5.6 Magnitude Earthquake Shakes San Jose Area

October 31, 2007

An magnitude 5.6 earthquake struck the San Francisco Bay Area last night, around 8 p.m., about 9 miles northeast of San Jose, according to the U.S. Geological Survey Earthquake Hazards Program. Ten aftershocks were felt, the largest with a magnitude of 2.1.

The event briefly halted local train services and prompted residents in downtown San Jose, Silicon Valley’s biggest city, to flee apartments. Local and state authorities responded to a number of calls concerning gas leaks, disrupted phone service, and broken water pipes, but little other quake-related damage was reported.

According to Air Worldwide, the earthquake was the largest earthquake to strike the San Francisco Bay area in nearly two decades.
“Though Tuesday’s earthquake, which occurred at a depth of 5.7 miles, posed little threat to property, it was felt over a wide region, including San Francisco, 50 miles north of San Jose, Stockton, 50 miles to the east, and Redwood Shores in Redwood City, 25 miles to San Jose’s north,” said Jayanta Guin, senior vice president of research and modeling. “This span of Northern California is home to eight major faults. The epicenter of Tuesday’s event was close to the intersection of the northern and central segments of the Calaveras Fault, which extends for 76 miles, splaying from the San Andreas Fault near Hollister and terminating, at its northern end, near Danville. It is one of the most geologically active and complex faults in the San Francisco Bay area. Its central segment, in particular, is characterized by abundant microseismicity, reflected by a high rate of seismic creep.”

According to the USGS, the central Calaveras Fault is capable of producing earthquakes no larger than M6.2. Historically, this fault has been the source of several moderate-magnitude earthquakes, starting in 1949 and ranging from M5.1 to M6.2. A similar sequence of earthquakes occurred between 1897 and 1911. Since 1858, eight earthquakes exceeding M5.0 have occurred within a 20 mile radius of Tuesday’s event.

Source: USGS, Air Worldwide

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