Chances of Major Earthquake in California are Increasing

PHOTO CREDIT: USGS, Esri, John Blanchard, The Chronicle
PHOTO CREDIT: USGS, Esri, John Blanchard, The Chronicle

The chances are increasing that a major quake — far larger than Loma Prieta — will hit California within the next 30 years, while the odds are decreasing that somewhat smaller but still dangerous jolts will strike in the same period, the state’s leading earthquake scientists warned Tuesday.

The probability of a magnitude 8 or larger quake in California, the experts said, has increased from 4.7 percent estimated in 2008 to 7 percent now.

One reason for the increased risk is that the scientists for the first time considered the probability that any two of the thousands of faults in California might rupture simultaneously in a “multifault” quake.

For example, there is now a real possibility that the Bay Area’s ever-dangerous Hayward and Calaveras faults, which have each ruptured separately in the past, could in the future rupture together, said Andrew Michael, a geophysicist at the U.S. Geological Survey in Menlo Park and one of the authors of the USGS report.

The findings resulted from the work of more than 100 seismic scientists who weighed the evidence from studies of more than 350 fault segments in the San Andreas system, which stretches from Humboldt County to the Mexican border, and applied massive new lines of evidence they had accumulated from past quakes since their last official forecast in 2008.

The experts zeroed in on specific California regions where earthquakes pose the most dangers and assessed their probabilities for future damaging quakes.

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SOURCE: SF Gate, David Perlman

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