is unlikely this is a Gorda plate event.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 5.0 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), 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.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly ~north-south trends of these red and blue stripes in the Pacific plate. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the North America plate, so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Juan de Fuca and Gorda plates.

    Global Strain

  • In a map below, I include a transparent overlay of the Global Strain Rate Map (Kreemer et al., 2014).
  • The mission of the Global Strain Rate Map (GSRM) project is to determine a globally self-consistent strain rate and velocity field model, consistent with geodetic and geologic field observations. The overall mission also includes:
    1. contributions of global, regional, and local models by individual researchers
    2. archive existing data sets of geologic, geodetic, and seismic information that can contribute toward a greater understanding of strain phenomena
    3. archive existing methods for modeling strain rates and strain transients
  • The completed global strain rate map will provide a large amount of information that is vital for our understanding of continental dynamics and for the quantification of seismic hazards.
  • The version used in the poster(s) below is an update to the original 2004 map (Kreemer et al., 2000, 2003; Holt et al., 2005).

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • n 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 lower right corner is a map that shows a comparison between the USGS Did You Feel It? reports and the USGS Modified Mercalli Intensity shakemap model. This comparison shows that the model is a decent fit for the reports from real people. If you felt the earthquake, please submit a report to the USGS here.
  • In the upper right corner I include a larger scale view of seismicity for this area. I highlight the important historic events (e.g. the 1991 Honeydew Earthquake and the 1992 Cape Mendocino Earthquake sequence.
  • Here is the map with a month’s seismicity plotted.


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  • Here is the map with a century’s seismicity plotted.


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  • Here is the map with a century’s seismicity plotted along with the Global Strain Map with a 30% transparency.


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  • Here is the educational interpretive poster from the 1992 Cape Mendocino Earthquake (report here).


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  • The USGS has been increasing the list of products that are produced in association with their earthquake pages. One of these products is an earthquake forecast (not a prediction as nobody can predict earthquakes yet) that lists the chance of an earthquake with a given magnitude over a certain period of time. The forecast for the M 5.6 earthquake is found here. These forecasts are updated periodically, so the information will change with time. Below is a table where I present the forecast as it was when I checked the page this morning (would be nice if the USGS would produce an easy to read table).

    • From the USGS:

    Be ready for more earthquakes

    • More earthquakes than usual (called aftershocks) will continue to occur near the mainshock.
    • When there are more earthquakes, the chance of a large earthquake is greater which means that the chance of damage is greater.
    • The USGS advises everyone to be aware of the possibility of aftershocks, especially when in or around vulnerable structures such as unreinforced masonry buildings.
    • This earthquake could be part of a sequence. An earthquake sequence may have larger and potentially damaging earthquakes in the future, so remember to: Drop, Cover, and Hold on.

    What we think will happen next

    • According to our forecast, over the next 1 Week there is a < 1 % chance of one or more aftershocks that are larger than magnitude 5.6. It is likely that there will be smaller earthquakes over the next 1 Week, with 0 to 11 magnitude 3 or higher aftershocks. Magnitude 3 and above are large enough to be felt near the epicenter. The number of aftershocks will drop off over time, but a large aftershock can increase the numbers again, temporarily.

    About our earthquake forecasts

    • No one can predict the exact time or place of any earthquake, including aftershocks. Our earthquake forecasts give us an understanding of the chances of having more earthquakes within a given time period in the affected area. We calculate this earthquake forecast using a statistical analysis based on past earthquakes.
    • Our forecast changes as time passes due to decline in the frequency of aftershocks, larger aftershocks that may trigger further earthquakes, and changes in forecast modeling based on the data collected for this earthquake sequence.

  • Gosh, almost forgot to include this photo of the seismic waves recorded on the Humboldt State University Department of Geology Baby Benioff seismometer. Photo Credit: Amanda Admire.


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USGS Landslide and Liquefaction Ground Failure data products

  • Below I present a series of maps that are intended to address the excellent ‘new’ products included in the USGS earthquake pages: landslide probability and liquefaction susceptibility (a.k.a. the Ground Failure data products).
  • First I present the landslide probability model. This is a GIS data product that relates a variety of factors to the probability (the chance of) landslides as triggered by this earthquake. There are a number of assumptions that are made in order to be able to produce this model across such a large region, though this is still of great value (like other aspects from teh USGS, e.g. the PAGER alert). Learn more about all of these Ground Failure products here.
  • There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). I spend more time discussing landslides and liquefaction in this recent earthquake report.
  • This model, like all landslide computer models, uses similar inputs. I review these here:
    1. Some information about ground shaking. Often, people use Peak Ground Acceleration, though in the past decade+, it has been recognized that the parameter “Arias Intensity” is a better measure of the energy imparted by the earthquake across the land and seascape. Instead of simply accounting for the peak accelerations, AI integrates the entire energy (duration) during the earthquake. That being said, PGA is a more common parameter that is available for people to use. For example, when I was modeling slope stability for the 2004 Sumatra-Andaman subduction zone earthquake, the only model that was calibrated to observational data were in units of PGA. The first order control to shaking intensity (energy observed at any particular location) is distance to the earthquake fault that slipped.
    2. Some information about the strength of the materials (e.g. angle of internal friction (the strength) and cohesion (the resistance).
    3. Information about the slope. Steeper slopes, with all other things being equal, are more likely to fail than are shallower slopes. Think about skiing. Beginners (like me) often choose shallower slopes to ski because they will go down the slope slower, while experts choose steeper slopes.
  • Areas that are red are more likely to experience landslides than areas that are colored blue. I include a coarse resolution topographic/bathymetric dataset to help us identify where the mountains are relative to the coastal plain and continental shelf (submarine).


  • Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
  • Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
  • An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
  • The liquefaction susceptibility map for the M 5.6 earthquake did not suggest that there would be possibly much liquefaction from this earthquake (probably due to the small magnitude). I discuss liquefaction more in my earthquake report on the 28 September 20018 Sulawesi, Indonesia earthquake, landslide, and tsunami here.
  • Here is a map that shows shaking intensity using the MMI scale (mentioned and plotted in the main earthquake poster maps). I present this here in the same format as the ground failure model maps so we can compare these other maps with the ground shaking model (which is a first order control on slope failure).


Other Report Pages

Some Relevant Discussion and Figures

  • 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.


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  • 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 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.

Geologic Fundamentals

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

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


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  • Here is another way to look at these beach balls.

  • 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. The following three animations are from IRIS.

    • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.


    • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.


    • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    Social Media

    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.
  • 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.
  • Dengler, L.A., and McPherson, R.C., 1993. The 17 August 1991 Honeydew Earthquake, North Coast California: A Case for Revising the Modified Mercalli Scale in Sparsely Populated Areas in BSSA, v. 83, no. 4, pp. 1081-1094
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • 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
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • 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.

Return to the Earthquake Reports page.


Earthquake Report: Gorda plate!

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I was at a workshop to develop a unified strategy for research and monitoring in the Klamath River estuary (led by the Yurok Tribe, Andreas Krauss) yesterday and missed feeling the first of two M 4.6-4.7 earthquakes. I was presenting the results from our tectonic geodetic studies as they moderate Sea Level Rise at the mouth of the Klamath River (and control sedimentation there). However, I was home in Manila, CA when the second earthquake hit. I felt a sharp and short motion (1-2 seconds max in duration). But I was exhausted still from the death of my cat Chicken. So, I needed to wait until today to put this report together.
The Cascadia subduction zone (CSZ) is a convergent plate boundary fault formed by the interaction between the downgoing oceanic Gorda plate (GP) beneath the continental North America plate (NAP). More about the CSZ can be found here.
The first M 4.6 was a thrust earthquake within the Gorda plate. The CSZ megathrust fault slab depth is about 15 km at the location of these 2 earthquakes. The M 4.7 earthquake is a strike-slip earthquake. Because of the predominant northeast striking left-lateral faults in the GP, I interpret this earthquake to be left-lateral.
My initial thought was that these two EQs could be related. So, I rummaged the literature to find papers that present research of static coulomb stress changes between earthquakes and faults similar to what we had yesterday. When an earthquake fault slips during an earthquake, the crust deforms elastically. This causes some regions to extend and other regions to compress. These extension/compression changes cause static stresses on faults to change. As the seismic waves travel through the crust, this can cause dynamic changes in stress along faults. Both of these types of stress change (static and dynamic) are very small. If there is a fault that is oriented correctly, has a high stress state (almost ready to slip during an earthquake), and has a sufficiently large enough stress change, the first earthquake may trigger a second earthquake.
Because the second earthquake happened long after the main seismic waves had stopped traveling through the region, the M 4.7 earthquake could not have been dynamically triggered by the earlier M 4.6 earthquake. However, based on my review of the literature, it appears that the M 4.6 may have triggered the M 4.7 earthquake.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5 (and down to M ≥ 4.5 in a second poster).
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 4.6 & 4.7 earthquakes. I include generic fault plane solutions for the other fault systems in the region.
I include posters that show either M 4.6 or M 4.7 MMI and Did You Feel It data. I also have version that include emag2 magnetic anomaly data. These mag anomaly data nicely show the structure of the oceanic crust formed at the Gorda spreading center (the anomalies are initially parallel to the spreading center; that these anomalies are parallel to the spreading center was some key evidence for the plate tectonic hypothesis prior to it being accepted as a theory).

  • 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 upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004). I placed a blue stars in the general location of today’s earthquakes.
  • In the upper left corner is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. This figure shows the predominant tectonic fabric in the GP (northeast striking left-lateral faults). More about this figure can be found below.
  • Below the Chaytor figure 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. Today’s earthquakes happened in the lower Gorda plate
  • In the lower left corner is a figure from Rollins and Stein (2010). In their paper they discuss how static coulomb stress changes from earthquakes may impart (or remove) stress from adjacent crust/faults. I place a blues star in the general location of today’s earthquakes.
  • To the left of the CSZ map is a figure from Yue et al. (2008) that shows the results of their static coulomb modeling. They model static changes in stress change from source earthquakes (strike-slip in upper panel and thrust faults in lower panel) imparted onto receiver faults. The lower left example (c) shows the stress change imparted following a thrust fault source earthquake imparted onto a left-lateral strike-slip receiver fault. If we rotate this panel counterclockwise (about 25°)to match the orientation of the M 4.6 earthquake, we may observe that the M 4.7 earthquake resides in the quadrant that saw an increase in static coulomb stress (colored red).
  • In the lower right corner is another example of static coulomb stress modeling from Lin et al. (2011). The left panel shows what static stress changes may be imparted from a source thrust fault onto left lateral strike-slip faults (“tear” faults are strike-slip faults that connect thrust faults). These left panels also show an increase in static coulomb stress in the lower left quadrant. I take these two examples as supporting evidence for my hypothesis that the M 4.6 earthquake triggered the M 4.7 earthquake.

M 4.6 MMI/DYFI


M 4.6 MMI/DYFI emag2


M 4.7 MMI/DYFI


M 4.7 MMI/DYFI emag2


USGS Earthquake Pages

  • Here is the Baby Benioff Seismograph from Humboldt State Univ. Dept. of Geology. See social media below.


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UPDATE: 2018.03.23 22:10 local time

  • I later noticed that there was a M 4.5 earlier on 2018.03.09 south of these two M 4.6 and 4.7 earthquakes. I here prepare an overlay analysis of the seismicity with the Yue et al. (2008) results compared to these 3 earthquakes in a sequence. The below figure has two panels representing the hypothetical static coulomb stress changes between these three earthquakes. Earthquake order number is labeled in cyan.
  • I orient the Yue et al. (2008) figures relative to the primary nodal plane strike preferred USGS interpretation. In other words, I use the orientation of the USGS preferred fault plane solution to orient the Yue et al. (2008) coulomb stress change figures.
  • These overlays are scaled relative to the published scale.
  • Note how the largest magnitude (M 4.7) earthquake is in a region of increased static coulomb stress from both the prior earthquakes. These stress changes are very small and the magnitudes are probably not scaled appropriately for the space, so this is possibly a conjectural interpretation.
    1. The left panel shows what stress changes might happen on left-lateral strike-slip receiver faults given a left-lateral strike-slip source fault. The M 4.7 EQ is in the region of increased static coulomb stress.
    2. The right panel shows what stress changes might happen on left-lateral striek-slip receiver faults given a thrust fault source earthquake. The M 4.7 EQ is AGAIN in the region of increased static coulomb stress.

Some Relevant Discussion and Figures

  • 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.


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  • 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).


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  • 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.

  • Here is the Yue et al. (2008) figure, along with their figure caption below.


    • Coulomb stress change for different combination of faults. The thick while line marks the source fault, and the white arrows indicate the focal mechanism. The black line and the black arrows represent the orientation of the receiving fault and its mechanism, respectively.

  • Here is the figure from Lin et al. (2011), along with their figure caption.


    • Maps showing Coulomb stress changes caused by an M = 7.0 earthquake on adjacent tear faults. The source is the same as in Figure 4. Coulomb stresses are calculated on (a–c) left‐lateral and (e–g) right‐lateral tear faults. Stress is sampled at depth of 1 km (Figures 6a and 6e), 10 km (Figures 6b and 6f), and 19.5 km (Figures 6c and 6g). (d) Cross section at the right end of the source earthquake (cross section position shown in Figure 6a). Note that left‐lateral tear faulting is favored in one position with respect to the thrust, while right‐lateral faulting is favored in the opposite position.

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Geologic Fundamentals

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

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


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  • 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. 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.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • Lin, J., R. S. Stein, M. Meghraoui, S. Toda, A. Ayadi, C. Dorbath, and S. Belabbes (2011), Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 Mw = 6.9 Zemmouri, Algeria, earthquake, J. Geophys. Res., 116, B03305, doi:10.1029/2010JB007654.
  • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Yue, H., Zhang, Z., Chen, Y.J., 2008. Interaction between adjacent left-lateral strike-slip faults and thrust faults: the 1976 Songpan earthquake sequence in Chinese Science Bulletin, v. 53, no. 16, p. 2520-2526
  • 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: 1992.04.25 M 7.1 Petrolia

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The 25 April 1992 M 7.1 earthquake was a wake up call for many, like all large magnitude earthquakes are.
I have some updated posters as of April 2021 (see below).
Here is my personal story.
I was driving my girlfriend’s car (Jen Guevara) with her and some housemates up to attend a festival at Redwood Park in Arcata. She lived in the old blue house at the base of the bridge abutment on the southwest side of HWY 101 as it crosses Mad River. The house burned down a couple of years ago, but these memories remain. We were driving along St. Louis and about to turn east to cross the 101 towards LK Wood. The car moved left and right. I pulled over as I thought we might have just gotten a flat tire. I got out, inspected the wheels, and there was no flat. We returned to our journey. When we arrived at the park, everyone was talking about how the redwood trees were flopping around like wet spaghetti during the earthquake. I then looked back in my memory and realized that, at the lumber mill that I had parked by when I got the imaginary flat tire, there were tall stacks of milled lumber flopping around. I had dismissed it that they were blowing in the wind. Silly me.
Later that night, I was at a reggae concert at the Old Creamery Building in Arcata. At some point, the lights flickered off and on. I figured that someone had accidentally brushed up against the light switch on the wall. BUT, this was the first of two large aftershocks.
Even later that night, actually the following morning, I was laying in bed with Jen. The house typically shook when large semi trucks crossed the 101 bridge. However, this time, the shaking had a much longer duration. This was the second of the two major aftershocks. I finally recognized this earthquake as an earthquake and not something else. To my credit, I was dancing during the first major aftershock.

    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.


Below is my updated interpretive poster for this earthquake.

    I include some inset figures in the poster.

  • In the upper left corner is a small scale map that shows the tectonic plates and their boundaries, along with USGS NEIC seismicity from 1921-2021 for earthquakes M > 6.5
  • In the upper right corner is a series of 4 panels highlighting earthquake intensity for the 3 main events in this sequence.
    • The 3 panels on the right show the Modified Mercalli Intensity (MMI) scale shaking intensity for the 7.2, 6.6, and 6.5 earthquakes. These are based on computer models that correlate MMI with distance from the earthquake.
    • The panel on the left is a larger scale map that shows the MMI contours for the 7.2 earthquake. I also plot the USGS “Did You Feel It?” results. These are data compiled from observations people made and reported to the USGS. Read more about the DYFI program here.
    • The plot in the bottom center-left shows both of these data: the USGS modeled intensities and the USGS DYFI intensities. Note how the intensity gets lesser with distance from the earthquake.
  • The lower right corner has two maps which are “Ground Failure” products from the USGS for this earthquake. Read more about the USGS ground failure products here. I explain these two products in greater detail below.
    • The map on the right shows the probability of (chance of) landslides to have been triggered by the M 7.2 earthquake shaking.
    • The map on the left shows the susceptibility for (chance for) liquefaction to have been generated by the M 7.2 earthquake shaking.
  • Above the Ground Failure maps is a larger scale view of the aftershocks from the Cape Mendocino Earthquake. Technically, many of these events are not aftershocks, but earthquakes on different faults than the M 7.2 source fault. These are earthquakes triggered by the stress changes following the 7.2 earthquake. The two second largest events (M 6.6 and 6.5) are triggered events on NW striking faults in the Gorda plate. More on these events in an upcoming paper. Stay tuned.


Shaking Intensity and Potential for Ground Failure

  • Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.

    • There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

    When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.


    Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
    Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
    An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
    Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


    Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.

  • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a high probability for landslides. This makes sense as the lower limit for earthquake triggered landslides is magnitude M 5.5 (from Keefer 1984).
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.


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The Cascadia subduction zone

  • 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.

1992 Cape Mendocino Earthquake and Tsunami

  • 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.


    •

Below is an updated interpretive poster for this earthquake sequence that focuses on the mechanisms.

  • I include the plot of tide gage data from Oppenheimer et al. (1993). These data are not in digital format, but are preserved in paper format. I hope to get some funding to head back to the east coast and scan the paper records to digitize them.
  • Note that the color format for the seismicity is based on depth. There are lots of events in the North America plate (see the green dots on the eastern end of the plot. There are also lots of Gorda plate and Mendocino fault events (the deeper events on the western side of the plot).
  • Look at the different types of earthquakes. What can you conclude about this sequence?


    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:

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.
  • 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.

Earthquake Report: 1700 Cascadia subduction zone 317 year commemoration

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Today (possibly tonight at about 9 PM) is the birthday of the last known Cascadia subduction zone (CSZ) earthquake. There is some evidence that there have been more recent CSZ earthquakes (e.g. late 19th century in southern OR / northern CA), but they were not near full margin ruptures (where the entire fault, or most of it, slipped during the earthquake).
I have been posting material about the CSZ for the past couple of years here and below are some prior Anniversary posts, as well as Earthquake Reports sorted according to their region along the CSZ. Below I present some of the material included in those prior reports (to help bring it all together), but I have prepared a new map for today’s report as well.

On this evening, 317 years ago, the Cascadia subduction zone fault ruptured as a margin wide earthquake. I here commemorate this birthday with some figures that are in two USGS open source professional papers. The Atwater et al. (2005) paper discusses how we came to the conclusion that this last full margin earthquake happened on January 26, 1700 at about 9 PM (there may have been other large magnitude earthquakes in Cascadia in the 19th century). The Goldfinger et al. (2012) paper discusses how we have concluded that the records from terrestrial paleoseismology are correlable and how we think that the margin may have ruptured in the past (rupture patch sizes and timing). The reference list is extensive and this is but a tiny snapshot of what we have learned about Cascadia subduction zone earthquakes. Brian Atwater and his colleagues have updated the Orphan Tsunami and produced a second edition available here for download and here for hard copy purchase (I have a hard copy).

Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct northeastwardly 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).


Today I prepared this new map showing the results of shakemap scenario model prepared by the USGS. I prepared this map using data that can be downloaded from the USGS website here. Shakemaps show what we think might happen during an earthquake, specifically showing how strongly the ground might shake. There are different measures of this, which include Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV), and Modified Mercalli Intensity (MMI). More background information about the shakemap program at the USGS can be found here. One thing that all of these measures share is that they show that there is a diminishing of ground shaking with distance from the earthquake. This means that the further from the earthquake, the less strongly the shaking will be felt. This can be seen on the maps below. The USGS prepares shakemaps for all earthquakes with sufficiently large magnitudes (i.e. we don’t need shakemaps for earthquakes of magnitude M = 1.5). An archive of these USGS shakemaps can be found here. All the scenario USGS shakemaps can be found here.
I chose to use the MMI representation of ground shaking because it is most easily comparable for people to understand. This is because MMI scale is designed based upon relations between ground shaking intensity and observations that people are able to make (e.g. how strongly they felt the earthquake, how much objects in their residences or places of business responded, how much buildings were damaged, etc.).
The MMI ground motion model 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. More on the MMI scale can be found here and here.


Here is the USGS version of this map. The outline of the fault that was used to generate the ground motions that these maps are based upon is outlined in black.


I prepared an end of the year summary for earthquakes along the CSZ. Below is my map from this Earthquake Report.

  • Here is the map where I show the epicenters as circles with colors designating the age. I also plot the USGS moment tensors for each earthquake, with arrows showing the sense of motion for each earthquake.
  • I placed a moment tensor / focal mechanism legend in the lower left 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.
  • In some cases, I am able to interpret the sense of motion for strike-slip earthquakes. In other cases, I do not know enough to be able to make this interpretation (so I plot both solutions).

    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. 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.
  • To the lower right of the Cascadia map and cross section is a map showing the latest version of the Uniform California Earthquake Rupture Forecast (UCERF). Let it be known that this is not really a forecast, and this name was poorly chosen. People cannot forecast earthquakes. However, it is still useful. The faults are colored vs. their likelihood of rupturing. More can be found about UCERF here. Note that the San Andreas fault, and her two sister faults (Maacama and Bartlett Springs), are orange-red.
  • To the upper right of the Cascadia map and cross section is a map showing the shaking intensities based upon the USGS Shakemap model. Earthquake Scenarios describe the expected ground motions and effects of specific hypothetical large earthquakes. The color scale is the same as found on many of my #EarthquakeReport interpretive posters, the Modified Mercalli Intensity Scale (MMI). The latest version of this map is here.
  • In the upper right corner I include generalized fault map of northern California from Wallace (1990).
  • To the left of the Wallace (1990) 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 lower right 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.


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 a version of the CSZ cross section alone (Plafker, 1972).


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.

Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.


Here is a photo of the ghost forest, created from coseismic subsidence during the Jan. 26, 1700 Cascadia subduction zone earthquake. Atwater et al., 2005.


Here is a photo I took in Alaska, where there was a subduction zone earthquake in 1964. These tree snags were living trees prior to the earthquake and remain to remind us of the earthquake hazards along subduction zones.


This shows how a tsunami deposit may be preserved in the sediment stratigraphy following a subduction zone earthquake, like in Cascadia. Atwater et al., 2005. If there is a source of sediment to be transported by a tsunami, it will come along for the ride and possibly be deposited upon the pre-earthquake ground surface. Following the earthquake, tidal sediment is deposited above the tsunami transported sediment. Sometimes plants that were growing prior to the earthquake get entombed within the tsunami deposit.


The NOAA/NWS/Pacific Tsunami Warning Center has updated their animation of the simulation of the 1700 “Orphan Tsunami.”
Source: Nathan C. Becker, Ph.D. nathan.becker at noaa.gov

Below are some links and embedded videos.

  • Here is the yt link for the embedded video below.

  • Here is the mp4 link for the embedded video below. (2160p 145 mb mp4)
  • Here is the mp4 link for the embedded video below. (1080p 145 mb mp4)


    • Here is the text associated with this animation:

    Just before midnight on January 27, 1700 a tsunami struck the coasts of Japan without warning since no one in Japan felt the earthquake that must have caused it. Nearly 300 years later scientists and historians in Japan and the United States solved the mystery of what caused this “orphan tsunami” through careful analysis of historical records in Japan as well as oral histories of Native Americans, sediment deposits, and ghost forests of drowned trees in the Pacific Northwest of North America, a region also known as Cascadia. They learned that this geologically active region, the Cascadia Subduction Zone, not only hosts erupting volcanoes but also produces megathrust earthquakes capable of generating devastating, ocean-crossing tsunamis. By comparing the tree rings of dead trees with those still living they could tell when the last of these great earthquakes struck the region. The trees all died in the winter of 1699-1700 when the coasts of northern California, Oregon, and Washington suddenly dropped 1-2 m (3-6 ft.), flooding them with seawater. That much motion over such a large area requires a very large earthquake to explain it—perhaps as large as 9.2 magnitude, comparable to the Great Alaska Earthquake of 1964. Such an earthquake would have ruptured the earth along the entire length of the 1000 km (600 mi) -long fault of the Cascadia Subduction Zone and severe shaking could have lasted for 5 minutes or longer. Its tsunami would cross the Pacific Ocean and reach Japan in about 9 hours, so the earthquake must have occurred around 9 o’clock at night in Cascadia on January 26, 1700 (05:00 January 27 UTC).

    The Pacific Tsunami Warning Center (PTWC) can create an animation of a historical tsunami like this one using the same too that they use for determining tsunami hazard in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model. The RIFT model takes earthquake information as input and calculates how the waves move through the world’s oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves and as they race around the globe one can also see the distance between successive wave crests (wavelength) as well as their height (half-amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC’s warning criteria, with blue-to-green representing no hazard (less than 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 to 100 cm or ~1 to 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 to 3 m or ~3 to 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (greater than 3 m or ~10 ft.).

    Toward the end of this simulated 24-hours of activity the wave animation will transition to the “energy map” of a mathematical surface representing the maximum rise in sea-level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional “beam” such that the tsunami was far more severe in the middle of the “beam” of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the “beam” have a much higher impact than those to either side of it.

    Offshore, Goldfinger and others (from the 1960’s into the 21st Century, see references in Goldfinger et al., 2012) collected cores in the deep sea. These cores contain submarine landslide deposits (called turbidites). These turbidites are thought to have been deposited as a result of strong ground shaking from large magnitude earthquakes. Goldfinger et al. (2012) compile their research in the USGS professional paper. This map shows where the cores are located.


    Here is an example of how these “seismoturbidites” have been correlated. The correlations are the basis for the interpretation that these submarine landslides were triggered by Cascadia subduction zone earthquakes. This correlation figure demonstrates how well these turbidites have been correlated. Goldfinger et al., 2012.


    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.


    Here is an update of this plot given new correlations from recent work (Goldfinger et al., 2016).


    Here is a plot showing the earthquakes in a linear timescale.


    I combined the plot above into another figure that includes all the recurrence intervals and segment lengths in a single figure. This is modified from Goldfinger et al. (2012).

    http://earthquake.usgs.gov/earthquakes/shakemap/global/shake/casc9.0_expanded_peak_se/

Earthquake Report: 2016 Summary Cascadia

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Here I summarize the seismicity for Cascadia in 2016. I limit this summary to earthquakes with magnitude greater than or equal to M 4.0. I reported on all but five of these earthquakes. I put this together a couple weeks ago, but wanted to wait to post until the new year (just in case that there was another earthquake to include).
I prepared a 2016 annual summary for Earth here.

    I include summaries of my earthquake reports in sorted into three categories. One may also search for earthquakes that may not have made it into these summary pages (use the search tool).

  • Magnitude
  • Region
  • Year

Earthquake Summary Poster (2016)

  • Here is the map where I show the epicenters as circles with colors designating the age. I also plot the USGS moment tensors for each earthquake, with arrows showing the sense of motion for each earthquake.
  • I placed a moment tensor / focal mechanism legend in the lower left 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.
  • In some cases, I am able to interpret the sense of motion for strike-slip earthquakes. In other cases, I do not know enough to be able to make this interpretation (so I plot both solutions).

    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. 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.
  • To the lower right of the Cascadia map and cross section is a map showing the latest version of the Uniform California Earthquake Rupture Forecast (UCERF). Let it be known that this is not really a forecast, and this name was poorly chosen. People cannot forecast earthquakes. However, it is still useful. The faults are colored vs. their likelihood of rupturing. More can be found about UCERF here. Note that the San Andreas fault, and her two sister faults (Maacama and Bartlett Springs), are orange-red.
  • To the upper right of the Cascadia map and cross section is a map showing the shaking intensities based upon the USGS Shakemap model. Earthquake Scenarios describe the expected ground motions and effects of specific hypothetical large earthquakes. The color scale is the same as found on many of my #EarthquakeReport interpretive posters, the Modified Mercalli Intensity Scale (MMI). The latest version of this map is here.
  • In the upper right corner I include generalized fault map of northern California from Wallace (1990).
  • To the left of the Wallace (1990) 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 lower right 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.


    Cascadia subduction zone: General Overview

  • Cascadia’s 315th Anniversary 2015.01.26
  • Cascadia’s 316th Anniversary 2016.01.26
  • Earthquake Information about the CSZ 2015.10.08

The big player this year was an M 6.5 along the Mendocino fault on 2016.12.08. Here I present an inventory of 8 earthquakes with M ≥ 5.0. There are a few additional earthquakes with smaller magnitudes that are of particular interest.

Please visit the #EarthquakeReport pages for more information about the figures that I include in the Earthquake Report interpretive posters below.

    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.
  • 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.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

Earthquake Report: Mendocino fault Update #1

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Today is one of my busiest days of the semester. I am administering a final and my two classes are presenting their video projects. Then, we had this M 6.5 earthquake in the wee hours and a M 7.7 (prelim USGS Mw) in the Solomons (I will post about this later).
Here is an update. For more background on the regional tectonics and the initial Earthquake Report, head here.

  • Here are the USGS websites for the main shock and larger aftershocks
  • 2016.12.08 14:47 UTC M 6.5
  • 2016.12.08 16:24 UTC M 2.4
  • 2016.12.08 16:32 UTC M 4.7
  • 2016.12.08 18:08 UTC M 2.9
  • and an earthquake that I will mention below
  • 2016.12.08 18:33 UTC M 4.3

There was an earthquake to the east on 2016.12.05. This M 4.3 seems to have occurred on a segment of the Mendocino fault. Here is my Earthquake Report for that earthquake. The M 4.3 was not a foreshock to the M 6.5 and probably did not affect the fault in that region.
UPDATE: 13:15 PST: figure notes:

  • I have prepared an updated map. I include some inset figures. I will update this report shortly as I need to get to my class to admin a final.
  • I include the USGS “Did You Feel It?” report maps for these two earthquakes on the right side of the map. I also include screen shots of the attenuation plots (these show how the ground motions diminish with distance from the earthquake).
  • In the upper left corner I include the first figure from Rollins and Stein (2010). In their paper they evaluate (as stated in the title), “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.” In this figure they present the major earthquakes in this region from 1976-2010. We have had several large earthquakes since this paper was published, but they behaved very similarly to the ones discussed in this paper. Note the location of teh 1994 Mendocino fault earthquake. Today’s M 6.5 happened in a region very close to the 1994 earthquake (albeit further offshore than the ’94 quake).
  • In the lower left corner is a figure where Rollins and Stein (2010) show their modeling results for the 1994 earthquake. The colors represent changes in static stress imparted upon faults/crust from rupture on the Mendocino fault in 1994. The 1994 earthquake was significantly larger than today’s M 6.5 (e.g. an M 7.0 is 32 releases ~32 times more energy than an M 6.0), so the static stress changes from today’s earthquakes could be assumed to be less than from the ’94 earthquake. Red designates faults/crust with an increased stress and blue designates a decreased stress. The left panel shows the model results when they generate bilateral rupture (the earthquake starts in one location and slips on either side of the fault) and the right panel shows the model results from unilateral rupture (the fault nucleates at one spot, but instead, slips to only one side of the fault). Note that the increased stresses extend about 1/4 to 1/2 a degree of longitude. This equates to about 15 to 30 miles or 28 to 56 km. The CSZ and SAF are at a much further distance than this, so would not “feel” this earthquake. However, there could be dynamic changes in stress (dynamic triggering), but they would be small and would not last very long (basically the time that the seismic waves are traveling through an area). SO, the fact that we have not had any earthquakes in these other regions suggests that the threat of dynamic triggering are over.

Below I plot the seismicity from the past week, with color representing depth and diameter representing magnitude (see legend). I use the USGS Quaternary fault and fold database for the faults.
I also include the shaking intensity contours on the map for the two earthquakes. Note how these MMI contours are quite different between the two earthquakes.
These contours are based upon 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.
I also include a figure from Rollins and Stein (2010). This figure shows how the 1994 earthquake (similar to today’s M 6.5) imparted stresses on the adjacent faults and crust.

  • UPDATE: I added a version of the Rollins and Stein (2010) figure of the regional earthquakes. I include their figure caption as blockquote below.


    • 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.

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


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

UPDATE 13:15 PST: Notes about the seismicity from the past month or so.
Here is something that I thought was interesting about this series of earthquakes in the past week or so. There were a couple earthquakes along the Blanco fracture zone that are too distant to impart changes in static stress on the CSZ, MF, or SAF (though people always want to seek relations where there are none). There was also a M 4.3 earthquake at the mouth of the Mattole River, near Petrolia. This is also a very very small earthquake that does not implicate other fault zones. However, following the M 6.5 today, there was an aftershock (at least 2) near the M 4.3. SO, I thought that perhaps these happened in a region that had seen an increase in static coulomb stress following the M 4.3. They are very close to the M 4.3. So, it seems reasonable that they were ready to go when the M 6.5 happened.

    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.
  • 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.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

Earthquake Report: Mendocino fault!

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I was awake and just logging into my laptop, still in bed, when I first felt some movement. The movement was slight and not impulsive, so I thought it was a small earthquake. Then the shaking reappeared. This is when I started counting. one one-thousand, two one-thousand…. twenty one-thousand. The S-Wave lasted about 20 seconds. I thought back to the 2010 earthquake that lasted about that long and it was a M 6.5 earthquake. SO, I immediately thought this was probably a mid M 6 earthquake. However, the shaking was subdued. So, it could be a larger earthquake further away. I logged into social media and people were already contacting me. A friend felt it shake for 2 minutes in southern Oregon (so I thought it might be a large earthquake on the Blanco fracture zone, especially since there were a couple up there recently).
UPDATE: Here is my Earthquake Report Update #1
I checked the USGS website here and saw that it was closer to me (Manila, CA), along the Mendocino fault. At first it was a M 6.8, but the location and magnitude changed to an M 6.5.
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. More on earthquakes in this region can be found in Earthquake Reports listed at the bottom of this page above the appendices.
The San Andreas fault is a right-lateral strike-slip transform plate boundary between the Pacific and North America plates. The plate boundary is composed of faults that are parallel to sub-parallel to the SAF and extend from the west coast of CA to the Wasatch fault (WF) system in central Utah (the WF runs through Salt Lake City and is expressed by the mountain range on the east side of the basin that Salt Lake City is built within).
The three main faults in the region north of San Francisco are the SAF, the MF, and the Bartlett Springs fault (BSF). I also place a graphical depiction of the USGS moment tensor for this 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.
The Cascadia subduction zone is a convergent plate boundary where the Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. The Juan de Fuca and Gorda plates are formed at the Juan de Fuca Ridge and Gorda Rise spreading centers respectively. More about the CSZ can be found here.
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 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.
This is a preliminary report and I hope to prepare some updates as I collect more information.

    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.
  • To the left of the CSZ map is the USGS Did You Feel It felt report map. This map is based upon reports submitted by real people. Note how the felt reports extend beyond the modeled estimates of MMI shaking as represented by the MMI contours on the map.
  • In the lower left corner is a figure from Dengler et al. (1995) that shows focal mechanisms from earthquakes in this region, along the Mendocino fault. Today’s earthquake is near the 1994 earthquake.
  • To the right of the Dengler et al. (1995) figure, I present a photo I took of the seismograph observed in Van Matre Hall on the Humboldt State University campus. This seismograph is operated by the HSU Department of Geology.
  • 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.


  • 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.


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  • 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.


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    References

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • 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.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].