Odds increasing that huge quake will hit California

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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.

In the Bay Area, the evidence shows that a quake with a magnitude of about 6 — the size of August’s Napa quake — is virtually certain to strike within the next 30 years, while the probability that an even larger one with a magnitude of roughly 6.7 is extremely high, or 72 percent, according to the report.

The forecast indicates that quakes with magnitudes equal to or greater than 5 or 6 are certain to strike somewhere in the Northern California region in the next 30 years; a 95 percent certainty that another one roughly the size of 1989’s Loma Prieta — or magnitude 6.9 — will strike within the same period; and 76 percent for a really large one with a magnitude of 7.

As they looked at new evidence for connections between faults in the San Andreas zone, the scientists examined evidence for past movements along the Hayward and Rodgers Creek faults, whose northern and southern strands appear almost joined together beneath San Pablo Bay. Here, too, is an example of a “multifault” quake possibly striking in the future, they said.

David Perlman is The San Francisco Chronicle’s science editor. E-mail: dperlman@sfchronicle.com