Category Archives: plate tectonics

Gorda plate earthquake animations

I have put together a couple animations of the earthquake epicenters preceding, during, and following the recent earthquake swarm in the Gorda plate. Here is the first page I put together for this M 6.8 earthquake. Here are some maps I put together to explain the epicenter pattern and how it might relate to faults in the Gorda plate.

There was also a M 5.2 extensional earthquake in the Gorda rise (where the Gorda plate is created, it is like the Mid-Atlantic Ridge). Looking at the animations below, one may notice that there were a couple earthquakes in the region of this M 5.2 prior to the 2014 March Gorda Earthquake Swarm. While it may be considered that the Gorda earthquakes triggered the M 5.2, it is evident that the fault that slipped during the M 5.2 was probably getting ready to rupture. This is true for most triggered earthquakes. The change in stress on faults due to loading from other earthquakes is very very small compared to the amount of stress necessarily applied to a fault to cause rupture. In other words, a fault needs to be close to earthquake failure in order to be triggered by another earthquake. Also, that these triggered earthquakes are limited in time because the change in fault stress is short lived. The 2010 Rollins and Stein paper (downloadable here) discusses some coulomb stress modeling they did following the 2010 M 6.5 earthquake in the Gorda plate. Here is a paper that discusses how a Cascadia subduction zone earthquake could trigger an earthquake on the San Andreas fault.

I used the USGS website to download the epicenters in this region. One of the options is to export the epicenters as a google kml file. These files display in google earth.

Click on the maps to open/download the video in a new window.

Here is an animation of the region offshore northern California for the time range 2014/1/16 through 2014/3/23. (~23 MB avi)

Here is an animation zoomed into offshore of Humboldt County. The USGS fault and fold database show the faults. There are also plate boundary faults plotted (they are less certain and less precise). (~28 MB avi)

Here is an animation of the earthquake swarm itself. Note that there are some epicenters that strike at the same instance as the mainshock. These are probably all ruptures on the same fault. There are many other aftershocks that are not on the same fault, but synthetic (trend in a parallel or subparallel direction as the main fault) or antithetic faults (trend in a conjugate direction). In some cases there are also aftershocks that occur at the same instance and apparently align along another fault (they line up). For faults close to the mainshock fault, these aftershocks may actually be on the mainshock fault but are plotted incorrectly due to the assumptions made in the location inversions (i.e. velocity models). People like Mark Williams or Anne Trehu (among others) may want to do some double differencing to these data to get better locations for these earthquake hypocenters.

Here I have slowed down the animation to better visualize the temporal relations between bursts in seismic energy.

Finally, here is an animation of the western USA that reveals something about the random nature of background seismicity. There are a few locations that show patterns that suggest faults ruptured. The recent Gorda plate earthquake swarm is one example. There were also a series of epicenters that plot in a northeast strike/trend just west of the mouth of the Mattole River. There also appears to be a swarm of earthquakes in the Yellowstone region, the Geysers (CA), and Salinas, CA.

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 focal mechanism, which shows it is either a northeast striking left-lateral strike slip earthquake or a southeast striking right lateral strike slip earthquake. This graphic shows that it is compressional and could be classified as an oblique 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.

This is the moment tensor, which shows it to be probably more clearly strike slip:

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:

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

Earthquake Catalog Search Tool

Here is the USGS earthquake catalog search tool website.

There are many useful choices one can make when they are searching for records of past earthquakes.

The basic search options are simple and obvious (the time and space).
The advanced search options provide some other constraints like depth, azimuthal gap, review status, event type, impact on people/buildings/shaking intensity, catalog, contributor, and product type.

There are several types and formats of output data.

    Basic Options:

  • These are pretty straightforward, right? Lat and Long coordinates need to be formatted in decimal degrees (e.g. 90.3667). Time is Universal Time (Greenwich Mean Time).
    Advanced Options:

  • Depth, Gap, and Review:
    Sometimes people are interested in earthquakes of a particular depth or “azimuthal gap.” Azimuthal gap relates to the geometrical relations between the seismometers and the earthquake hypocenter.
  • Event Type:
    This allows one to discriminate their search results with regard to whether the record is the result of an earthquake or some other source. Not all records may have these attributes (i.e. “not reported”.
  • Impact:
    This allows one to restrict their search results to earthquakes that had a particular impact upon people, their infrastructure, or “Did You Feel It” Surveys.
  • Catalog:
    There are many different catalogs and they are managed for different reasons. Some catalogs include records that are processed differently than others, with different criteria (certainty, quality, or otherwise). Therefore, the catalogs that have a higher standard for some aspect of these records would have a smaller amount of records. The catalogs that do not have any discrimination criteria therefore are larger (more records).
  • Contributor:
    There are several large academic and govt groups that operate the seismometer networks that contribute to the records in these catalogs. Catalogs may have records from many different contributors, others may not.
  • Product Type:
    This is a list of the possible types of data that will be represented by the output data.
  • Output Options:
    The main options (format) that I might use are CSV (comma delimited text file that could be imported into a GIS and converted to a geodatabase or shapefile) and KML (that could be opened in google earth). The color (by age or depth) and layer ordering (time or magnitude) can also be important if one is trying to tell a particular story regarding age, depth, or magnitude.