San Francisco Bay Area rattled as 2 quakes occur 8 seconds apart

SAN JOSE, Calif. — A one-two punch of earthquakes rattled nerves in the San Francisco Bay Area on Monday morning, and scientists say the 3.5 and 4.0 magnitude temblors under the city of El Cerrito were reminders of the danger in an area notorious for its seismic risk.

The double jolts — accompanied by two smaller tremors — were too small to release much energy along a restless stretch of the Hayward Fault. This is the fault line most likely to be the source of the Bay Area's next "Big One," with a 1-in-3 chance of a 6.7 rupture in the next 30 years, according to the U.S. Geological Survey.

While Monday morning's surprise caused no significant damage, it was a welcome reminder even for scientists to be prepared — especially if the soaring price of gas has your needle near "empty."

"I had to pop in my car and drive to work — with almost no gas," said Keith Knudsen, deputy director of the Earthquake Science Center at Menlo Park's USGS, who was eating breakfast at his Albany home when the 5:33 a.m. PDT temblor struck. "If there's a big earthquake, with loss of power, it's good to have more than fumes."

Witnesses described feeling a small precursor to the larger quake, an observation later confirmed by the USGS.

The epicenter was five miles below El Cerrito's Mira Vista Golf & Country Club, where Superintendent Tim Youngberg was meeting with the greens staff in the pro shop when the earthquake struck.

"First it was a little shake, a pause, then a big one after that. A pretty good jolt," Youngberg said. After finding no damage, he said "everything's good now. We had a good laugh about it."

USGS's David Schwartz said that the 3.5 quake was a foreshock that occurred 8 seconds before the 4.0 quake. This was followed by a 2.0 aftershock at 6:03 a.m. and a 1.1 shaker at 6:29 a.m.

This El Cerrito-based patch of instability is familiar to geologists, who think it is a spot where different types of rocks get stuck — then suddenly release.

A similar locked-up stretch of the Hayward Fault is found under Berkeley, which last fall experienced four earthquakes of 3.0 magnitude or greater.

There are other stretches of the Hayward Fault where rocks glide past each other more smoothly, so are less likely to trigger earthquakes, said Knudson.

On Monday in Berkeley, a second-floor apartment on Addison Street was flooded when a pipe to a hot-water heater sheared off, said Deputy Fire Chief Gil Dong. The Berkeley department got about five calls for alarms at commercial buildings, but no other damages or injuries were reported, he said.

Similar smatterings of shelves askew and other minor disarray were reported through a flood of social-media postings, but there were no known reports of serious injury or damage. Bay Area Rapid Transit stopped its trains for five minutes after the earthquake to perform routine track inspections. Riders experienced 10-minute delays.

The 74-mile-long Hayward Fault is part of the greater San Andreas fault system, a zone capable of generating significantly destructive earthquakes through densely-populated areas, including the cities of Richmond, El Cerrito, Berkeley, Oakland, San Leandro, Hayward, Fremont, and San Jose.

In 1868, a deadly quake on the Hayward Fault struck with a magnitude of 7. Scientists say that historic records of the fault's seismic activity show a roughly 160-year "recurrence interval" of large temblors — suggesting it's time to get ready for another "Big One."

There's not a lot of vertical movement along this fault. Rather it is caused by horizontal slippage of two continental plates. If you stood for eons on the western Pacific Plate and looked east, you'd see the Sierras slowly moving south.

Monday's events "are not unusual ... and it doesn't cause us to run screaming for the hills," said Knudson. "But they should cause us to think about the supplies that we need, and to plan for our families."