Earthquake Report: Gulf of Alaska!

I was asleep in bed, trying to catch up to prevent myself from getting ill, when there was a large earthquake in the Gulf of Alaska (GA), offshore of Kodiak, Alaska. When I wakened, I noticed a fb message from my friend Scott Willits notifying me of an M 8.2 earthquake in Alaska, posted at 2:20 AM local time. I immediately got up to check on this and was surprised that there was not a tsunami evacuation going on. I live in the small town of Manila (population ~700), on the North Spit (a sand spit west of Arcata and Eureka, CA). I live above 10 m in elevation and do not consider myself exposed to tsunami risks, local or distant (especially given that (1) the CSZ locked zone is mostly under land here and (2) that the part of the locked zone that is not under land is in shallow water; so our local tsunami will probably be much smaller than further north, like Crescent City or Brookings). I have been involved in tsunami education and outreach for over 15 years and prepared the first tsunami hazard map for northern CA (working with Dr. Lori Dengler and the Redwood Coast Tsunami Work Group). Needless to say, I am cogent and aware about the tsunami risk here in norcal.

https://earthquake.usgs.gov/earthquakes/eventpage/us2000cmy3/executive

SO. I soon discovered that the GA earthquake happened in the Pacific plate, far from the subduction zone and that the earthquake was a strike-slip earthquake. Both of these facts explained why the sheriff had not been at my door earlier this morning. In addition, the magnitude had been adjusted to M 7.9 (no longer a Great earthquake, just a Large earthquake; earthquake classes are defined here). However, there were some small tsunami waves observed (see below) as reported by the National Tsunami Warning Center (see social media below).
This earthquake appears to be located along a reactivated fracture zone in the GA. There have only been a couple earthquakes in this region in the past century, one an M 6.0 to the east (though this M 6.0 was a thrust earthquake). The Gulf of Alaska shear zone is even further to the east and has a more active historic fault history (a pair of earthquakes in 1987-1988). The magnetic anomalies (formed when the Earth’s magnetic polarity flips) reflect a ~north-south oriented spreading ridge (the anomalies are oriented north-south in the region of today’s earthquake). There is a right-lateral offset of these magnetic anomalies located near the M 7.9 epicenter. Interesting that this right-lateral strike-slip fault (?) is also located at the intersection of the Gulf of Alaska shear zone and the 1988 M 7.8 earthquake (probably just a coincidence?). However, the 1988 M 7.8 earthquake fault plane solution can be interpreted for both fault planes (it is probably on the GA shear zone, but I don’t think that we can really tell).
This is strange because the USGS fault plane is oriented east-west, leading us to interpret the fault plane solution (moment tensor or focal mechanism) as a left-lateral strike-slip earthquake. So, maybe this earthquake is a little more complicated than first presumed. The USGS fault model is constrained by seismic waves, so this is probably the correct fault (east-west).
I prepared an Earthquake Report for the 1964 Good Friday Earthquake here.
UPDATES Below is a list of all the reports associated with this earthquake sequence.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5. More about the plate boundary can be found in that report.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 7.9 earthquake, in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. Slab 2.0 is due out later this year!

  • I include some inset figures.

  • In the upper left corner, I place a map created by Dr. Peter Haeussler, USGS, which shows the historic earthquakes along the Alaska and Aleutian subduction zones. I place the epicenter from today’s earthquake as a cyan star.
  • To the right of this map, I include first the USGS map that shows their interpretation of where the fault is (the red line) and then I include the USGS fault slip model (color = slip in meters).
  • In the upper right corner is a map from IRIS that shows seismicity with color representing depth.
  • In the lower right corner, I include a low angle oblique view of the subduction zone, showing how the Pacific plate is subducting beneath the North America plate.
  • In the lower left corner, I include a map that shows the magnetic anomalies in the GA region. I include USGS seismicity from 1918-2018 for earthquakes M ≥ 5.5.


  • UPDATE 12:45 my local time
  • The USGS updated their MMI contours to reflect their fault model. Below is my updated poster. I also added green dashed lines for the fracture zones related to today’s M 7.9 earthquake (on the magnetic anomaly inset map).


  • These are the observations as reported by the NTWC this morning (at 4:15 AM my local time).


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  • Here is an educational video from IRIS about the tectonics in Alaska.

Some Relevant Discussion and Figures

  • Here is a map for the earthquakes of magnitude greater than or equal to M 7.0 between 1900 and 2016. This is the USGS query that I used to make this map. One may locate the USGS web pages for all the earthquakes on this map by following that link.


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  • Here is a cross section showing the differences of vertical deformation between the coseismic (during the earthquake) and interseismic (between earthquakes).


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  • Here is a figure recently published in the 5th International Conference of IGCP 588 by the Division of Geological and Geophysical Surveys, Dept. of Natural Resources, State of Alaska (State of Alaska, 2015). This is derived from a figure published originally by Plafker (1969). There is a cross section included that shows how the slip was distributed along upper plate faults (e.g. the Patton Bay and Middleton Island faults).


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  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
  • Strike Slip:

  • Compressional:

  • Extensional:

  • This figure shows what a transform plate boundary fault is. Looking down from outer space, the crust on either side of the fault moves side-by-side. When one is standing on the ground, on one side of the fault, looking across the fault as it moves… If the crust on the other side of the fault moves to the right, the fault is a “right lateral” strike slip fault. The Mendocino and San Andreas faults are right-lateral (dextral) strike-slip faults. I believe this is from Pearson Higher Ed.


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

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


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