Earthquake rattles Bay Area with more than 30 tremors striking area

The area has a history of earthquake swarms, when quakes repeatedly strike a small area

A series of small earthquakes rattled the San Francisco Bay Area on Monday morning in an area that has had a lot of seismic activity in recent months.

The most powerful of Monday's quakes was a magnitude 4.2 that struck shortly after 7 a.m. just south of San Ramon, according to the U.S. Geological Survey.

At least a dozen other smaller quakes struck in the same area starting around 6:30 a.m. and continuing for more than an hour.

There were no immediate reports of major damage. Bay Area Rapid Transit said delays could result from trains briefly running at reduced speeds during routine track safety inspections.

Shaking was felt more than 30 miles (48 km) away in San Francisco and across the East Bay cities of Oakland and Richmond.

San Ramon has been shaken by multiple recent earthquakes. The Contra Costa County area recorded 87 quakes at magnitude 2 or above in November and December, according to a San Francisco Chronicle analysis of USGS data last month.

The area has a history of earthquake swarms, when quakes repeatedly strike a small area, and don’t follow a typical mainshock-aftershock sequence, according to seismology experts.