Even Without Quake, Alert Called a Success : Seismology: Odds grow slim of a Parkfield temblor within experts’ deadline. Officials praise response by state and local agencies.

The director of the state Office of Emergency Services said Thursday that even if no earthquake shakes the hamlet of Parkfield, this week’s prediction experiment was overwhelmingly positive and in similar circumstances such an alert would be issued again.

On Monday night, after a magnitude 4.7 earthquake struck near Parkfield in Monterey County, the state and the U.S. Geological Survey issued an “A level” alert to seven Central California counties warning that there was a 37% chance that a magnitude 6 earthquake would occur within 72 hours. The alert was set to expire at 10:28 p.m. Thursday.

By midafternoon Thursday, the statistical chance for a magnitude 6 quake had fallen to less than 5%, but Emergency Services Director Richard Andrews said “the promptness of the initial alert notification, the response by the local and state agencies, the cooperation and understanding of the media in all these counties and the very high level of public support” made the procedure a success.

The alert was announced 33 minutes after the Monday night temblor in accord with a plan that called for a warning if smaller quakes in the region fit a pattern that scientists believe indicates a pending magnitude 6 quake.

The Parkfield region has had sizable temblors at an average interval of 22 years, more often than any other point along the San Andreas Fault. The last such quake hit in 1966. In 1985, scientists predicted that there was a 95% chance of another one by the end of this year.

At a cost of $19 million, the U.S. Geological Survey has installed extensive instrumentation in the Parkfield area in an attempt to measure what occurs before, during and after such a quake.

“We’re essentially committed to continuing this experiment through January,” Andrews said, “and we will between now and then review the whole effort to see whether we should continue in the same way.”

Even if there is no magnitude 6 temblor by Jan. 1, he said, the experiment may be continued if “the Geological Survey still has the same degree of confidence in the area as a good site for gathering data and feels there is a good chance of predicting an earthquake there.”

Andy Michael, the U.S. Geological Survey geophysicist who oversees the Parkfield program, said that he was not discouraged even without an earthquake occurring during the alert.

“We knew we’d be wrong sometimes,” he said. “If we were wrong 10 times, then maybe we’re doing something wrong. But after this, I don’t know of any immediate changes we could make.”

Michael noted that Monday’s magnitude 4.7 quake was centered three miles southeast of the three known foreshocks to previous magnitude 6 temblors in the Parkfield area. Even so, he said, “I’m not sure we know enough to change” the size of the area where small quakes prompt public alerts.

“If the same thing happens next Monday . . . we would go through the exact same process,” he said.

The USGS recorded eight tiny earthquakes, all less than magnitude 1.5, between 9 p.m. Wednesday and 3:15 a.m. Thursday at shallow depths in the vicinity. These were not of sufficient strength to trigger a new alert.

Meanwhile, in Southern California, two earthquake scientists questioned whether much would be established about the validity of quake prediction techniques even if a magnitude 6 jolt occurred near Parkfield.

“Even then, it’s not going to prove that we predicted the earthquake on the basis of a good understanding of how earthquakes work,” said Ken Hudnut of the U.S. Geological Survey’s Pasadena office. “We don’t have a fundamental understanding of the physics of how earthquakes start or stop.”

Egill Hauksson, a seismologist at Caltech, echoed this view. “Parkfield was picked and people were willing to put 10 years of their careers on the line with this experiment because it seemed to be so easy, it seemed to be so obvious,” he said.

“Now predicting a Big One on the southern San Andreas (east of Los Angeles), that’s a whole other story,” Hauksson said.

A federal-state seismic review panel is scheduled to announce its re-evaluation of the chances of the Big One in mid-November.