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4.0 Bay Area quake occurred along dangerous Hayward fault

March 5, 2012 | 11:16 am


Seismologists said the 4.0 earthquake that rattled the Bay Area on Monday is a reminder of the danger posed by the Hayward fault.

Experts have long said the Hayward Fault is overdue for a major temblor, which could be destructive given it runs under numerous population centers.

The Hayward fault runs along the eastern side of San Francisco Bay and has long been considered one of the most dangerous faults in California. Although it gets less attention than the San Andreas fault, which crosses the city of San Francisco (and extends well into Southern California), the Hayward fault is near population centers in Fremont, Oakland and dozens of other cities.

"The Hayward fault is a very, very complicated fault in terms of having parts of it that move all the time and parts of it that are stuck. And it's the stuck parts that really - that's where the stress builds up," David Schwartz of the U.S. Geological Survey told KQED.

"We know that the Hayward fault is the really important fault in the Bay Area," Schwartz added in an interview with KGO-TV.  "These earthquakes, these 4's, are just an indication of ongoing activity, ongoing stress on the fault. They do nothing to relieve the likelihood of something larger happening."

Four small earthquakes were recorded Monday morning north of San Francisco, but only one was widely felt, according to the USGS.

A shallow magnitude 4.0 earthquake hit the El Cerrito area at 5:33 a.m. 

According to the USGS' "Did You Feel It?" service, the temblor was felt from Santa Cruz to beyond Santa Rosa. It was particularly reported by Oakland, Berkeley and other East Bay communities close to the epicenter. (See map above).

BART temporarily halted service for track inspection but the transit service quickly resumed.

According to the USGS, the epicenter was a mile from East Richmond Heights, two miles from Richmond, four miles from Berkeley and 13 miles from San Francisco City Hall.

There were several smaller temblors before and after in nearby Richmond.

In the past 10 days, there have been two earthquakes magnitude 3.0 and greater centered nearby.


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Image: Hayward fault. Credit: USGS