3.3 quake on San Andreas Fault shakes Bay Area

(06-28) 14:08 PDT SAN FRANCISCO -- A small earthquake on the San Andreas Fault rattled California from Santa Rosa to San Luis Obispo on Monday, but appeared to cause no damage.

The tremor struck at 7:47 a.m. 5 miles beneath the ocean northwest of the San Francisco Zoo. The U.S. Geological Survey estimated the quake's magnitude at 3.3.

Two smaller quakes followed - one with a magnitude of 1.2 at 8:05 a.m., and the second at 10:07 a.m. with a magnitude of 2.1.

Ross Stein, a geophysicist at the geological survey in Menlo Park, said that instruments indicated the quake appeared to have a component of "thrusting," where one side moves upward above the other.

The San Gregorio Fault, which cleaves the Earth's crust west of the San Andreas and lies roughly parallel to it, eventually adjoins the San Andreas to the north - perhaps somewhere off Bolinas, Ross said.

"There's probably a certain amount of compression between the two faults, and that would account for the movement in this quake," Stein said. "There seems to be a cluster of seismicity in the area since the 1970s, and although this quake is nothing special, it's nothing to be ignored."

San Francisco residents, from the Sunset and Richmond districts to downtown, said they felt their homes move very briefly.

Gwendolyn Tornatore, spokeswoman for the zoo, said because there were no keepers on duty at the time of the quake, it was impossible to know whether any animals were disturbed.

"I live only six blocks from the zoo and it was scary - a short, sharp jolt," she said. "But it did no damage."

Mary Currie, spokeswoman for the Golden Gate Bridge, Highway and Transportation District, said she was at her office near the span when she felt "kind of a roll through my office building" for no more than two seconds.

"It felt like a quick roll-by, if you will," she said.

BART stopped all trains for a few minutes, then resumed service when it was apparent there was no damage.

E-mail David Perlman at dperlman@sfchronicle.com. Chronicle staff writer Henry K. Lee contributed to this report.

This article appeared on page C - 2 of the San Francisco Chronicle