Newfound fault raises chances of major California earthquake

December 06, 1990|By Los Angeles Times

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.

The findings, which have yet to be confirmed officially by the U.S. Geological Survey, would boost those estimates to 90 percent and 15 percent, respectively.

Mr. Jackson and his colleagues found evidence of the fault's existence after analyzing data from an array of ground instruments used in conjunction with space satellites. The research group measured motion along the offshore fault at about a half-inch, Mr. Jackson said.

"It's at least as fast as some sections of the San Andreas Fault," he said. "If you were to have an earthquake the size of the Loma Prieta quake, which is not unthinkable at all, it could cause widespread damage in Los Angeles or San Diego and in many of the towns in between."

That San Francisco Bay area earthquake Oct. 18, 1989, measured 7.1 on the Richter scale.

Baltimore Sun Articles
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.