SAN FRANCISCO (AP) — The 6.0-earthquake that damaged buildings and left scores of people injured in California's wine country was the largest temblor to hit the San Francisco Bay Area since the 6.9-magnitude Loma Prieta earthquake in 1989.
Loma Prieta occurred on the extremely active San Andreas Fault. Seismologists say Sunday's quake near Napa occurred on the lesser-known West Napa Fault, which has not been well-mapped.
"If you had put a bunch of seismologists and geologists together in a room and asked them where the next magnitude-6.0 quake would occur in the Bay Area, this would likely be the fifth fault they would name or the sixth," said Jack Boatwright, a geophysicist with the U.S. Geological Survey. "The amount we don't know is overwhelming."
Here are five things to know about faults in California:
WHAT ARE FAULTS?
— Faults are fractures between two blocks of rock that form the Earth's crust. A big earthquake can result when the blocks move. But faults can be difficult to discover when there is nothing on the surface such as visible fissures to indicate their presence. At least part of the West Napa Fault falls into this category, according to Boatwright. An earthquake such as the temblor that struck Sunday can help scientists spot and study faults. The speed at which one side of the fault slides past the other determines earthquake activity. The fastest moving faults have more and larger earthquakes.
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