A seismologist at the University of California at Los Angeles has found evidence of an undersea fault off the coast of Southern California that, if confirmed by the U.S. Geological Survey, could further increase the probability of a major earthquake in the area.

David D. Jackson, a geophysics professor, said an earthquake along the 100-mile fault, which parallels the San Andreas Fault, could affect cities from Santa Barbara to the Mexico border.

The U.S. Geological Suvey estimates the risk of a Southern California earthquake measuring 7 or higher on the Richter scale at 10 percent within the next five years and 60 percent within the next 30 years.

Making allowances for the new fault means geologists' estimates of the total amount of fault movement in Southern California must be "increased by about a third," said Jackson. "This means that for a 30-year time interval, the probability of a major earthquake of magnitude of 7 or above would be increased by about 10 percent."

Researchers found the fault causes the Santa Barbara Channel Islands to slip northwest by about a half-inch a year, Jackson reported at an American Geophysical Union meeting in San Francisco last week.