August 21, 2010 in Nation/World

Southern California ‘Big One’ overdue

Study finds large quakes undercounted
Rong-Gong Lin Ii Los Angeles Times

Sarah Robinson, 23, a graduate student at Arizona State University, runs along a trench at Bidart Fan Site on the Carrizo Plain north of Los Angeles. She is a member of a team of geologists who are trying to construct a history of earthquakes.
(Full-size photo)

LOS ANGELES – Southern California is long overdue for a major earthquake along the San Andreas fault, according to a landmark study of historic seismic activity released Friday.

The study, produced after several years of field studies in the Carrizo Plain area about 100 miles northwest of Los Angeles, found that earthquakes along the San Andreas fault have occurred far more often than previously believed.

For years, scientists have said major earthquakes occurred every 250 to 450 years along this part of the San Andreas. The new study found big temblors on the fault every 88 years, on average.

The last massive earthquake on that part of the fault was in 1857, leading scientists to warn that another such temblor is likely in Southern California.

“The next earthquake could be sooner than later,” said Lisa Grant Ludwig, a University of California, Irvine, earthquake expert and co-author of the study, which was published online in the journal Geology. “It was thought that we weren’t at risk of having another large one any time soon. Well, now, it might be ready to rupture.”

Other seismic experts described the revelation as a major change in the way they think about earthquake risks along the southern San Andreas fault.

Thomas Jordan, director of the Southern California Earthquake Center, said the fault is “locked and loaded. It’s been a long time since an earthquake has occurred on that fault – over 150 years.”

To reach the new conclusion, scientists dug trenches deep into the Carrizo Plain. They used carbon dating and sophisticated imaging technology known as lidar to find signs of earth movements. They were able to detect earthquakes dating back to the 15th century, creating a far more complete record than had previously been known.

The research found that earlier examinations of the San Andreas had badly undercounted the number of major earthquakes. Those were based on observations made in the 1970s when scientists used measuring tape to look for evidence of past earthquakes.

“Now we have better techniques,” Grant Ludwig said. “We can see there’s actually more earthquakes.”

Scientists now estimate that earthquakes occurred on that section of the fault in 1417, 1462, 1565, 1614 and 1713.

The finding adds weight to the view of many seismologists that the San Andreas has been in a quiet period and that a major rupture is possible. A 2009 study, which Grant Ludwig also participated in, suggested that the San Andreas was overdue for a rupture. But Friday’s report offers a much more grim estimate of how frequently quakes have occurred on that segment of the fault.

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