The San Andreas fault has rumbled much more frequently over the last 700 years than commonly believed, and the next major earthquake could happen soon, researchers have found.
"If you're waiting for someone to tell you when we're close to the next San Andreas earthquake, just look at the data," said University of California-Irvine seismologist Lisa Grant Ludwig, in a statement.
A report she and other researchers have written is due out early next month in the journal Geology.
UC Irvine and Arizona State University researchers charted earthquakes going back 700 years by digging trenches and taking charcoal samples. The research team found quakes have occurred on one portion of the fault--the Carrizo Plain--every 45 to 144 years, which is much more frequent than the widely accepted belief that an earthquake occurs every 250 to 400 years, and also more frequent than the 235-year average used in recent seismic hazard evaluations.
Ms. Ludwig said the finding "puts the exclamation point" on the need for California's residents, legislators and regulators to be ready.
The results of the study have been available to the scientific community for several years, said Mary Lou Zoback, vice president of earthquake risk applications for Risk Management Solutions (RMS). The new likelihoods "are already incorporated in our models," said Ms. Zoback.
Researchers said their findings concluded not all earthquakes were as strong as had been thought, but they still ranged between magnitude 6.6 and 7.9.
However, the last big quake happened 153 years ago--a 7.8 magnitude earthquake at Fort Tejon, Calif. It was one of the strongest earthquakes ever recorded in the United States and left a long rupture scar along the fault, according to the Southern California Earthquake Data Center. Yet only two people died as a result of the temblor as California at that time was sparsely populated.
Standard homeowners, renters and business insurance policies do not cover damage from earthquakes. Coverage is available either in the form of an endorsement or as a separate policy. The California Earthquake Authority, a privately funded organization, writes the most earthquake insurance in the country. Only about 12 percent of California residents currently have earthquake coverage, down from more than 33 percent in 1996, according to the Insurance Information Institute.