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.
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.
The fault that produced a 4.0-magnitude earthquake in Fremont early Tuesday morning is expected to produce a major earthquake “any day now” and Bay Area residents should be prepared, a U.S. Geological Survey scientist said.
The 2:41 a.m. earthquake on the border of Fremont and Union City occurred on the Hayward Fault at a depth of 5 miles. The epicenter was at a spot just north of the intersection of Niles Canyon Road and Mission Boulevard.
The quake caused some BART delays early Tuesday while work crews checked the tracks, but appears to have caused no major damage. At least 13 smaller quakes or aftershocks had been reported near the same location as of 6:42 a.m., the largest of which was a 2.7-magnitude at 2:56
While damage from the quake was minimal, scientists warn that a much larger one is expected on the Hayward Fault, which extends from San Pablo Bay in the north to Fremont in the south and passes through heavily populated areas including Berkeley, Oakland, Hayward and Fremont.
The last big earthquake on the fault, estimated to have a 6.8-magnitude, occurred in 1868, according to the USGS.
It killed about 30 people and caused extensive property damage in the Bay Area, particularly in the city of Hayward, from which the fault derives its name. Until the larger 1906 earthquake, it was widely referred to as the “Great San Francisco Earthquake.”
"The population is now 100 times bigger in the East Bay, so we have many more people that will be impacted,” said Tom Brocher, a research geophysicist with the USGS.