SAN FRANCISCO -- A previously unknown undersea fault is causing the islands off Southern California to slip northwestward at about a half-inch a year, possibly building up seismic strain for a major earthquake that could affect several Southern California cities, a scientist reported here yesterday.
The fault, which appears to run 20 to 50 miles offshore and parallel to the San Andreas Fault, has been slipping without the usual small quakes to relieve strain. This makes a quake of about magnitude 7 a real possibility, said seismologist David D. Jackson of the University of California, Los Angeles, at a meeting of the American Geophysical Union.
Mr. Jackson said that the new evidence for the fault raises the estimate of overall quake risk from a known fault in Southern California by at least 50 percent. Before this new evidence, the U.S. Geological Survey estimated the risk of a quake of magnitude 7 or higher in Southern California at 60 percent over the next 30 years, or 10 percent within the next five years.