A small earthquake that rattled parts of the South Bay on Monday morning occurred on a previously unknown fault line, 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. 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 Monday morning's quake is a precursor to a larger temblor, Boatwright said.
Personnel at several police departments and the USGS said they had not received reports of any damage from Monday's quake. The trembling was felt mildly by residents in various parts of the Bay Area, including in Mountain View and as far north as San Francisco.