Magnitude 4.0 earthquake rattles San Francisco Bay area

SAN FRANCISCO (Reuters) - A 4.0 magnitude earthquake shook the San Francisco Bay area on Tuesday and spooked residents, although officials said there were no immediate reports of serious damage or injuries.

The quake struck around 2:40 a.m. local time and was centered just outside of Fremont, some 30 miles southeast of San Francisco, according to the U.S. Geological Survey.

A series of about a dozen smaller quakes followed, with magnitudes ranging from 1.0 to 2.7 through the early morning hours, the USGS said.

The shaking caused brief delays across the Bay Area Rapid Transit train system as workers checked the tracks for damage, the system said.

"The glasses in the kitchen started clattering, which woke me up," Elsi Wu, a visitor to San Francisco, told local broadcaster KRON4. "It was terrifying."

Last August, the bay area's key wine-producing town of Napa was struck by a 6.0 magnitude earthquake that injured more than 100 people, damaged historic buildings, set some homes on fire and caused millions of dollars' worth of damage.

California, which sits along a series of seismic faults, is forecast to experience a much more powerful earthquake at some point, but scientists do not know when it might come or how strong it might measure.

(Reporting by Curtis Skinner; Editing by Susan Heavey)