New Fault Discovered in East Bay
Mon Nov 25, 8:23 PM ET

Tony Russomanno

As the swarm of earthquakes (news - web sites) near San Ramon continued Monday, geologists could see that they were not on the Calaveras Fault, as originally believed.

"They've begun to spread and grow out in the east and northeast direction, perpendicular to the Calaveras fault," said United States Geological Survey geologist Dr. David Schwartz.

The swarm is on a previously unknown fault -- one of thousands believed to lie undiscovered around the Bay Area. A similar fault produced a swarm of quakes near Alamo in 1990.

The perpendicular fault is caused by the Calavaras trying to meet up with the Concord Fault on the other side of Mt. Diablo. Geologists say a swarm typically stops after a few days or weeks. But that's little comfort to a rattled East Bay.

"Earthquakes every 12 hours get a little unnerving after a while," said San Ramon resident Delphine Langille.

The ground started shaking Sunday morning near San Ramon. The strongest temblors were at 3.9 and a 3.8. A 3.5 hit Monday morning, with aftershocks of 2.8 and 1.9.

The northern segment of the Calavaras Fault near San Ramon has not had a sizable earthquake (news - web sites) since the USGS (news - web sites) has been keeping records. The central segment produced a magnitude 6.1 earthquake in 1984, which severely damaged homes around Anderson Reservoir in Morgan Hill. The current San Ramon swarm is giving people only a taste of what is possible.

"It's pretty scary," said Danville resident Georgie Daring. "I'm apprehensive, wondering if the bigger one is yet to come."

Scientists say a bigger one is coming, but maybe not right away. There's a 70% chance of a major quake somewhere in the Bay Area within the next 30 years. For the northern segment of the Calaveras Fault, the odds are placed at 10%.

The fact that it's been seismically quiet means that both sides of the fault are sticking," Schwartz said. "They're locked. The stresses are building up."

• New Fault Discovered in East Bay