M 6.8 Gorda plate earthquake 2014/03/10

That was exciting. I counted over 40 seconds of motion in Manila. This is a wake up call for all of us, including me.

Check back here for updates. I am posting some basic material to begin with. We had another earthquake in this region last year. Here is a page i put together for that earthquake.

Don’t forget to get to the USGS online to fill out the “Did You Feel It?” form. The information provided helps geologists and seismologists learn about the ground shaking response for earthquakes in this region. They also can apply this information elsewhere (in some ways).

The Cascadia subduction zone is formed where the Gorda and Juan de Fuca plates subduct northeastward beneath the North America plate. Here is a figure that Alan Nelson put together. I have updated it with material from Jason Chaytor’s 2004 paper.

This is most likely an earthquake in the underlying Gorda plate. The Gorda is losing the battle between the JdF plate to the north and the Pacific plate to the south, both of which are colder, older, and more dense (basically, they form a vise that is squeezing Gorda so much that it deforms internally). This internal deformation results in the formation of left lateral strike slip faults in the southern GP that form on preexisting faults (originally formed at the Gorda rise, where the Gorda plate crust is created).

Here is a map of the epicenter, about 70 km west of eureka:

This is the moment tensor, which shows it is either a northeast striking left-lateral strike slip earthquake or a southeast striking right lateral strike slip earthquake. Given what we know about the regional tectonics, I would interpret this to be a left lateral earthquake. It plots just southwest of the 1980 M 7.2 Trinidad earthquake. There was a focal mechanism earlier that matched this moment tensor and then a later focal mechanism that was incorrect. I have removed both of them as the moment tensor is a more reliable measure of the sense of motion on the fault.

Here is a map showing historic seismicity. The largest circle to the northeast of the epicenter in orange is the 1980 Trinidad earthquake.

Here is a map from Rollins and Stein (2010) showing the faults and tectonics of the Gorda plate. Today’s M 6.9 is probably somewhere in the right step of the dashed fault labeled “B.”

Here is the seismograph from Jamie Shuttmutt (downloaded from here)

Here is the seismograph from HSU dept. of Geology as taken by Jamie Shuttmutt

Here is the seismograph from the UC Berkeley Jacoby Creek seismometer, posted by Lori Dengler:

Here is a map showing the Modified Mercalli Shaking Intensity for the region. The contours and color over land have the same color scale. Note the increased shaking in the Humboldt Bay region. This is probably the result of the underlying material here (sediments vs bedrock).

Here is the shakemap, with colors designating the Modified Mercalli Shaking Intensity (scale bar in the legend):

This is the map of shaking intensity which changes as more people fill out their “Did You Feel It” (link above):

Here is the map from the Southern California Earthquake Center (SCEC). There are several aftershocks in red. There was a M3.3 foreshock. The aftershocks appear to align with the northeast striking faults in this region of the Gorda plate. These are likely triggered earthquakes on different faults than the mainshock (so may not be considered aftershocks, but triggered seismicity).

Here is a primer for the different types of earthquake faults:

These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.
Mw = 5 Trinidad Chaytor

Here is the pager information (shows the potential exposure, economic or human, to the ground shaking).

Category(s): cascadia, College Redwoods, earthquake, education, geology, humboldt, plate tectonics, subduction

17 Responses to M 6.8 Gorda plate earthquake 2014/03/10

  1. … this was very informative, and timely too!

  2. Yeah way to network! Information has been relayed to KMUD and broadcast on the air! 91.1, 88.1, 90.3, 99.5 FM

    Daniel Dempsey says:

    Nicely done, Jay. will use some of this for discussion in Phys Geog come Tuesday’s presentation. Encouraging those students who are excited by such information to sign up for your “Hazards” course.

    Cristie Wold says:

    Kevin shared this site with me! Great info, especially as I was out of town for this one!! Thanks for the good update!

    Suzanne Courteau says:

    Thanks very much for posting this! It helps a lot to know. If you would, would you mind explaining why the depth of the aftershocks ranged from 0.1 km to around 22.8 km? Why, why, why Mister Patton Science Guy?

    (Thanks!)

    • it is interesting that there are these deep aftershocks. the depths may have significant error since they are offshore… but i am not a seismologist and cannot place numbers on this uncertainty. 0.1 km seems suspicious. maybe that was on a small eq and was not really evaluated much.

      the main shock was rather deep, for the gorda crust in this location. possibly rupturing into the upper mantle. the water depth is 3 km at the epicenter. the crust is probably 5-7 km thick there… the usgs depth is set at 16.6 km. that is pretty deep, so it makes sense that some of the aftershocks would also be deep. in recent years we have been observing earthquakes with increasing deeper hypcenters. there was one offshore the Okhotsk peninsula that was ~600 km deep http://earthjay.com/?p=627 . we have also seen large subduction zone (tohoku-oki 2011 and sumatra-andaman 2004) and strike slip (sumatra outer rise 2012 http://earthjay.com/?p=301 ) earthquakes that appear to have ruptured into the mantle. this was unexpected…

    Eileen Weppner says:

    Thanks for being there with the info, Jay!!

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