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.
- 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.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
- 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.
- 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 >>>
Seismicity Profile
What is an earthquake? What causes earthquakes and where do they happen? How are earthquakes recorded and measured? Learn more about 'The Science of Earthquakes' at: https://t.co/JAQv4cc2KC pic.twitter.com/pJ2IfQ76bs
— USGS Earthquakes (@USGS_Quakes) January 4, 2023
- 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).
- 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.
- 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.
- 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).
Aftershock Patterns
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.
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).
Mapped Geology
- 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..
Earlier Report Interpretive Posters
- 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 poster includes seismicity from the past ~5 decades, for temblors M > 3.0. I also include the map and cross section as explained above. On the left is a map that shows the possible shaking intensity from a future CSZ earthquake.
- More about the materials on this poster can be found on this page.
- Hemphill-Haley, E., 1995. Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern coastal Washington in GSA Bulletin, v. 107, p. 367-378.
- Nelson, A.R., Shennan, I., and Long, A.J., 1996. Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America in Journal of Geophysical Research, v. 101, p. 6115-6135.
- Atwater, B.F. and Hemphill-Haley, E., 1997. Recurrence Intervals for Great Earthquakes of the Past 3,500 Years at Northeastern Willapa Bay, Washington in U.S. Geological Survey Professional Paper 1576, Washington D.C., 119 pp.
- 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.
- Here is a link to the embedded video below, showing the week-long seismicity in April 1992.
- 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).
- 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.
Some Relevant Discussion and Figures
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
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.
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.
- 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.
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.
- 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.
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.
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.
- 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.
- 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).
- Next let’s look at the stress changes following the 2005 M 7.2 earthquake.
- 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.
- 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.
Source models for earthquakes S and T, 10 January 2010, M = 6.5, and 4 February 2010, Mw = 5.9.
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].
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.
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.
- Here is a video that Ross Stein prepared as part of their analyses for this earthquake sequence. They include this in their Temblor report on this sequence here.
- 1700.09.26 M 9.0 Cascadia’s 315th Anniversary 2015.01.26
- 1700.09.26 M 9.0 Cascadia’s 316th Anniversary 2016.01.26 updated in 2017 and 2018
- 1992.04.25 M 7.1 Cape Mendocino 25 year remembrance
- 1992.04.25 M 7.1 Cape Mendocino 25 Year Remembrance Event Page
- Earthquake Information about the CSZ 2015.10.08
- 2022.12.20 M 6.4 Gorda plate
- 2020.05.18 M 5.5 Gorda Rise
- 2018.07.24 M 5.6 Gorda plate
- 2018.03.22 M 4.6/4.7 Gorda plate
- 2017.07.28 M 5.1 Gorda plate
- 2016.09.25 M 5.0 Gorda plate
- 2016.09.25 M 5.0 Gorda plate
- 2016.01.30 M 5.0 Gorda plate
- 2015.12.29 M 4.9 Gorda plate
- 2015.11.18 M 3.2 Gorda plate
- 2014.03.13 M 5.2 Gorda Rise
- 2014.03.09 M 6.8 Gorda plate p-1
- 2014.03.23 M 6.8 Gorda plate p-2
- 2010.01.10 M 6.5 Gorda plate
- 2019.08.29 M 6.3 Blanco transform fault
- 2018.08.22 M 6.2 Blanco transform fault
- 2018.07.29 M 5.3 Blanco transform fault
- 2015.06.01 M 5.8 Blanco transform fault p-1
- 2015.06.01 M 5.8 Blanco transform fault p-2 (animations)
- 2021.12.20 M 5.7 & 6.2 Mendocino fault
- 2020.03.09 M 5.8 Mendocino fault
- 2018.01.25 M 5.8 Mendocino fault
- 2017.09.22 M 5.7 Mendocino fault
- 2016.12.08 M 6.5 Mendocino fault, CA
- 2016.12.08 M 6.5 Mendocino fault, CA Update #1
- 2016.12.05 M 4.3 Petrolia CA
- 2016.10.27 M 4.1 Mendocino fault
- 2016.09.03 M 5.6 Mendocino
- 2016.01.02 M 4.5 Mendocino fault
- 2015.11.01 M 4.3 Mendocino fault
- 2015.01.28 M 5.7 Mendocino fault
- 2015.11.01 M 4.3 Mendocino fault
- 2015.01.28 M 5.7 Mendocino fault
- 2020.03.18 M 5.2 Petrolia
- 2019.06.23 M 5.6 Petrolia
- 2017.03.06 M 4.0 Cape Mendocino
- 2016.11.02 M 3.6 Oregon
- 2016.01.07 M 4.2 NAP(?)
- 2015.10.29 M 3.4 Bayside
- 2018.10.22 M 6.8 Explorer plate
- 2017.01.07 M 5.7 Explorer plate
- 2016.03.19 M 5.2 Explorer plate
- 2017.06.11 M 3.5 Gorda or NAP?
- 2016.07.21 M 4.7 Gorda or NAP? p-1
- 2016.07.21 M 4.7 Gorda or NAP? p-2
Cascadia subduction zone
General Overview
Earthquake Reports
Gorda plate
Blanco transform fault
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
Uncertain
Social Media
#EarthquakeReport for M 6.4 #Earthquake in Mendocino triple junction (Triangle of Doom) region
early to tell (if we learned from last year) left or right lateral strike-slip prob in Gorda plate
read more from last year's reporthttps://t.co/aS9ySr9YIPhttps://t.co/9HKHnpuwE9 pic.twitter.com/YZeimi6AC9
— Jason "Jay" R. Patton (@patton_cascadia) December 20, 2022
#EarthquakeReport for M 6.4 #Earthquake in Mendocino triple junction (Triangle of Doom) region
aftershocks suggest left-lateral strike-slip in Gorda plate
felt broadly, about 92%g in Ferndale
read more from last year's reporthttps://t.co/aS9ySrs7WXhttps://t.co/9HKHnpMFSh pic.twitter.com/wt4UduAuvt
— Jason "Jay" R. Patton (@patton_cascadia) December 20, 2022
#EarthquakeReport for M 6.4 #Earthquake offshore of #HumboldtCounty #California
intensity summary: @USGS_Quakes model vs Did You Feel It? observations
PGA in g units from https://t.co/KM7lTGSzX7
report forthcoming, 2021 review: https://t.co/aS9ySr9YIPhttps://t.co/9HKHnpuwE9 pic.twitter.com/K2JiOEKJTm
— Jason "Jay" R. Patton (@patton_cascadia) December 22, 2022
#EarthquakeReport for M 6.4 #Earthquake in #Humboldt County #California
interpretive poster showing aftershocks and comparison with '22 sequence
no foreshocks@USGS_Quakes slip/GNSS model compared with GNSS observations
report forthcoming, '22 report: https://t.co/aS9ySr9YIP pic.twitter.com/NfYkUuW13J
— Jason "Jay" R. Patton (@patton_cascadia) December 22, 2022
#EarthquakeReport for M6.4 #Earthquake offshore northern #California#FerndaleEarthquake
hypocenters from @USGS_Quakes
i plotted USGS Slab2 depths https://t.co/HdW0ZOzted
i traced Gorda slab from Guo 2021 B-B' https://t.co/t8gXg1jaYYread report herehttps://t.co/0rRNL3TfNk pic.twitter.com/Mni4dbD8Oo
— Jason "Jay" R. Patton (@patton_cascadia) December 24, 2022
#EarthquakeReport for M6.4 #Earthquake in #HumboldtCounty #California
Gorda intraplate left-lateral strike-slip earthquake
see: https://t.co/t8gXg1jaYYsome tensional mechanisms
possibly 2 main faults involved (?) outlined in white
read report herehttps://t.co/0rRNL3TfNk pic.twitter.com/FQripjzAa0
— Jason "Jay" R. Patton (@patton_cascadia) December 26, 2022
#EarthquakeReport for M 6.4 #Earthquake in northern @California
updated plot: hypocenters compared to Gorda crust and the @USGS_Quakes Finite Fault Model showing that most of the slip occurred in the NAP (not sure this is correct)
read more herehttps://t.co/0rRNL3TfNk pic.twitter.com/Nca4IimQ3z
— Jason "Jay" R. Patton (@patton_cascadia) December 26, 2022
#EarthquakeReport for M6.4 #Earthquake in northern #California
geology from CDMG '99 and McLaughlin et al. '00. units are labeled, so no legend (abt 30 units in each data set)
lack of upper plate structures oriented with 6.4 seismicity
read report here:https://t.co/0rRNL3TfNk pic.twitter.com/iyCQBCsxf4
— Jason "Jay" R. Patton (@patton_cascadia) December 26, 2022
a triple junction is defined as where three plate boundaries meet, not where three plates meet (though that is also true). the types of triple junctions (e.g., RRR, TTT, RFF) refer to the types of faults that meet there. https://t.co/zfP2DidN6Ihttps://t.co/CLUfzwNanj pic.twitter.com/t9O5RFstfY
— Jason "Jay" R. Patton (@patton_cascadia) December 30, 2022
#EarthquakeReport for M6.4 & 5.4 #Earthquakes in the #Triangleofdoom #Mendocinotriplejunction
M6.4 – left-lateral strike-slip (in crust?)
M5.4 – right-lateral s-s (in mantle?)updated aftershock map and hypocenter profile
read the report herehttps://t.co/0rRNL3TfNk pic.twitter.com/HNpiZRBP8a
— Jason "Jay" R. Patton (@patton_cascadia) January 3, 2023
The #earthquake stopped campus clocks at 2:34 AM. pic.twitter.com/PeQNwvL4I0
— Cal Poly Humboldt (@humboldtcalpoly) December 22, 2022
Good morning Redwood Coast CA. Did you feel the magnitude 6.4 quake about 7.5 miles southwest of Ferndale at 2:34 am? The #ShakeAlert system was activated. See: https://t.co/zwOapjTWaA pic.twitter.com/eMSUAT3inw
— USGS ShakeAlert (@USGS_ShakeAlert) December 20, 2022
A M6.4 earthquake has occurred south of Eureka, CA in northern CA (Humboldt Co.). Additional shaking from aftershocks is expected in the region. We are continuing to monitor this event, so check back for additional information. #Humboldt #earthquake pic.twitter.com/DpaIlz3RGV
— California Geological Survey (@CAGeoSurvey) December 20, 2022
A M6.4 earthquake & several aftershocks hit the coast near Ferndale, CA. Epicenter is close enough to land that strong shaking & some ground/structure damage is expected. #earthquake #Humboldt pic.twitter.com/YcO3mVEJCI
— Brian Olson (@mrbrianolson) December 20, 2022
#EarthquakeReport for M 6.4 #Earthquake in Mendocino triple junction (Triangle of Doom) region
felt broadly at least intensity MMI 8
read more from last year's reporthttps://t.co/aS9ySr9YIPhttps://t.co/9HKHnpuwE9 pic.twitter.com/8qAaSK6i9y
— Jason "Jay" R. Patton (@patton_cascadia) December 20, 2022
#EarthquakeReport for M 6.4 #Earthquake in Mendocino triple junction (Triangle of Doom) region
modest chance for eq triggred landslides
high likelihood for eq induced liquefactionread more from last year's reporthttps://t.co/aS9ySr9YIPhttps://t.co/RXs6q07wjX pic.twitter.com/zI9cPfnRUG
— Jason "Jay" R. Patton (@patton_cascadia) December 20, 2022
A few more clean signals pic.twitter.com/53tL2Rixkb
— Brendan Crowell (@bwcphd) December 20, 2022
That was a big one. Power is now out in #ferndaleca. House is a mess. #earthquake pic.twitter.com/YEmcv1Urhp
— Caroline Titus (@caroline95536) December 20, 2022
About 50,000 PG&E customers are without power in Humboldt after that earthquake, which was a preliminary magnitude 6.4.https://t.co/TLWiUpfEGp
— North Coast Journal (@ncj_of_humboldt) December 20, 2022
Road Closure: State Route 211 at Fernbridge, Humboldt County is CLOSED. The bridge is closed while we conduct safety inspections due to possible seismic damage. pic.twitter.com/601oOQRz2o
— Caltrans District 1 (@CaltransDist1) December 20, 2022
FERNBRIDGE EARTHQUAKE DAMAGE: Damage to Fernbridge following the 6.2 magnitude #earthquake in Humboldt County. Main road to Ferndale currently closed off by CalTrans as crews inspect for additional damage. pic.twitter.com/4BPOSvZrN9
— Austin Castro (@AustinCastroTV) December 20, 2022
Auto solution FMNEAR (Géoazur/OCA) with regional records for the M 6.3 – OFFSHORE NORTHERN CALIFORNIA – 2022-12-20 10:34:25 UTC (Loc EMSC used to trigger inversion).https://t.co/UHDsc1hVXA
Thanks to the seismic records provided in particular by IRIS, SCEDC pic.twitter.com/VOEflZWynp— Bertrand Delouis (@BertrandDelouis) December 20, 2022
Mw=6.4, NEAR COAST OF NORTHERN CALIF. (Depth: 9 km), 2022/12/20 10:34:25 UTC – Full details here: https://t.co/nC4QZqppm0 pic.twitter.com/QW0ggaT4dE
— Earthquakes (@geoscope_ipgp) December 20, 2022
strong #Earthquake offshore California, United States Of America
Felt by at least 9.0 m. people.
More than 130k people live in regions, where damage can be expected.
Severe damage is expected in an area affecting more than 50k people.https://t.co/9Ku6UPu3gQ pic.twitter.com/6UN4tIasme— CATnews (@CATnewsDE) December 20, 2022
The area where this quake occurred is quite active. These images from @EarthScope_sci IRIS Earthquake Browser show earthquakes in the area of M4+, M5+, M6+ and M7+. pic.twitter.com/y3xafBrCfd
— Wendy Bohon, PhD 🌏 (@DrWendyRocks) December 20, 2022
In fact, there have already been 20+ aftershocks of M2.5+! Again, this is normal and expected.
PSA: aftershocks are just smaller earthquakes that occur after a larger quake.
Here’s more info https://t.co/byWunrqSqZ
— Wendy Bohon, PhD 🌏 (@DrWendyRocks) December 20, 2022
— Robert Martin (@NordBob) December 20, 2022
Following the M6.4 mainshock, there have been well over 20 recorded aftershocks above M2.5. pic.twitter.com/Gej6rRNDlG
— EarthScope Consortium (@EarthScope_sci) December 20, 2022
Saw this on Facebook from someone in Eureka after tonight's quake. A reminder to "Secure Your Space" by tethering heavy furniture to the wall for this exact reason. #earthquake pic.twitter.com/1CiYbLOQcE
— Brian Olson (@mrbrianolson) December 20, 2022
Some people in Los Angeles and Tacoma really need to chill out and have less caffeine before bed 🧐🤔 https://t.co/5zIRhUR6eq pic.twitter.com/iURHlVVuOS
— Austin Elliott (@TTremblingEarth) December 20, 2022
Just took a cruise down Main Street #ferndaleca. Couldn’t see one broken window. Many store owners replaced broken ones after 6.2 on this same day in 2021. Also today’s #earthquake shook north/south. #earthquakeca pic.twitter.com/Ua1nMx0UuJ
— Caroline Titus (@caroline95536) December 20, 2022
Ferndale M6.4 strike-slip earthquake and aftershocks so far, all lining up along the left-lateral nodal plane of the focal mechanism. pic.twitter.com/lfWX5Qz4FF
— Harold Tobin (@Harold_Tobin) December 20, 2022
M6.4 #earthquake near Ferndale, CA: Seismicity for today (red), the past year (orange) and back to 1982 (green-blue-purple). Views from above/south/east. Today's events may be in upper part of down-going, Gorda plate. pic.twitter.com/WdvPq85LJP
— Anthony Lomax 😷🇪🇺🌍🇺🇦 (@ALomaxNet) December 20, 2022
Cal OES is coordinating with local and tribal governments to assess the impacts of the Earthquake and supporting with resources, mutual aid and damage assessment. State Agency response including Cal OES, Cal Fire, Cal Trans, Cal CGS, CHP in support of local efforts
— California Governor's Office of Emergency Services (@Cal_OES) December 20, 2022
Peak ground acceleration plot of seismic stations that recorded shaking from last night's M6.4 earthquake in Humboldt County. Notably, several values are *WELL* above the predicted envelope given distance from the epicenter. Is this real? Any explanations? Near-field effect? pic.twitter.com/QHdAMlglbM
— Brian Olson (@mrbrianolson) December 20, 2022
A brief explainer about the M6.4 earthquake near Ferndale in Northern California pic.twitter.com/3Ar03QFlC3
— Wendy Bohon, PhD 🌏 (@DrWendyRocks) December 20, 2022
Gov. Newsom & State officials provide updates on the M6.4 earthquake today near Ferndale in Humboldt County. #earthquake #Eureka @Cal_OES @GovPressOffice https://t.co/xHAkna9UYw
— California Geological Survey (@CAGeoSurvey) December 20, 2022
Cindy Pridmore representing CGS at today's press conference on the M6.4 Ferndale earthquake. She noted quakes of this size aren't uncommon here & people should be aware of continuing aftershocks, especially if they are in structures already damaged by the quake. @CAGeoSurvey pic.twitter.com/hRrJkLT7Tz
— Brian Olson (@mrbrianolson) December 20, 2022
Small teams of CGS geologists are currently out in the Ferndale, Rio Dell, & Eureka areas documenting structural & ground damage from this morning's M6.4 earthquake. Seeing where damage occurs helps us understand how shaking intensity & damage are related. #earthquake #humboldt
— California Geological Survey (@CAGeoSurvey) December 20, 2022
The @USGS_Quakes aftershock forecast for the M6.4 event in Northern California is out.
MOST LIKELY – “There will likely be smaller aftershocks within the next week with up to 24 M3+ aftershocks. M3+ aftershocks are large enough to be felt nearby.” https://t.co/7o2iJhozp0
— Wendy Bohon, PhD 🌏 (@DrWendyRocks) December 20, 2022
Important info for folks that live in Earthquake country 👇🏻 https://t.co/ZGWNf1zYpr
— Wendy Bohon, PhD 🌏 (@DrWendyRocks) December 20, 2022
Sadly, two reported deaths.
https://t.co/s2By2z6zDh— Jason "Jay" R. Patton (@patton_cascadia) December 20, 2022
This Magnitude 6.4 earthquake in California and subsequent power outage got me wanting to share this new video guide on small scale solar back up now.
This is the short version of the video based on this step-by-step guide – https://t.co/af5okVx2P7#photovoltaics #prepper pic.twitter.com/oxmYbiNQ9i
— Lonny Grafman (@LonnyGrafman) December 20, 2022
Seismicity map of today's Ferndale earthquakes with red outline (suggesting EW plane may be fault), and the events from exactly a year ago in purple. A bit confusing, since the M6.2 from a year ago appears to have been relocated significantly from its original offshore location. pic.twitter.com/K3HvECUt4l
— Jascha Polet (@CPPGeophysics) December 20, 2022
Still not seeing many images of damage, but based on anecdotes from folks in quake zone it does sound like there was damage to some structures & especially to infrastructure. I suspect that ongoing widespread regional power outages are reason we haven't heard more yet.#earthquake https://t.co/ACvlDpvqJR pic.twitter.com/ejHBRaMOHJ
— Daniel Swain (@Weather_West) December 20, 2022
Before (May 2018) & After (today) photos of the old Humboldt Creamery building in Loleta (across from the Cheese Factory). Old brick buildings perform so badly during earthquakes. I hope the cheese factory is safe.🧀🥛 #FerndaleEarthquake #earthquake pic.twitter.com/aMbbpsrsz6
— Brian Olson (@mrbrianolson) December 20, 2022
Some excellent 5-Hz GNSS velocities for the Ferndale EQ showing some strong site amplification at @EarthScope_sci site P168 (peak 35 cm/s). Closest seismic site KNEE is in good agreement. pic.twitter.com/OSskpMEMjX
— Brendan Crowell (@bwcphd) December 21, 2022
Wow! Extreme ground accelerations, well above 1 g, during the recent M6.4 earthquake near Ferndale, Caifornia, recorded in Rio Dell: https://t.co/HmJa5feZ3g
(preliminary data processing) https://t.co/7RGCxkHmZ5 pic.twitter.com/4FiIYo5EDC— Pablo Ampuero (@DocTerremoto) December 20, 2022
Governor @GavinNewsom proclaimed a state of emergency for Humboldt County to support the emergency response to today’s 6.4 magnitude earthquake near the City of Ferndale. https://t.co/EieUtBovqT
— Office of the Governor of California (@CAgovernor) December 21, 2022
'Significant' Damages in Rio Dell Area, Says Humboldt Office of Emergency Services; 11 Injuries, Two Dead from Medical Emergencies https://t.co/ruNr3ma5tN
— Lost Coast Outpost (@LCOutpost) December 20, 2022
Road damage from Northern California earthquake, in Rio Dell pic.twitter.com/P9eSSX4kRU
— EthanBaron (@ethanbaron) December 21, 2022
Over 3 million people in California & Oregon received #ShakeAlert-powered alerts during today’s M6.4 quake near Ferndale, CA. #ShakeAlert is success because of: @Cal_OES @OregonOEM @waEMD @waDNR @CAGeoSurvey @OregonGeology @OHAZ_UO @UW @PNSN1 @CaltechSeismo @BerkeleySeismo @USGS pic.twitter.com/wWL4N6aMxI
— USGS ShakeAlert (@USGS_ShakeAlert) December 20, 2022
Watching observations from this morning’s #earthquake come in: Some from our @CAGeoSurvey geologists and others gleaned from news reports and social media by our GIS professionals. Most of these are damage reports so far. Incredibly valuable spatial data! pic.twitter.com/JUPIV2AAtR
— Tim Dawson (@timblor) December 20, 2022
Real-time GNSS displacements recorded by GSeisRT for the Ferndale M6.4 event on Dec. 20th. @EarthScope_sci pic.twitter.com/HZz6F758l5
— Jianghui Geng (@GengJianghui) December 21, 2022
Our field teams were out documenting structural & ground damage yesterday to help us understand the shaking effects from yesterday's M6.4 Ferndale earthquake.
Most vulnerable to any strong shaking are "unreinforced masonry" buildings like the old Humboldt Creamery in Loleta. 1/7 pic.twitter.com/ZCInnRJv6k— California Geological Survey (@CAGeoSurvey) December 21, 2022
Learn more about the M6.4 earthquake near Ferndale, CA in this @USGS featured story : https://t.co/T5EYMvlKK5 @USGS_Quakes @CAGeoSurvey @Cal_OES @OregonOEM @OHAZ_UO @PNSN1 @waDNR @waShakeOut @ShakeOut @ECA @CalConservation @CaltechSeismo @BerkeleySeismo @ListosCA @FEMARegion9 pic.twitter.com/mgPPGQeM54
— USGS ShakeAlert (@USGS_ShakeAlert) December 21, 2022
Regarding the North Coast earthquake, my undergrad Geography advisor, Eugenie Rovai (Rio Dell local), did social geography research after the 1994 earthquake, and wrote about how history affected the capacity for each community to recover. https://t.co/eY4LXrGekl pic.twitter.com/HuWTkzyjVJ
— Zeke Lunder ~ The Lookout (@wildland_zko) December 22, 2022
The earthquake waves from the M6.4 Ferndale quake were recorded by seismic stations across North America. By the time the waves move away from the region where the earthquake occurred they are much too small to feel but not too small to measure. pic.twitter.com/s7UYGUjPey
— Wendy Bohon, PhD 🌏 (@DrWendyRocks) December 22, 2022
The supercomputer has finished chugging. Here is a preliminary simulation of how yesterday’s M6.4 earthquake might have focused shaking in specific areas. Event page here: https://t.co/UA9LAh0bJ2 pic.twitter.com/bdR9ZFYapn
— USGS Earthquakes (@USGS_Quakes) December 21, 2022
What’s the difference between geologic hazard and risk? What are the USGS National Seismic Hazard Maps, and how are they used? Find out in this introduction to the National Seismic Hazard Maps: https://t.co/biDoY1ewWx#SeismicHazards #Earthquakes pic.twitter.com/T48ytJ7gKJ
— USGS (@USGS) December 22, 2022
CA worked night and day and — less than 48 hours after a strong earthquake in Humboldt County — power has been restored to all communities.
Thank you @Cal_OES, @CaltransHQ, @CAL_FIRE, @CA_EMSA, and @CHP_HQ for helping recovery efforts.https://t.co/JIaFUWJO9A
— Office of the Governor of California (@CAgovernor) December 23, 2022
And the corresponding map view.
High-precision relocations of M≥2 1982 to 2021/12 done with NLL-SSST-coherence (https://t.co/EwE8DRzwvU), past year done with NLL-SSST.
Earthquake arrival data from https://t.co/7TWxvNHnee pic.twitter.com/KVN606rFfC
— Anthony Lomax 😷🇪🇺🌍🇺🇦 (@ALomaxNet) December 22, 2022
The 12/20/22 M6.4 earthquake has produced a nice aftershock sequence that illuminates the fault that likely ruptured. A nice zone northeast of the epicenter. @CAGeoSurvey found no surface rupture, so this is a seismologist’s earthquake with lots to learn. pic.twitter.com/XJB8MuJPno
— Tim Dawson (@timblor) December 24, 2022
ARIA has processed interferograms with 23 December Copernicus Sentinel-1 covering M6.4 Ferndale earthquake. Geocoded UNWrapped (GUNW) interf. files available from NASA ASF archive. @iamgracebato did InSAR time-series with MintPy to mitigate atmosphere in attached map. pic.twitter.com/6zqOpGFzD0
— Advanced Rapid Imaging & Analysis (ARIA) (@aria_hazards) December 24, 2022
HUMBOLDT OES: Around 70 Local Buildings Deemed Unsafe in the Wake of the Quakes, in Total; Here is the Big List of Resources for People Who Need Help https://t.co/yTizDPCnE5
— Lost Coast Outpost (@LCOutpost) January 3, 2023
- 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
- 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.
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
- Sorted by Magnitude
- Sorted by Year
- Sorted by Day of the Year
- Sorted By Region
I was returning from New Orleans where I was attending the American Geophysical Union Fall Meeting. There was a short layover in Denver and I had a short time to find some food, which is challenging with my dietary restrictions. I cannot recall precisely, but I got some notification from my CGS crew about a magnitude M 6.2 earthquake offshore of the Mendocino triple junction. One of these notifications was from Cindy as we both collaborate to prepare quick reports for earthquakes in California. These reports are sent upstream to management in our organization and others. I was unavailable to contribute this time. Needless to say, I was sad to have missed experiencing this good sized shaker for myself. This is the first earthquake of this size that I have missed (in Humboldt) since I moved here in 1991. Last week or so, their analyses were produced publicly and the earthquake catalog was updated. What we discovered is that there were two closely spaced (in time but not space) earthquakes, an M 5.7 and and M 6.2. https://earthquake.usgs.gov/earthquakes/eventpage/nc71127029/executive It was complicated for the seismologists to work out because the seismic waves of the two events overlapped in time. i.e., the waves from the first quake were still passing through the Earth when the waves from the second quake started. Basically, there was initially an M 5.7 strike-slip earthquake along the Mendocino transform fault zone about 20 km (12.5 miles) offshore. About 10 or 11 seconds later, there was an M 6.2 strike-slip earthquake within the Gorda plate, below the megathrust fault. Here is a plot from the USGS. Each horizontal squiggly line is the seismograph record from an individual seismometer. They are plotted with the seismometer closest to the earthquake on the bottom row and the furthest seismometer on the uppermost row. The P wave (primary wave) is the first of four major types of seismic waves. Next comes the S (secondary) wave, then the Love waves, and finally the Raleigh waves. The P wave arrives at closer seismometers before it arrives at more distant seismometers. Because of this, we generally call this type of plot a travel time plot. In the above plot we can see how the M 6.1 P waves are arriving while the M 5.7 S waves are still being transmitted. The M 5.7 is clearly a right-lateral strike-slip event given the aftershock pattern and the known location and type of the Mendocino fault system (a right-lateral strike-slip fault. Earthquake mechanisms (the “beach balls”) show two possible ways that the earthquake could have slipped. We use aftershock patterns and existing mapped faults to help us interpret which of these [nodal] fault planes is the more likely one. If we look at the earthquake poster below, we see that the M 6.2 earthquake is an almost pure strike-slip earthquake. The two possible fault planes are one that is oriented in the northwest direction (would be right-lateral) and one that is in the northeast direction (would be left-lateral). So, while most of our experience with the Gorda plate is with northeast oriented (striking) left-lateral strike-slip faults (e.g., 1980, 2010, 2014, etc.) it is possible that there are other faults, sub-parallel to the post-1992 seismicity trends, where the M 6.2 and other aftershocks were hosted. I mention these northwest trending faults in a recent Earthquake Report here. Something that is interesting is that the onshore events from this 20 Dec 2021 sequence are just to the north of the aftershocks from the 1992 sequence. They are at similar depths as those ’92 quakes and have similar earthquake mechanisms. As Spock would say, Fascinating. Dr. Anthony Lomax, famous for his work locating the hypocenter for the 1906 San Francisco Earthquake, has been developing excellent tools for seismologists ever since. He recently applied one of his new tools to locate earthquakes to the Mendocino triple junction region. I present some of his figures below. 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.
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.
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.
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.
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.
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. 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. 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. 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. 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. 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.
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.
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.
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.
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.
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.
(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?” 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. 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: 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.
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.
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.
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. — Jason "Jay" R. Patton (@patton_cascadia) March 19, 2020 This 3-D representation shows earthquake locations of the 03/18/20, Magna sequence. The largest circle is the magnitude 5.7 main shock, at a depth of about 7.5 miles (12 km), and the other circles are aftershocks that had occurred through 1:30 pm MDT.https://t.co/5YuwS7G8Rm pic.twitter.com/uPDuoiRX3l — Utah Geological (@utahgeological) March 18, 2020 UGS geologists are on the ground documenting the geologic effects of today's earthquakes. More information will be added as our field teams continue their investigations.https://t.co/0U7ga954RD#utahearthquake pic.twitter.com/La1oJnIIhy — Utah Geological (@utahgeological) March 19, 2020
I was in Humboldt County last week for the Redwood Coast Tsunami Work Group meeting. I stayed there working on my house that a previous tenant had left in quite a destroyed state (they moved in as friends of mine). Here is a seismic selfie from Riley, a student at Humboldt State University (taking a geology course). This photo was posted on the HSU Dept. of Geology facebook page.
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. 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. From the USGS:
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: 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.)
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.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy — Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
This morning there was a small earthquake in a region of northern California between two major faults that are part of the Pacific-North America plate boundary. The M 4.3 earthquake occurred between the San Andreas fault (SAF) to the west and the Maacma fault (MF) to the east. There are no mapped earthquake faults in this region. I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1917-2017 with magnitudes M > 4.0.
Geologic sketch map of the northern Coast Ranges, central California, showing faults with Quaternary activity and basin deposits in northern section of the San Andreas fault system. Fault patterns are generalized, and only major faults are shown. Several Quaternary basins are fault bounded and aligned parallel to strike-slip faults, a relation most apparent along the Hayward-Rodgers Creek-Maacama fault trend.
Maps showing the regional setting of the Rodgers Creek–Maacama fault system and the San Andreas fault in northern California. (A) The Maacama (MAFZ) and Rodgers Creek (RCFZ) fault zones and related faults (dark red) are compared to the San Andreas fault, former and present positions of the Mendocino Fracture Zone (MFZ; light red, offshore), and other structural features of northern California. Other faults east of the San Andreas fault that are part of the wide transform margin are collectively referred to as the East Bay fault system and include the Hayward and proto-Hayward fault zones (green) and the Calaveras (CF), Bartlett Springs, and several other faults (teal). Fold axes (dark blue) delineate features associated with compression along the northern and eastern sides of the Coast Ranges. Dashed brown line marks inferred location of the buried tip of an east-directed tectonic wedge system along the boundary between the Coast Ranges and Great Valley (Wentworth et al., 1984; Wentworth and Zoback, 1990). Dotted purple line shows the underthrust south edge of the Gorda–Juan de Fuca plate, based on gravity and aeromagnetic data (Jachens and Griscom, 1983). Late Cenozoic volcanic rocks are shown in pink; structural basins associated with strike-slip faulting and Sacramento Valley are shown in yellow. Motions of major fault blocks and plates relative to fi xed North America, from global positioning system and paleomagnetic studies (Argus and Gordon, 2001; Wells and Simpson, 2001; U.S. Geological Survey, 2010), shown with thick black arrows; circled numbers denote rate (in mm/yr). Restraining bend segment of the northern San Andreas fault is shown in orange; releasing bend segment is in light blue. Additional abbreviations: BMV—Burdell Mountain Volcanics; QSV—Quien Sabe Volcanics. (B) Simplifi ed map of color-coded faults in A, delineating the principal fault systems and zones referred to in this paper.
EVOLUTION OF THE SAN ANDREAS FAULT.
Earthquake shaking hazards are calculated by projecting earthquake rates based on earthquake history and fault slip rates, the same data used for calculating earthquake probabilities. New fault parameters have been developed for these calculations and are included in the report of the Working Group on California Earthquake Probabilities. Calculations of earthquake shaking hazard for California are part of a cooperative project between USGS and CGS, and are part of the National Seismic Hazard Maps. CGS Map Sheet 48 (revised 2008) shows potential seismic shaking based on National Seismic Hazard Map calculations plus amplification of seismic shaking due to the near surface soils.
Map showing location and geologic setting of the Franciscan Coastal Belt in the northern California Coast Ranges. Inset shows magnetic anomalies (in magenta) of Figure 2 in and near the Coastal Belt, mapped occurrences of basalt (black dots and areas), and associated fossil localities (numbered white X’s; listed in Table 1). Map units: fc—Franciscan False Cape terrane; KRt—Franciscan King Range terrane; Cob—Franciscan Coastal Belt, undivided; Yg—Franciscan Yager terrane; Cnb— Franciscan Central Belt; Eb—Franciscan Eastern Belt; um—ultramafi c rocks; MTJ— Mendocino triple junction; GVg—Great Valley Group; T—Tertiary cover; Q—alluvium, largely Quaternary. Stippled pattern near Fort Ross shows outcrop of Ohlson Ranch Formation. Magenta arrows labeled PAC and GOR show relative plate motion of Pacific and Gorda plates, respectively, relative to the North American plate (McCrory, 2000). Tiny box labeled MH—Marin Headlands (area of Fig. 8). Coastal Belt thrust is shown with thrust teeth. Geology was compiled and simplifi ed from Jennings (1977), Blake et al. (1992), Blake et al. (2002), Jayko et al. (1989), McLaughlin et al. (2000), U.S. Geological Survey and California Geological Survey (2006), and geologic mapping by R.J. McLaughlin northeast of Clear Lake and south of Willits. Southern part of the Coastal Belt thrust west and south of Willits is from (1) mapping by McLaughlin,
Filtered magnetic map of the Coastal Belt. See Langenheim et al. (2011) for details of filtering that places anomalies over magnetic sources and enhances anomalies for which sources are exposed or near surface. Magenta lines—margins of the belt, with the San Andreas fault on the west and the Coastal Belt thrust and other faults on the east. Dashed dark green lines—depositional contacts. Red lines— boundaries between the terranes of the Coastal Belt: Coastal Belt, undivided (Cob), False Cape terrane (fc), King Range terrane (KRt), and Yager terrane (Yg). The Wheatfield Fork terrane (WFt) is too narrow to show at the scale of the figure, but its extent along the eastern boundary of the Coastal Belt is circled in dark blue. Thin dark blue dotted lines separate structural domains discussed in text and shown in figure 6. Blue line—profile location of model shown in Figure 5B.
Structural domains and dips interpreted from filtered magnetic anomalies (Fig. 2). Layer dip from asymmetry of magnetic anomaly is shown. Dark blue lines separate domains discussed in text. Anomalies within area east of the Coastal Belt thrust may be caused by magnetic layers in the Coastal Belt beneath a thin sheet of Central Belt rocks in the hanging wall of the thrust. Anomalies colored green, blue, and lavender are discussed in text. WFt— Wheatfield Fork terrane. Dashed green line is outline of onshore Eel River basin.
I was driving around Eureka today, running to the appliance center to get an appliance (heheh). I got a message from a long time held friend (who lives in Salinas, CA). They asked me if I was OK, given that there was an earthquake up here. I thought I had not felt it because I was driving around. However, after looking at the USGS website, I learned the earthquake happened earlier, while I was back working on my house. The main reason I did not feel it is because it was too far away.
this happens regularly. earthquake notifications are automatic as epicenter locations are identified from incoming seismic waves in the seismic network. sometimes the named arrivals (eg. p wave, s wave, and the many other arrivals) are miss-correlated between stations. this miss-correlation then leads to earthquakes in the database that are not real. Today’s M 5.7 earthquake was along the western part of the Mendocino fault (MF), 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. Here is the USGS website for this earthquake. This is a preliminary report and I hope to prepare some updates as I collect more information.
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.
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.
Strike Slip: The 25 April 1992 M 7.1 earthquake was a wake up call for many, like all large magnitude earthquakes are. I plot the seismicity for a week beginning April 25, 1992, with color representing depth and diameter representing magnitude (see legend).. 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.
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.
Observed and predicted coseismic displacements for the Cape Mendocino main shock (epicenter located at star).
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.
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)
(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 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). Please visit the #EarthquakeReport pages for more information about the figures that I include in the Earthquake Report interpretive posters below.Earthquake Report: M 5.7 & 6.2 Mendocino triple junction
I got home about 3 am the next morning and did not have energy to prepare an earthjay report. Though I started working on it the next day. However, I soon learned that this was a complicated earthquake and I decided to await additional analyses by the Berkeley Seismmo Lab and the USGS.
https://earthquake.usgs.gov/earthquakes/eventpage/nc73666231/executive
The interpretation for the type of earthquake for the M 6.2 is a little more complicated.
There are two reasons why I interpret the M 6.2 to be right-lateral (of course, I could be wrong).
Below is my interpretive poster for this earthquake
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Shaking Intensity and Potential for Ground Failure
Some Relevant Discussion and Figures
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
Cascadia subduction zone
General Overview
Earthquake Reports
Gorda plate
Blanco transform fault
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
Uncertain
Social Media
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
Earthquake Report: Mendocino triple junction
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.
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?
WHy?
Well, there are two kinds of earthquake triggering.
* note, i corrected this caption by changing the word “relationships” to “relations.”
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 include some inset figures. Some of the same figures are located in different places on the larger scale map below.
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
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
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.
Further North
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.
The main take away is that we are not at a greater risk because of these earthquakes.
Cascadia subduction zone
General Overview
Earthquake Reports
Gorda plate
Blanco transform fault
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
Uncertain
Social Media
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
Earthquake Report: Salt Lake City
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
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 include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
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:
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.
Basin and Range
General Overview
Earthquake Reports
Utah
Idaho
Nevada
Social Media
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
Earthquake Report: Mendocino fault
As I was grabbing a bite at Taqueria Bravo in Willits, I checked in on social media and noticed my friend Dave Bazard had posted moments earlier about an earthquake there. I had missed it by about 2 hours or so.
https://earthquake.usgs.gov/earthquakes/eventpage/nc73351710/executive
Yesterday’s earthquake was a right-lateral strike-slip earthquake on the Mendocino fault system. The Mendocino fault is a strike-slip fault formed by the eastward motion of the Gorda plate relative to the westward motion of the Pacific plate. The last major damaging earthquake on the MF was in 1994.
Interestingly, this was the 6 year commemoration of the 2014 M 6.8 Gorda plate earthquake (the last large earthquake in the region).
Also, there was a similarly sized event on the MF in 2018.
Big “take-aways” from this:
Below is my interpretive poster for this earthquake
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
Cascadia subduction zone Earthquake Reports
General Overview
Earthquake Reports
Gorda plate
Blanco transform fault
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
Uncertain
Social Media
References:
Basic & General References
Specific References
Return to the Earthquake Reports page.
Earthquake Report: Mendocino triple junction
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 USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
Magnetic Anomalies
Global Strain
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
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.
Be ready for more earthquakes
What we think will happen next
About our earthquake forecasts
USGS Landslide and Liquefaction Ground Failure data products
Other Report Pages
Some Relevant Discussion and Figures
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
Geologic Fundamentals
Compressional:
Extensional:
Cascadia subduction zone
General Overview
Earthquake Reports
Gorda plate
Blanco fracture zone
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
Uncertain
Social Media
References:
Return to the Earthquake Reports page.
Earthquake Report: Laytonville (northern CA)!
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).
About 75% of the relative plate motion is accommodated along the SAF and its synthetic sister faults in the northern CA region. The rest of the plate boundary motion is accommodated along the Eastern CA shear zone and Walker Lane, along with the Central Nevada Seismic Belt, and the Wasatch fault systems. In Northern CA, there is about 33-37 mm/yr strain accumulated on the SAF plate boundary system. About 18-25 mm/yr is on the SAF, 8-11 mm/yr on the MF, and 5-7 mm/yr on the Bartlett Springs fault system (Geist and Andrews, 2000).
The three main faults in the region north of San Francisco are the SAF, the MF, and the Bartlett Springs fault (BSF). I also place a graphical depiction of the USGS moment tensor for this earthquake. The SAF, MF, and BSF are all right lateral strike-slip fault systems. There are no active faults mapped in the region of Sunday’s epicenter, but I interpret this earthquake to have right-lateral slip. Without more seismicity or mapped faults to suggest otherwise, this is a reasonable interpretation.Below is my interpretive poster for this earthquake.
I use the USGS Quaternary fault and fold database for the faults. I outlined the Vizcaino Block, which many interpret to be a prehistoric subduction zone accretionary prism from a time before the San Andreas existed.
I plot the USGS fault plane solutions (moment tensors in blue) for some relevant historic earthquakes.
I include some inset figures.
Below are some earthquake report posters for earthquakes in this region.
This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendicino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendicino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendicino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system. (Image modified after original illustration by Irwin, 1990 and Stoffer, 2006.)
interpretation of aeromagnetic anomalies, and 1:62,500 scale topography, and (2) that east of Point Arena is from photogeologic interpretation that resulted in a greater extent of mélange assigned to the Central Belt.
San Andreas fault earthquake reports
General Overview
Earthquake Reports
Northern CA
Southern CA
Eastern CA
References:
Earthquake Report: Mendocino fault! (northern California)
Once I got home, after work, I noticed that lots of people were discussing how they were confused about the earthquake notifications from the USGS. Apparently, there were two M 5.X earthquakes in the USGS earthquake online system for a while. Then there was one. This is a common occurrence and I prepared an explanation for some people Here is what I wrote for these people on social media:
seismologists are monitoring the process and review these data for quality, looking for mistakes, and refining magnitude estimates, moment tensor and focal mechanism solutions, location estimates, casualty estimages (PAGER alerts), and all the derivative data products (intensity, PGA, PGV, etc. maps and data).
sometimes these earthquakes are from data in the same location as the real earthquake (like today) and sometimes they are “picked” from seismic data from remote earthquakes.
some of these earthquakes are listed here:
https://earthquake.usgs.gov/earthquakes/errata.php
See the figures from Rollins and Stein (2010) 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 (JDFP and GP, respectively) 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.
There was a good sized (M 6.5) MF earthquake late last year on 2016.12.08. I present my poster for that earthquake below. Here is my report for that earthquake. Here is the updated report.
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 have placed several inset figures.
Compressional:
Extensional:
Update
Cascadia subduction zone Earthquake Reports
General Overview
Earthquake Reports
References
Earthquake Report: 1992.04.25 M 7.1 Petrolia
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.
Here are some additional blogs about this earthquake.
Below is my interpretive poster for this earthquake.
I include some inset figures in the poster.
Below is my updated interpretive poster for this earthquake.
I include some inset figures in the poster.
Shaking Intensity and Potential for Ground Failure
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.
The Cascadia subduction zone
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
1992 Cape Mendocino Earthquake and Tsunami
Below is an updated interpretive poster for this earthquake sequence that focuses on the mechanisms.
Here is the USGS website for all the earthquakes in this region from 1917-2017 with M ≥ 6.5.
Cascadia subduction zone
General Overview
Earthquake Reports
Gorda plate
Blanco fracture zone
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
References
Earthquake Report: 2016 Summary Cascadia
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).
Earthquake Summary Poster (2016)
I include some inset figures in the poster.
Cascadia subduction zone: General Overview
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.
Here are the web pages for these earthquakes. The first link goes to the USGS page and the second link goes to the Earthquake Report page.
Cascadia subduction zone:
General Overview
2016 Earthquake Reports
Gorda plate
Mendocino fault
North America plate
Explorer plate
References