Quake Prediction Error Can’t Shake 2 Scientists’ Core Beliefs : SCIENCE FILE: An exploration of issues and trends affecting science, medicine and the environment

In April, Charles G. Sammis, chairman of the USC geology department, said he expected a magnitude 6.0 to 6.5 earthquake to occur in Central California by July 9. He based his prediction on an upsurge since 1987 of quakes above magnitude 3.0.

The earthquake didn’t happen, one in a long line of failed quake prognostications.

“The fact that the window ran out shouldn’t confuse people,” Sammis said a bit defiantly in a later interview. “Seismic activity is still increasing in the area. There is every reason to believe there will be a moderate earthquake.

“But,” he acknowledged, “the prediction was a failure.”

Some question whether Sammis should have made his prediction public. Allan Lindh, a leading earthquake scientist with the U.S. Geological Survey, said after Sammis’ views were reported this spring that there was no more than a 5% probability that the predicted quake would occur in the period stated.

Lindh contended that Sammis should have put his theory before the California Earthquake Prediction Evaluation Council, consisting of about a dozen quake experts from throughout the state. “It would have been analyzed, and the conclusion would have been that there wasn’t a sound enough basis for it,” he said. “Not enough research had been done.”

But Tom Henyey, director of the Southern California Earthquake Center at USC, said Sammis deserved credit, not blame, for making the prediction.

“The methodology that’s being applied here is still in a research mode, and it’s going to take some time testing various earthquake patterns here in California and elsewhere in the world before we can determine whether this approach has any merit,” Henyey said.

“I think it’s to Sammis’ credit that he’s willing to step forward and make a public prediction, while the technique is still in its infancy. I think the public wants to know how science is done, and they’d rather not be kept in the dark. . . . The public is intelligent enough to sort out issues of this type.”

Sammis and graduate geophysicist David D. Bowman had plotted the upsurge in bursts of mild quakes on a graph that they said represented an accumulation of strain in an area including San Luis Obispo, Paso Robles and Parkfield, the hamlet where a magnitude 6 earthquake has been predicted to hit since 1985.

But in the interview after the prediction failed to occur, Sammis and Bowman acknowledged that the quake record had anomalies that should have alerted them to the possibility of error.

“If we made a mistake, it was that we were overly optimistic [about] how well we could constrain the dates,” Sammis said. “We were a bit naive.”

As with weather, he noted, earthquakes may reflect “a touchy physical system” in which “if you change a little something, it affects everything.”

Besides, he said, the assumption that there are firm recurrent intervals between earthquakes may not be valid. “It’s more of a chaotic phenomenon,” he said.

But, Bowman insisted, “we still think [the quake] is going to happen.”

Lindh responded: “It certainly will happen. Quakes are inevitable in California. Any community in coastal California has at least a 1% to 2% chance a year of having strong ground motion.”

But even if the quake had occurred by July 9, it might have been coincidence, he said. “It wouldn’t have proved that what they had done was right.”