Two faults may be connected, USGS says

The Calaveras and Hayward faults may be connected and could generate even bigger earthquakes than previously feared, according to a geologist participating in the American Geological Union meeting this week in San Francisco.

The Hayward Fault’s destructive capabilities were realized in 1868 when it shook so violently that it leveled several East Bay communities and San Francisco with a magnitude 7 quake.

On Oct. 30 a magnitude 5.6 quake shook the Calaveras fault beneath San Jose, the largest earthquake to hit the Bay Area since the 1989 Loma Prieta quake and a reminder about another dangerous fault, according to the U.S. Geologic Survey.

The two faults run beneath some of the most populated areas of the Bay Area, including Oakland, Berkeley, Hayward, Fremont and San Jose.

The epicenter of the Oct. 30 quake was at a critical point on the Calaveras where scientists now feel confident that the Hayward Fault meets its South Bay cousin, according to USGS geologist Russ Graymer.

“If you’re thinking about the Hayward Fault and the Calaveras Fault, you’re really thinking about one thing,’’ Graymer said. “If you look at this evidence it seems pretty clear.’’

The Alum Rock quake, as the Oct. 30 event is called, broke at a critical junction between the two faults and moved south through San Jose.

The new information is both good news and bad news, according to Graymer.

The two faults, acting as one, could generate larger earthquakes than previously thought but could generate less of them, Graymer said.

Scientists estimate that the Hayward fault snaps every 140 years and 2008 is the 140th anniversary of the 1868 quake, Graymer said. The anniversary is another topic of discussion at the weeklong AGU meeting.

Faults don’t work on schedules Graymer said, but the new information gives scientists new tools to study the probability of upcoming quakes. Scientists have predicted a 25 percent chance of a major rupture along the Hayward Fault in the next 30 years.

“We just don’t have the data,’’ Graymer said about predicting earthquakes.

The AGU meeting continues every day through Friday at Moscone Center in San Francisco.

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