Warning After 2nd Quake Is Strongest Ever

Sunday morning’s unusual seismic occurrence--separate big earthquakes 20 miles apart in rural San Bernardino County--resulted in the most strongly worded earthquake advisory ever issued by the state.

Clearly taken by surprise by the twin temblors, which included the most powerful in California in 40 years, scientists joined the state Office of Emergency Services in the unprecedented public statement warning Southern California residents to prepare themselves for at least a large aftershock.

The advisory declared an official determination that “a major earthquake sequence has begun” and is expected to continue. It said there was a greater than 50% chance of an aftershock sometime Sunday above magnitude 6, and warned Southern Californians “to the extent possible (to) stay off the freeway system.”

“This is the first time we’ve ever been this specific,” said Tom Mullins, a spokesman for the Office of Emergency Services. “This time, because of the uncertainty of the scientists, because of how seriously this was being taken, we felt it was prudent.”

The advisory was toned down later Sunday, after freeway inspections found no evidence of damage, but scientists continued to stress that the odds were about 50-50 of another damaging aftershock of magnitude 6 or greater hitting the area within the week.

The power of the initial main shock--the strongest in California since the magnitude 7.7 Tehachapi quake in 1952--was underscored when a Caltech seismologist, surveying the zone by helicopter, found a surface rupture extending 44 miles across the desert north of Yucca Valley.

Some pieces of ground had slid 18 feet in opposite directions, equal to the maximum horizontal displacement recorded in the 1906 great quake that destroyed much of San Francisco.

“We’re just lucky this was way out in the desert,” said seismologist Kerry Sieh.

In the advisory announced Sunday morning, the 50% likelihood of another strong quake was five times higher than expressed in the 10-year history of such public quake advisories. and There has never been any mention of staying off the freeways.

Within hours, authorities backed away from some details, saying they had meant the freeway warning to be taken more seriously in San Bernardino and Riverside counties than in Los Angeles.

But some scientists had wanted to go even further, after being surprised by the second quake.

Thomas H. Heaton, head of the U.S. Geological Survey’s Pasadena office, was concerned that the second, less powerful Big Bear quake was within 10 miles of a segment of the San Andreas Fault believed to be a likely locale of the eventual Big One predicted by scientists. He urged that the Sunday morning advisory at least allude to the possibility of a larger quake on the San Andreas.

But another Geological Survey scientist, Lucile M. Jones, backed by Caltech experts, said the quake had not occurred close enough to the San Andreas to fall within guidelines set several years ago by a study group for such warnings.

Discussion of the advisory’s language--and obtaining approval from Richard Andrews, head of the Emergency Services office--delayed the advisory’s release until more than an hour after the Big Bear quake.

A statement released Sunday afternoon by Jones, Heaton and six other Geological Survey and Caltech scientists, meanwhile, said that Sunday’s first main shock at 4:58 a.m. had been a shallow magnitude 7.4 quake centered on an extension of a fault that moved April 22 in the 6.1 Joshua Tree earthquake.

Unlike the April quake, Sunday’s first main shock near Landers, six miles north of Yucca Valley, was centered less than two miles underground and caused a noticeable surface rupture.

From his helicopter survey, Sieh found the surface rupture to be 44 miles long, beginning northeast of Yucca Valley and running to within 23 miles of Barstow.

The rupture appeared to run along two faults--the Johnson Valley Fault and the Camp Rock Emerson, farther north.

Sieh said the horizontal offset was at least 10 feet along the entire stretch, and reached 18 in at least one spot. He found that the left, or western side, of the faults had moved to the northwest, while the right, or eastern side, had moved to the southeast.

The surface inspections near Landers were getting started when a second, magnitude 6.5 main shock struck at 8:04 a.m. close to Big Bear Lake, 20 miles west of the first quake. At first, startled scientists couldn’t agree what to call the second temblor--an aftershock or a new quake.

But within minutes, Jones announced that seismologists had concluded that the Big Bear quake was a second main quake, not an aftershock, and had been “triggered” by the first in a way that the scientists do not yet completely understand.

A few scientists joined Heaton in worrying out loud that the triggering process--which transfers seismic strain from one fault to another--could continue to the nearby San Andreas, unlocking a jammed segment that has had no major quakes since about 1680.

Lori Dengler, professor of geology at Humboldt State University, said in an interview: “We know that one earthquake has already triggered a second one and it’s moving close to the San Andreas. It’s sort of like a kick. We know there’s a big section of fault that’s stuck. . . . If you kick it, does that make it more likely to go? That’s always the concern.”

She called the official advisory “a fairly conservative statement” in that it did not mention that possibility.

But in a later statement, drafted by Egill Hauksson of Caltech and signed by seven other scientists, it was noted that the fault on which the Big Bear quake occurred was northeast-trending, a different direction than the San Andreas follows.

Of the most powerful 7.4 shock, Heaton said: “This was certainly a big one, but it’s not what’s been called the Big One.”

Nonetheless, it was felt far and wide. Motion of water in swimming pools was reported in Boise, Ida.

And both main shocks were generating a large number of aftershocks. Of the hundreds recorded, at least 10 were magnitude 5 or greater. There were 30 other aftershocks in the magnitude 4 range.

“Numerous damaging aftershocks should be expected from these two main shocks,” said the later statement. “Based on the generic aftershock model, the chance of further damaging aftershocks (magnitude 6.0 or greater) in the (next) week is about 50%.”