SAN JOSE, CALIF. -- Scientists working with new data have significantly raised the odds of a major earthquake in the San Francisco Bay Area by the year 2020, the U.S. Geological Survey said yesterday.

The odds are two out of three, or about a 67 percent chance, of the region being struck by a quake of 7 on the Richter scale within the next 30 years along one of four Bay Area fault segments, the USGS said. Two years ago, the USGS assigned a 50 percent probability to a Bay Area earthquake of that size.

The change stems primarily from new data about two sections of the Hayward fault, which runs from Mission San Jose through Berkeley to San Pablo Bay, and a northward extension of the Hayward Fault, called the Rodgers Creek Fault, which runs from San Pablo Bay north, passing just to the east of Santa Rosa. The other fault examined by the panel is the San Francisco Peninsula section of the San Andreas Fault.

The new estimate represents an upward revision of the odds by the Working Group on California Earthquake Probabilities, a 12-person panel of the National Earthquake Prediction Evaluation Council (NEPEC), which advises the USGS.

Lloyd Cluff, a member of the working group, said the odds are "significantly greater" than they were in 1988. "We have to be careful that those who came through unscathed on Loma Prieta earthquake don't become overly confident they can survive anything," he said.

Thomas V. McEvilly, chairman of NEPEC, said the probability figure should serve as a reminder that the danger of a major Bay Area earthquake has not dropped because of last October's quake.

"The tendency is to think 'Now it's over,' that the chances are reset to zero for another 50 years," he said. "That's not the case at all. The point is that the probabilities are as high or higher for another earthquake as they were before Loma Prieta."