Category Archives: Large Earthquakes

Finite-Fault Model for the Apr 25, 2015 Mw 7.9 Nepal Earthquake

According the USGS web site, they have analyzed selected P, SH and long-period surface waves to construct a kinematic model of this large earthquake. The NEIC solution (Lon. = 84.7 deg.; Lat. = 28.2 deg., Dep. = 15.0 km), has a fixed depth at 15 km. The fault plane dips 10° and strikes  295° (just north of west). The maximum displacement is ~ 3m (Figure accessed Sunday 26 April, 2015).

http://earthquake.usgs.gov/earthquakes/eventpage/us20002926#scientific_finitefault

[From the USGS] Cross-section of slip distribution. The strike direction is indicated above each fault plane and the hypocenter location is denoted by a star. Slip amplitude is shown in color and the motion direction of the hanging wall relative to the footwall (rake angle) is indicated with arrows. Contours show the rupture initiation time in seconds.

[From the USGS] Cross-section of slip distribution. The strike direction is indicated above each fault plane and the hypocenter location is denoted by a star. Slip amplitude is shown in color and the motion direction of the hanging wall relative to the footwall (rake angle) is indicated with arrows. Contours show the rupture initiation time in seconds.

The 2013 Great, Deep Sea of Okhotsk Earthquake (Mw 8.3)

On May 24th, 2013 the largest known deep earthquake (~600 km) with Mw 8.3 occurred beneath the Okhotsk sea. Such an event may reshape our understanding of deep earthquakes.  I don’t know many details about the nature of deep earthquakes, I’m wondering if the mineralogical phase change associated with the 660-km discontinuity could contribute to these deep events? The more generally accepted explanation for deep earthquakes is nucleation by a phase transition within the subducted material, for example:

http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2013/20130524.pdf

One interesting thing I found in the poster is that the shaking intensity distribution shows an interesting pattern. Can we use it to learn about the earth structure above the source?

Splay faulting during the 2010, Mw 8.8 Maule, Chile Earthquake

The authors describe their use of off-shore observations to demonstrate the existence of splay faulting in the shallow regions of the Chilean Subduction Zone involved in the Mw 8.8, 2010 Maule, Chile earthquake. A splay fault is a relatively steep fault that connects the plate boundary interface with the seafloor that when activated during a large earthquake, can enhance tsunami excitation. Based on previous work noted in the paper, the most likely location of splay faults is along the boundary between outer and inner wedges, which is where the authors, Liessr and others, observed the seismic activity in the Chile study.

Specifically, the authors deployed a 30-station ocean-bottom seismometer network for three months and analyzed the offshore data in concert with observations from another 33 land-based seismic stations. They used a a 2.5 dimensional velocity model derived from seismic reflections profiles and previous local earthquake studies to locate the aftershocks.

Good data coverage provides a road to good results, which in this case includes several interesting outcomes. The aftershock locations illuminate a 50 km long linear structure extending from the plate boundary interface to the seafloor that coincides with a splay fault outcrop (on top of the wedge). The P-wave speed distribution (estimated from active-source and tomographic results) suggests that the splay fault begun to branch off with an angle of 7⁰-8⁰ from the plate boundary interface at ~20 km depth and ~67 km away from the deformation front. Finally, it’s important to mention that the southern part of the study area it does not appear that the main shock experienced any activity associated with a splay fault.

Please see the papers for details:  http://geology.gsapubs.org/content/41/12/e309.full

Thamer

ERI Moment Tensor Links

The GlobalCMT group is not the only group performing routine moment tensor analysis. The USGS has a suite of methods they apply (teleseismic, regional, W-phase) and the Earthquake Research Institute of Japan also provides both regional (for Japan) and global solutions:

http://wwweic.eri.u-tokyo.ac.jp/WPHASE/global/

and

http://wwweic.eri.u-tokyo.ac.jp/WPHASE/japan/

L.A. Preparing for the next “Big One”

The March 1st, 2015 edition of EOS featured an article about Los Angeles and their recent effort to prepare for a large earthquake in southern California. Over the past year, Lucy Jones a seismologist from the USGS has been working at the L.A. city hall helping to devise a plan to help the city prepare for the next big earthquake. The article focused mainly on discussing what steps the community thought were most important to help minimize damage from future earthquakes. The “Resilience by Design” report released in December focused on retrofitting vulnerable structures, preserving access to water, and preserving telecommunications. To complete the suggested tasks will likely cost billions of dollars. I think this article raises an interesting public issue on how much time and money should be invested in earthquake preparedness measures that although expensive could potentially save thousands of lives and billions of dollars when the next large earthquake inevitably strikes. Should all cities be developing similar preparedness plans?

Reading this article also prompted me to think a lot about earthquake prediction and a book I recently read, Predicting the Unpredictable by Susan Hough.  The book was a very interesting and enjoyable read ( I would highly recommend it and would be happy to let anyone borrow it). It describes the highs and lows felt by the seismological community over the past several decades when there was a hope that we would be able to reliably make short-term predictions about large earthquakes. While reliable short-term predictions may not be possible we can use statistical seismology to provide likelihood of when events will occur.