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Major earthquakes on San Andreas happen more frequently than previously thought

January 23, 2009 |  4:34 pm

Fault Large earthquakes have rumbled along the southern section of the San Andreas fault more frequently than previously believed, suggesting that Southern California could be overdue for a strong temblor on the notorious fault line, according to a new study.

The Carrizo Plain section of the San Andreas has not seen a massive quake since the much-researched Ft. Tejon temblor of 1857, thought to have been about magnitude 7.9, which is considered the most powerful earthquake to hit Southern California in modern times.

But the new research by UC Irvine scientists, to be published next week, found major quakes occurred there roughly every 137 years over the last 700 years. Until now, scientists have believed big quakes have occurred along the fault roughly every 200 years. The findings are significant because seismologists have long believed this portion of the fault is capable of sparking the so-called “Big One” that officials have for decades warned will eventually occur in Southern California.

“That means it’s been long enough since 1857 that we should be concerned about another great earthquake that ruptures through this part of the fault,” said Ken Hudnut, a geophysicist at the U.S. Geological Survey in Pasadena, who was not involved in the study.

Many scientists thought the Carrizo area produced relatively infrequent earthquakes -– but ones on the massive scale of the Fort Tejon temblor.

The new work suggests the area produces more quakes but also ones of a smaller magnitude than Ft. Tejon, said Ray Weldon, a University of Oregon geologist who was not involved in the research, but reviewed the paper for the Journal of Geophysical Research. Still, experts warned, such temblors would likely be at least the size of the 1994 Northridge quake, which had a magnitude of 6.7.

“The most significant thing to come out of the work is recognition that there’s a greater variety of earthquake sizes that occur on the San Andreas,” Weldon said. “But even moderate earthquakes on the San Andreas can cause considerable damage, so the overall hazard and risk has gone up.”

-- Jia-Rui Chong

Photo: U.S. Geological Survey