Fault running from San Diego to LA could cause 7.3 magnitude quake

Posted March 10, 2017

An natural disaster fault running from San Diego Bay to Los Angeles is capable of producing a magnitude-7.4 quake that could affect some of the region's most densely populated areas, according to a study released Tuesday.

For the study, U.S. Geological Survey postdoctoral fellow Valerie Sahakian, and colleagues used sonar imaging to map the fault lines of the system that run from Los Angeles to San Diego.

Southern California could be overdue for a major natural disaster along the Grapevine north of Los Angeles, according to a sobering new study by the U.S. Geological Survey.

Officials forecast a 16 percent chance that a magnitude 7.5 or larger quake will occur on the southern San Andreas Fault in the next 30 years.

The fault system most famously hosted a 6.4-magnitude quake in Long Beach, Calif. that killed 115 people in 1933.

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Researchers processed data from previous seismic surveys, and supplemented it with high-resolution underwater topography data gathered offshore by Scripps researchers between 2006 and 2009 along with seismic surveys conducted aboard Scripps research vessels in 2013. The fault is broken into four main strands separated by three so-called stepovers, or horizontal breaks that are less than 2 kilometers wide.

In the event of the theoretical 7.4 quake, both the the Newport-Inglewood and Rose Canyon faults would have to rupture. That natural disaster was an especially large one-magnitude 7.9-and its severity could explain why the fault has been fairly inactive since then, as seismologist Egill Hauksson tells the Los Angeles Times.

'We are fortunate that seismic activity in California has been relatively low over the past century, ' said Tom Jordan, Director of the Southern California Earthquake Center and a co-author of the study. The theoretical "Cholame" epicenter of the 1857 quake is marked with the large red dot.

There has been a long drought of major earthquakes on the southern San Andreas fault, which has slowly been accumulating strain as the Pacific plate grinds northward against the North American plate. The smaller the stepover, the more likely that a rupture of one fault will be able to jump to the next. Geological evidence of ancient earthquakes suggests that the fault has ruptured between three and five times in the past 11,000 years.

'The response will be orders of magnitude larger than Hurricane Katrina or Super Storm Sandy, ' said Lt. Col. Clayton Braun of the Washington State Army National Guard.