New seismic fault revealed near California nuclear plant

SAN FRANCISCO - An unknown seismic fault has been detected on the ocean floor a half-mile from Pacific Gas & Electric Co.'s nuclear power plant in Diablo Canyon in central California. A company report on the discovery says the plant could safely withstand a magnitude- 6.5 quake on the fault.

The fault zone was detected more than two years ago by scientists at the U.S. Geological Survey working with the utility and was promptly reported to the federal Nuclear Regulatory Commission, but its existence did not become widely known until Tuesday.

William Ellsworth, the USGS scientist who developed the method used to determine the fault's dimensions, said it is "not a major fault." The Shoreline Fault Zone runs for 14 miles offshore in three segments, roughly from the vicinity of a coastal feature called Point Buchon northwest of the plant to well out in San Luis Obispo Bay.

Ellsworth said the USGS and the utility have long had a standard cooperative agreement to work jointly on seismic research. He said the fault was found by scientists gathering seafloor data to assess probabilities for major quakes in the Bay Area.

The fault, Ellsworth said, runs vertically about 6 miles beneath the seafloor and is known as a "strike-slip" fault, which means that in its motion one side would slip past the other.

The fault lies about 3 miles inshore from the well-known Hosgri Fault, whose discovery in 1971 by scientists forced PG&E to upgrade the plant's design.