August 8, 2011

MIT Researchers Reconstruct Baja’s ‘Easter Earthquakes’

MIT researchers report that they have reconstructed Baja California's 2010 "Easter Earthquakes."

The earthquake that shook Baja California took placed on April 4, 2010, sending tremors throughout a region 40 miles south of the U.S. - Mexico border.

The months after the 7.2-magnitude earthquake triggered aftershocks that shook Los Angeles.

"The southern San Andreas Fault has not had a major earthquake on it since the 1600s," Thomas Herring, professor of geophysics in the Department of Earth, Atmospheric and Planetary Sciences at MIT, said in a statement.

"So every time there's an earthquake down in that region, everyone gets very worried that it's going to rupture northward and continue propagating up. That is a very reasonable fear."

The researchers mapped the earthquake's path by using satellite images, GPS data, seismic recordings and laser altimetry. 

The team found that a smaller tremor occurred 15 seconds before the main 7.2-magnitude shock, challenging conventional wisdom that the strength of an earthquake can be determined in the first few seconds of onset; and the main earthquake ruptured two faults that geologists previously through were inactive.

Geologists observed that as an earthquake ripples through the Earth, it releases stress on some surrounding faults but increases it on others.

"Understanding how stresses in the Earth release and build up could help us understand how one earthquake sets off another over a long time scale," Herring said in a press release. "Getting more refined at understanding these interactions is something I think we can make progress on in the next decade."

The team took advantage of the many data resources there to draw up detailed analyzes of the Easter Earthquake.  The team analyzed seismic data from ground-based sensors spread throughout Southern California and nearby regions of Mexico.

Herring said that typically within the first few minutes of a major earthquake, seismometers become "saturated," and it is difficult to gauge the magnitude or scope of very large earthquakes from seismic data alone.

The researchers were able to draw up a simple mode of how the earthquake propagated from all their data. 

According to the team's analyzes, the quake started as a small tremor along a fault, about 6.2-miles below the surface of the Earth.

Brendan Meade, associate professor of planetary sciences of Harvard University, said the combination of data showed that most of the earthquake activity occurred deep within the fault system.

"The San Andreas Fault lies at the heart of the incredibly complex Southern California fault system," Meade, who was not involved in the research said in a statement.. "This paper shows that accurate characterization of the complex nature of fault systems requires not only field observations, but also inferences from remotely sensed data."

Herring said in the long term, the Easter tremor may be part of a slow tectonic rippled that will inevitably split a seam up the length of California.

"This is a relatively young area, so new faults are still forming," Herring said in press release. "Baja California probably started opening up five to six million years ago, and since then it's sort of been unzipping, working its way up. Los Angeles will probably be toast in about 10 million years."

The group published its findings in the July 31 online edition of Nature Geoscience.


Image Caption: A view looking southeast along the surface trace of the San Andreas fault in the Carrizo Plain. In this image, a road has been cut going through the fault (from left to right). Image: USGS


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