UNION CITY -- A 3.0 earthquake struck about half a mile northeast of Union City at 10:22 a.m. Saturday.
There were no reports of damage, but people in 32 ZIP codes reported feeling light to weak shaking, said a United States Geological Survey spokeswoman. Most reports were from Union City and Hayward.
"Usually you don't get damage unless the earthquake is 5.0 magnitude or higher," said Susan Garcia. The temblor was near the Hayward Fault, which extends through the East Bay from south Fremont to San Pablo Bay. More than 2.2 million people live near the active fault, which last had a major earthquake in 1868.
"There's a 31 percent chance of a 6.0 or greater earthquake occurring on that fault," Garcia said. "This serves as good reminder that earthquakes do happen in this area, and we should be prepared."
The USGS collects reports from residents at www.earthquake.usgs.gov.
"It's a wonderful tool for readers to bookmark and go on and report what they feel after an earthquake. In the event of a large earthquake, that will help responders go to areas that have the most damage," she said.
Saturday's quake also is a reminder that the 25th anniversary of the Bay Area's last major earthquake, Loma Prieta, takes place in October. That 6.9-magnitude earthquake had its epicenter in the Santa Cruz Mountains but caused damage throughout much of the Bay Area.
Contact Rebecca Parr at 510-293-2473 or follow her at Twitter.com/rdparr1.