SAN FRANCISCO, California (CNN) -- A 4.2-magnitude earthquake shook the San Francisco area Friday at 4:42 a.m. PT (7:42 a.m. ET), the U.S. Geological Survey reported.
Under the USGS classification, that magnitude of quake is considered "light," which it says usually causes minimal damage.
"We had quite a spike in calls, mostly calls of inquiry, none of any injury, none of any damage that was reported," said Capt. Al Casciato of the San Francisco police. "It was fairly mild."
The quake was centered about two miles east-northeast of Oakland, at a depth of 3.6 miles, the USGS said. Oakland is just east of San Francisco, across San Francisco Bay.
An Oakland police dispatcher told CNN the quake set off alarms at people's homes. The shaking lasted about 50 seconds, said CNN meteorologist Chad Myers.
According to the USGS, 4.2-magnitude quakes are felt indoors and may break dishes and windows, and overturn unstable objects. Pendulum clocks may stop.
