Morning earthquake reveals newfound fault

A small earthquake that rattled parts of the Bay Area Monday morning occurred on a previously unknown fault, according to Jack Boatwright, a geophysicist with the U.S. Geological Survey.

The magnitude 4.3 quake hit at 10:40 a.m. and was centered about 11 miles north of Morgan Hill and 16 miles east-southeast of San Jose, according to the USGS.

Personnel at several police departments and the USGS said they had not received reports of any damage. The trembling was felt mildly by residents in various parts of the Bay Area, including in San Francisco.

Boatwright said the small, unmapped fault that appears to have produced the earthquake is believed to be a “splay fault” off the Calaveras Fault, which is itself a branch off the San Andreas Fault. The fault crosses the hills east of San Jose, an area that is difficult to map.

Because the new fault is not considered a major fault, experts at the USGS do not believe this morning’s quake is a precursor to a larger temblor, Boatwright said.

Visit earthquake.usgs.gov for more information.

Earthquake ground motion is highly influenced by the epicenter and underlying soils. The USGS website shows the variance in ground motion from the 1906 quake.

“Just a reminder to all of us to get our emergency supplies current,” Bob Moss, a resident of the Barron Park area of Palo Alto, told neighborhood organizations in an e-mail this morning. Moss did not feel this morning’s quake.