Earthquake, San Francisco Bay area, causes shaking but little damage

A light earthquake hit the southern San Francisco Bay area Friday afternoon, triggering a few false fire alarms but causing no significant damage.

The magnitude 4.1 quake occurred along the Calaveras fault, with an epicenter located 13 miles east southeast of San Jose in the San Felipe Hills.

The Calaveras fault is one of several that slice through the bay area and are capable of damaging earthquakes. These faults are related to the region's granddaddy, the San Andreas fault, which marks the boundary between two large, moving patches of Earth's crust – the Pacific and North American plates.

The quake struck at 4:10 p.m. Pacific time. Slightly more than 8,000 residents in the region, including people in San Francisco, which is some 60 miles north of the quake's epicenter, reported shaking to the US Geological Survey's "Did You Feel It?" website.

"Everything just rolled and jiggled in our office," said Petra Albrecht, a coroner investigator for the Santa Clara County Medical Examiner-Coroner's Office, in an interview with the San Jose Mercury News.

Magnitude 4 quakes on the Calaveras fault are common and tend to do little more than vigorously rattle windows and dishes, seismologists say.

Indeed, researchers classify the fault as "weak," requiring relatively small buildups in stress to trigger quakes. This is what makes it one of the most active faults in the northern San Andreas fault network, geophysicists say.

But the fault also is capable of more-damaging temblors.

In 1911, a magnitude 6.5 quake along the Calaveras knocked down chimneys and cracked brick walls in several communities, including San Jose and Santa Clara. One brick structure at the Lick Observatory on Mt. Hamilton was so badly damaged it had to be replaced.

A magnitude 6.2 quake struck in 1984, causing at least $10 million in damage. The fault delivered another sharp jolt to the region in 2007, when a magnitude 5.6 quake occurred.

Some 92 percent of all the temblors since the 1984 quake have occurred on a 21-mile segment of the 80-mile-long fault, seismologists conclude from measurements. This portion includes the epicenters of the 1911 and 1984 quakes, located near a city called Morgan Hill.