State Warns of Increased Quake Chance Near San Jose

An advisory that the area south of San Jose that was rocked Monday by a moderate earthquake faces “a slightly increased likelihood” of a severe quake, ranging up to 6.5 magnitude, within the next five days was issued Tuesday by the California Office of Emergency Services.

In its notice issued for officials in Santa Clara, Santa Cruz, San Benito and Monterey counties, the Office of Emergency Services said it was acting on the advice of scientists from the U.S. Geological Survey, U.C. Berkeley and state Department of Conservation’s Division of Mines and Geology.

Estimate Given

Within minutes of the advisory, one Geological Survey scientist, speaking on a not-for-attribution basis, said that by the time it was issued, the estimated chances of a quake of this strength had declined from about 2% Monday to less than 1% Tuesday and would continue to decline through the five-day period.

The scientist did not disagree with the decision to issue the advisory, however. An Office of Emergency Services spokesman said that no matter how small the actual odds, the office believed that it would “be remiss in performing our duties” if some word of even a slight risk were not promptly put out.

Spokesman Tom Mullins said that county authorities were told to make sure that their emergency plans were in order, that they had the necessary personnel on hand should a severe earthquake occur and to put themselves into a general higher state of awareness.

Most Surprised

Checks with the authorities in the affected area found most saying they were surprised by the advisory, they had never been told such an advisory might be issued, but that they were alerting police and other agencies.

Gerald Verwolf, acting emergency operations coordinator for Monterey County, said, “When you get right down to it, anyone who has lived in California for any length of time realizes we live in earthquake country.”

He said that while the county has no formal plan for dealing with such an advisory, officials there had been put into “a heightened level of awareness.”

Geoff Paulsen, emergency planning coordinator of Santa Clara County, said he had no checklist for what to do in case of such an advisory, so he simply informed the cities within his jurisdiction and advised county department heads to carry portable radios with them for the next five days in case something happened.

Paulsen said he also ordered preparation of the county’s Emergency Operations Center, which is normally used for regular administrative office space. Otherwise, he said, “it’s nothing we’re getting too excited about.”

Mullins said the scientists who recommended issuing the advisory are members of a working group that has several times since 1982 urged authorities to issue similar advisories of possible impending seismic or volcanic activity. So far, none of the advisories--most of which have been issued up to now by the Geological Survey--has been followed by the larger events said to be possible.

For instance, on May 25, 1982, the Geological Survey issued a “notice” regarding the possibility of volcanic activity near Mammoth Lakes in the Eastern Sierra. Two years later, it said that seismic activity had lessened and that a volcanic eruption was not an immediate threat.

On June 17, 1985, the Geological Survey, following three quakes in the 4 range in the San Diego area, advised that a small chance existed of a larger quake within five days. No such quake occurred.

Nothing Happened

On July 22, 1986, the Geological Survey, following 5.5 and 6.1 earthquakes north of Bishop, said that more quakes in the 6 range, and an even larger temblor of 7.0, was possible. Again, no such quakes occurred.

On Oct. 4, 1987, following a strong aftershock of the 5.9 Whittier Narrows quake in the Los Angeles area, the Geological Survey said that an aftershock sequence “is likely to extend over several days and possibly weeks.” In fact, the aftershocks tailed off sharply.

Both state and federal authorities cautioned Tuesday that the odds against anything happening now are great, but nonetheless, there is a slightly greater chance than is generally the case.

“It’s a little more likely because this quake (Monday, centered 16 miles south of San Jose) was on the San Andreas fault and that about doubled the usual chance of the quake being a foreshock of a larger one,” said the Geological Survey scientist.

“But it was probably on the order of 2% yesterday, and it reduces as the five-day period goes on. One-quarter of such larger shocks occur within the first hour, and by the time you’re a day away, it’s one-third that chance,” the scientist said.

Mark A. Stein reported from San Francisco and Kenneth Reich from Los Angeles.