Quake Prediction Is Too Strong, Expert Says : Seismology: He says the science of forecasting is too unreliable to warn of a 47% chance for a magnitude 7 temblor in Southern California in the next five years.

A top earthquake expert who was among those who recently assigned a 47% chance of a magnitude 7 or larger earthquake hitting Southern California in the next five years now says that figure is too high.

Thomas H. Heaton, who last November spoke on behalf of a panel of experts that made the prediction, said Friday: “We haven’t come to the point where we can rely on these numbers with any degree of reliability. . . . We should have said: ‘We don’t know what the number is.’ ”

Heaton’s comments came during a sharp debate at a meeting in Pasadena of the California Earthquake Prediction Evaluation Council.

The director of the state Office of Emergency Services, Richard Andrews, and the state geologist, James F. Davis, defended the probabilities, taking issue with Heaton, past head of the Pasadena field office of the U.S. Geological Survey.

Another dispute arose at the meeting over Andrews’ and Davis’ plan to issue “imminent” earthquake alerts for magnitude 7 or larger quakes in Southern California if a magnitude 6 earthquake strikes along southern segments of the San Andreas Fault.

Heaton criticized the use of the word “imminent” because he said it connotes that something is going to happen immediately, when scientists cannot be sure.

But Andrews said the state must use terms such as “imminent” if it hopes to prevail on local authorities and the public to go along with disruptions in normal activities, such as the possible closure of schools during alert periods.

The California Earthquake Prediction Evaluation Council is charged with setting the criteria for short-term earthquake predictions, advisory statements and alerts issued by the Office of Emergency Services and the Geological Survey.

Three scientists who are also members of the group joined in Friday’s impromptu exchange.

One, James Brune of the University of Nevada, Reno, joined Heaton in expressing concern that officials have overstated probabilities and may be too anxious to issue “imminent” alerts.

Another, David Jackson of UCLA, said that when the devastating consequences of a big earthquake in Southern California are considered, the state is justified in setting high probabilities and using dramatic language such as “imminent” to capture the public’s attention.

Expressing puzzlement over the debate was Keiiti Aki of USC, head of the Southern California Earthquake Center, a group that did much of the research and writing on the probabilities issued Nov. 30 by the panel of scientific experts commissioned by the Geological Survey and the Office of Emergency Services.

The panel set a range of probabilities, declaring that there was a 5% to 12% chance of a magnitude 7 or stronger quake each year in the near future and up to 47% chance in the next five years. No lower end of the range for the five-year period was given.

Aki said he had asked Davis before the announcement whether he was concerned about the use of a range of numbers rather than one number and the state geologist had told him he was not.

In any case, a range was needed, he said, because, “it is very difficult to come up with a consensus among scientists.”

Heaton said it would have been better had the panel voted on a specific number rather than a range of probabilities. But Davis responded that it would have appeared unscientific to the public if it had been told scientists voted on such a matter.

Heaton said he became concerned the next day when the news media had emphasized the highest numbers, and Brune said he was taken aback when he saw Andrews in a television interview saying the chances of a magnitude 7 or greater earthquake in five years was 50-50.

Andrews said he had rounded off the numbers he had gotten at the briefing and felt that he and other officials had done a good job communicating with the public.