Geologists Downgrade Earthquake Risk

Scientists from the U.S. Geological Survey, having put Central California on alert for a strong earthquake near the hamlet of Parkfield, on Monday downgraded the chance that a magnitude 6 quake will occur by early Wednesday morning. They said the probability had fallen to 1 in 8, and would fall to 1 in 20 if no such quake occurred by this morning.

The assessment represented a major change from Sunday, when after a swarm of 25 temblors--the strongest a moderate 4.8--federal and state authorities declared their highest-level earthquake alert in a seven-county region surrounding Parkfield. Scientists said the odds of a strong earthquake within a 72-hour period were 1 in 3.

Geological Survey spokeswoman Pat Jorgenson said instrument recordings along the San Andreas Fault segment near Parkfield, 180 miles northwest of Los Angeles, showed very little seismic activity as Monday wore on.

Other instruments in the vicinity--measuring any creep of the sides of the fault, subterranean strain and water well levels--also showed normal readings. Jorgenson said the chances of a sizable earthquake would be elevated if anomalies in these categories show up, even without an increase in small quakes.

The A-level alert issued Sunday in the seven counties--Monterey, Fresno, San Benito, Kings, Kern, San Luis Obispo and Santa Barbara--marked only the second time that the Geological Survey and the state Office of Emergency Services have issued the high-grade warning in an earthquake forecasting experiment begun in 1985.

Last year, under an advisory procedure that predated the system of alerts, the agencies said there was a 50% chance of a magnitude 6 aftershock of the Landers and Big Bear earthquakes within 24 hours. No such temblor occurred.

Altogether, there have been at least 11 notices, advisories, or alerts in the state since 1982. Through Monday evening, none of the potential events had occurred.