Panel Sees Less Chance of Major Earthquake : Seismology: Scientists say their estimate of 60% probability in the next 30 years is too high for southern San Andreas fault. They will issue new figures next year.

Predictions of a 60% chance that a major earthquake will strike on the southern San Andreas fault in the next 30 years are too high, and a new probability to be announced next year will be lower, leading members of a scientific panel said Wednesday.

The panel, charged with assessing quake probabilities for Southern California, is divided over the chances of a big quake--magnitude 7 or larger--occurring on any of the other 300 important faults in the Southland. The scientists have delayed a Dec. 2 announcement of Southern California earthquake probabilities at least until March to try to reconcile the differences.

Keiiti Aki of USC, a co-chairman of the panel and science director of the Southern California Earthquake Center, said panel members had tentatively concluded that such a quake was 90% likely on some Southern California fault in the next 30 years. But, he said Wednesday, that estimate was too high and was the source of some disagreement among scientists.

As for the San Andreas Fault, the scientists think a probability lower than the 60% figure issued in 1988 is advisable for two main reasons.

First, Aki said that scientists have concluded that the recurrence of big quakes on the San Andreas historically has been too irregular to justify assigning a 60% probability in a 30-year span.

Of seven quakes of magnitude 7 or larger to shake Southern California since 1811, only one in the 182-year period to 1993 was on the San Andreas. That was the 8.3 quake at Ft. Tejon in 1857.

The second reason for reducing the San Andreas probabilities is that some large earthquakes of past centuries may have been double-counted by researchers because they affected more than one segment of the fault, said the Earthquake Center’s executive director, Tom Henyey. For this reason, single quakes may have been mistakenly recorded as separate temblors.

Fewer quakes in the past translates into a probability of fewer, within specific time spans, in the future, the scientists suggested.

The director of the state Office of Emergency Services, Richard Andrews, said he is pleased with the decision by 40 scientists connected with the panel to delay the announcement of new probabilities.

“I had some real concern about the way things were stated (in a draft of the report),” Andrews said. “There was an opportunity for substantial misinterpretation.”

Aki said he is concerned that scientists may be inclined to overstate the probability of a quake occurring on any single fault, and that when all of the faults of Southern California are combined into one probability figure, the distortion would grow.

Accordingly, he and Henyey said that except for the San Andreas Fault, they may not issue a precise percentage probability for a big quake in Southern California in the foreseeable future. Rather, they may identify the risks in more general terms. Henyey said he thinks that using 90% would be “alarmist.”

The scientists did not say how much they intend to reduce the probabilities for the San Andreas and San Jacinto faults. The San Jacinto, running south and east of San Bernardino through the Hemet area, was assessed a 50% probability of a magnitude 7 or larger quake in the next 30 years.

The panel of scientists, from the U.S. Geological Survey and a number of major universities, was commissioned to reassess the probabilities after the 1992 Landers earthquake awakened fears of an imminent “Big One” in the Southland.

Lucile M. Jones of the U.S. Geological Survey, who has worked with the panel, said Wednesday that determining the overall probability figure has proved to be by far the most difficult part of the mission.

“It’s important to take the time to do it right and that’s what we’re doing,” Jones said. “There are a lot of reasons to think we could have ended up with too high a number. We all agreed on the methodology, but we came out with a number that seemed screwy.”

Meanwhile, panel members said, it appears clear that the probabilities they issued Nov. 30, 1992, stating there was a 5% to 12% chance of a major quake in Southern California each year and up to a 47% chance of it occurring within five years, also no longer apply.

These figures were issued soon after the magnitude 7.6 Landers earthquake had engendered fears of an imminent Big One. Now that a year has passed without anything significant occurring seismically, those estimates have been superseded, the scientists said.