Rattled Nerves : Quake Advisories Are Sometimes on Shaky Ground

Within 45 minutes of Wednesday’s magnitude 6.1 earthquake, scientists and state officials were issuing a public advisory that there could be a larger temblor, even a magnitude 7.5, in the next three days along the southern San Andreas Fault.

In 10 years of volcanic and earthquake advisories in California, this was the quickest such warning to be issued after a seismic event. Most others have taken days.

Although the reliability of the entire process is questionable--no major quake has yet followed an advisory--state officials defend the warnings as useful in putting local authorities and the public on guard.

Wednesday night, there was some confusion caused by different pronouncements over what the precise chances of a big quake were in the designated region from the San Gorgonio peak to the south end of Salton Sea.

In various television interviews, Caltech seismologist Kate Hutton put the chances at 10% to 25%, while U.S. Geological Survey seismologist Lucile Jones said they were closer to 10%. Another seismologist, Caltech’s Steve Bryant, kept emphasizing that the chances had diminished substantially after the first hour passed without disastrous occurrences.

The official advisory to six Southern California counties as announced by Paul Flores, deputy director of the California Office of Emergency Services, put the chance at 10%.

By Thursday, with aftershocks migrating northward toward the Morongo Valley Basin, away from the San Andreas Fault, Jones said the probability of a larger earthquake had declined to 5% over the next two days.

The differences underscore the uncertainties of the warning process. There have been at least eight such advisories in the state in the last several years, and often authorities have backed off from them before the warning period elapsed.

Richard Andrews, director of the Office of Emergency Services, Thursday said he would prefer that percentages not be issued in the advisories. He suggested that general phrases, such as “significant likelihood,” be used instead.

Andrews said the advisories help warn authorities to assemble emergency equipment and personnel and may make the public a little more cautious than normal, but scientists should be careful not to convey an impression that they know more about probabilities than they do.

“Not enough is known and numbers imply a precision that I’m not sure is really there,” he said.

But Andrews said the advisories will continue. The state Emergency Services Office must give its consent before an advisory can be issued.

“In all the advisories we’ve issued, we’ve seen no evidence of public panic,” Andrews said.

The advisory issued late Wednesday night came quickly for two reasons.

First, the earthquake occurred in an area where some scientists believe the “big one” in Southern California is most likely to strike, so they were particularly anxious to speak about the dangers.

Secondly, the scientific task force, which includes some of Southern California’s most eminent quake experts, just two years ago developed specific criteria for issuing a warning in this area, and Wednesday night’s quake precisely fit those criteria.

The task force, chaired by Jones and by Caltech’s Kerry Sieh, estimated in late 1990 that the chances of a great quake stronger than magnitude 7.5 in the San Gorgonio-Salton Sea portion of the San Andreas Fault at 40% over the next 30 years.

It also noted that historically, about half of California quakes over magnitude 5.0 along strike-slip faults such as the San Andreas have been preceded by foreshocks. Foreshocks are defined as coming within 10 kilometers (6.2 miles) and three days of the main shocks.

Through an elaborate series of calculations based on quake records, the task force then assigned some probabilities that earthquakes of certain magnitudes near specific segments of the San Andreas Fault would presage larger temblors.

In the San Gorgonio segment, they said, a 6.1 quake would be followed by a magnitude 7.5 temblor between 5% and 25% of the time. In the Palm Springs segment, the same prediction would be made after a 5.2 temblor.

Wednesday night, when the 6.1 earthquake struck, it was in the Palm Springs segment a few miles east of the San Gorgonio segment, and the fault it happened on was within five miles of the San Andreas Fault. Jones and other scientists felt that the criteria had largely been met to issue a warning.

They also noted that the 6.1 quake had been preceded by a foreshock, a magnitude 4.6 temblor that occurred 2 1/2 hours earlier.

With the support of the Office of Emergency Services, the scientists lost no time in issuing their advisory as soon as the press and television cameras arrived at Caltech’s earthquake center. Their only trouble was with the percentage within the 5% to 25% range.

By Thursday, with the aftershock pattern showing most of the hundreds of small quakes occurring farther away from the San Andreas Fault than the main shock, Jones said it appeared that the direction of the apparent main shock was toward the north.

The quake appeared “pointed toward” Morongo Valley Basin communities, such as Joshua Tree and Yucca Valley, where most of the structural damage and injuries occurred, and away from the San Andreas and the Coachella Valley communities of Desert Hot Springs and Palm Springs, where there was comparatively less damage, she said.