Monthly Archives: December 2013

Landslide or Fault Tip?

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there were some early reports that used the fault tip movement as a measure of the slip on the fault during the 11 march 2011 Tohoku-Oki earthquake. amazing measurements made with hi res bathy data.
i saw a phenomenal 3-D view of the high resolution bathymetry in that region at the AGU meeting last week. it appears as though this previous interpretation of the fault tip may instead be a landslide. based on the 3-D view, it appears that the rupture tip was further up slope on the accretionary prism and did not extend to the trench in the latitude where the landslide is (though it did reach the trench further south, where there was less slip).
there were some submarine benchmarks that contributed to these estimates of slip, but it appears as the slip estimates that include the landslide as a measure are probably over estimates. the slip (>50 m) was large, but given the large tsunami, people accepted these interpretations. i now question these larger slip estimates with this new information. also, the landslide may be consistent with the large tsunami run ups there. we will need to see what the tsunami modelers come up with when they account for this new interpretation.
based on bathymetry and seismic reflection data offshore, Cascadia does not have these type of landslides in the recent past (in the last few 100’s of thousands of years), so probably is not at risk of these larger localized tsunamis. what was once interpreted as a landslide (the “Humboldt slide”) is not a landslide, but a series of steps formed by sedimentation and erosion.
Here is an article that discusses this from a tsunami and slip modeling perspective.

M 4.1 Gorda plate 2013/12/17

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Good Morning Humboldt County!
This earthquake awakened me early today after just a couple hours sleep. Very exciting and another wake up call to make sure I am prepared for a Cascadia subduction zone earthquake. The Cascadia subduction zone is formed where the Gorda and Juan de Fuca plates subduct northeastward beneath the North America plate.
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 that form on preexisting faults (originally formed at the Gorda rise, where the Gorda plate crust is created). We recently had an earthquake (just ~2 months ago) that also was in the Gorda plate. There are some good maps that show the different tectonic models used to explain this internal deformation.
The moment tensor suggests a slightly compressional earthquake (probably on a northeast striking fault). The focal mechanism supports this in that it shows oblique slip (combination of strike slip and compression). These are interesting in that they are rotated slightly clockwise compared to other earthquakes in this region. This may be due to the deep hypocenter, where the Gorda would be compressing due to the bending (where it would be extensional in the shallow part of the plate, extending due to the bending).
Here is the USGS page for this earthquake.
Here is the updated moment tensor:

Here is one focal mechanism:

Here is another focal mechanism:
Here is the USGS page that describes what a moment tensor is.
Here is a USGS web page that discusses what a focal mechanism is and what it can tell us about the earthquake it was constructed from.
Here is their graphic that illustrates how to interpret focal mechanisms.

This is a map of the region with historic earthquakes plotted as gray circles. The epicenter of today’s earthquake is orange:

This is a map showing the “did you feel it” plotted on top. Today’s epicenter is the red star in the center of the map.
Here is a map with PGA (peak ground acceleration, in units of %g, where g = gravity, or 9.8 m/s^2):
Here is the record from the HSU benioff seismometer, taken by Vanessa Crandell and posted by the HSU Dept. of Geology:
Here is the record from Jamie Schutmutt’s garage seismometer:

Here is the seismogram from Berkeley’s Jacoby Creek Seismometer (as plotted by Lori Dengler from HSU Dept. of Geology):

Simuelue Swarm 2013/12/01-02

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Here is a series of earthquakes in the region of the 2005 great Sumatra-Andaman subduction zone earthquake. These earthquake epicenters plot very close to the epicenter of the 2005 earthquake.
The island just northwest of today’s earthquake is named Simeulue. The island is located at the southern terminus of the 2004 earthquake and the northern limit of the 2005 earthquake. Records of past earthquakes are recorded in the growth rings (like the rings of a tree) of coral micro atolls (Aaron Meltzner, Belle Philibosian, Kerry Sieh, Danny Natawidjaja, and Richard Briggs have worked on these). This region is called the Simeulue Saddle because not only did the middle of the island not deform vertically during either 2004 nor 2005 earthquakes, but also for past earthquakes (as measured in the coral growth rings). We think this may be a persistent boundary for subduction zone earthquakes along this subduction zone.
These earthquakes are actually not at the saddle, but i thought it was a fun name for the swarm.
Here is the USGS page for the M5.4 earthquake.
Here is the USGS page for the M6.0 earthquake.
Here is the focal mechanism for the M5.4 from 12/2, revealing a thrust or reverse mechanism. The other earthquakes were too small and did not get moment tensors.

Here is the focal mechanism for the M6.0 from 12/1, revealing a thrust or reverse mechanism. The other earthquakes were too small and did not get moment tensors.

Here is a map of these recent earthquake epicenters. Lake Toba, a caldera left behind from the largest volcanic eruption in the Pleistocene, is the lake due east of these earthquakes.

Here is the Modified Mercalli Shaking Intensity from the M6.0 on 12/1:

Here is a map of the historic earthquakes in this region: