SAN FRANCISCO, June 6 (UPI) -- Residents of Contra Costa County east of San Francisco got a reminder early Friday that they live in an earthquake zone when three small quakes hit.
The quakes involved a small unknown fault, the Oakland Tribune reported.
David Schwartz, a geologist who heads the U.S. Geological Survey San Francisco Bay Area Earthquake Hazards Project, said there are many small faults in the hills in the East Bay area. A similar sequence of small quakes occurred in 2006.
The first quake, at 1:15 a.m., registered 3.0 on the Richter scale, the second, at 2:02 a.m., was magnitude 3.5 and the third, at 2:16 a.m., 1.7. There was no reported damage from any of the quakes.
"The hills have been folded and faulted for several millions years, so they're always being stressed and every once in a while a little patch decides to give a light jolt," said Schwartz.
The USGS reported that 782 people called in to report shaking.