The fault that produced a 4.0-magnitude earthquake in Fremont early Tuesday is expected to produce a major earthquake "any day now" and Bay Area residents should be prepared, a U.S. Geological Survey scientist has warned.
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 were reported near the same location, the largest of which was a 2.7-magnitude at 2:56 a.m.
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.
"We keep a close eye on the Hayward Fault because it does sit in the heart of the Bay Area and when we do get a big earthquake on it, it's going to have a big impact on the entire Bay Area," Brocher said.
While a 2008 report put the probability of a 6.7-magnitude or larger earthquake on the Hayward-Rodgers Creek Fault system over the next 30 years at 31 percent, Brocher said the reality is a major quake is expected on the fault "any day now."
"The past five major earthquakes [on the fault have been about 140 years apart, and now we're 147 years from that 1868 earthquake, so we definitely feel that could happen any time," Brocher said.
Brocher urged residents to take steps to prepare for a major
The USGS shake map shows residents in the areas close to Fremont and Union City experienced light shaking in Tuesday morning's event, while weaker shaking might have been felt in areas as far south as Santa Cruz, up the Peninsula and as far east as Livermore.
Residents throughout the Bay Area reported feeling the quake, with responses concentrated in the East and South Bay, according to the USGS.
Brocher said this morning's 4.0 earthquake was not likely to have much of an impact one way or the other on the likelihood of a major earthquake occurring on the same fault.