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Bishop Temblors May Be Precursor of Larger Quake

July 22, 1986|THOMAS H. MAUGH II | Times Science Writer

The series of earthquakes that has rattled Chalfant Valley near Bishop since July 3, culminating in Monday's temblor of about 6.1 in magnitude, is part of a decade-long increase in seismic activity in that area and may be a precursor of a larger quake there, experts said.

"The level of seismic activity since 1978 in the region immediately surrounding the valley has been about 20 times greater than the level during the previous decade," said geologist Alan Ryall of the Center for Seismic Studies in Arlington, Va.

However, any link between the Chalfant Valley quakes and the July 8 temblor in Palm Springs and the July 13 quake off the coast of Oceanside is tenuous, at best, seismologists said.

"There is a theory that sometimes you get an overall stress increase in the western United States that triggers a number of earthquakes in widely separated areas," according to Ed Corbett of the University of Nevada at Reno.

However, no quake larger than magnitude 7 has ever been associated with such a wide-scale buildup of stress, Corbett emphasized.

"But that's only a theory," Caltech geologist Lucille M. Jones said. "No one has ever been able to show that there is a significant correlation. Every time someone sets out to try to find such a correlation, the studies have shown that the earthquake process is random."

The problem in trying to link the recent quakes, Jones said, "is that they occurred in such different geologic regimes."

"So why should they be related when they are that far apart? It often seems like they are," he said, "because we do get these clusters, but we need a lot more data to understand it better."

In the Chalfant Valley, most of the increased geologic activity is probably caused by growing stress in the complex series of faults along the western flank of the White Mountains, Ryall said.

The Chalfant Valley quakes were probably not related to the nearby volcanic field around Mammoth Lakes called the Long Valley caldera, according to geologist Bruce Bolt of the University of California, Berkeley. In 1980, an underground flow of molten rock in the caldera was associated with a series of magnitude 6 earthquakes.

Aside from the Long Valley caldera, the primary geological feature of eastern California is a fault zone characterized by a chain of historical quakes, most of which averaged magnitude 7 or greater. The region, stretching from Winnemucca, Nev., south through the Owens Valley to the Mojave Desert, has been called the Central Nevada Seismic Belt or the East California Seismic Belt.

The belt is associated with the upthrust forces that created the Sierra Nevada Mountains.

1872 Quake Killed 27

The largest quake to strike the region, estimated at magnitude 8, hit the town of Lone Pine in the Owens Valley on March 26, 1872, killing 27 people out of a population of 300. The quake left a scarp--a steep slope caused when the earth on one side of a fault rises above the level on the other side--more than 100 miles long, whose northern terminus is just south of Bishop.

Last weekend's quakes were centered about 15 miles from Bishop, almost due north of the scarp.

According to geologist Robert Wallace of the U.S. Geological Survey in Menlo Park, Calif., different segments of the East California Seismic Belt have been struck by quakes of magnitude 7 or larger in 1915, 1932, 1954 and 1982.

"The only part of that belt that has not yet been broken is the segment between the north end of the 1872 fault and (the area of) the 1932 activity in Nevada," Wallace said.

Wallace calls this segment the White Mountain Seismic Gap.

"We think that this gap is capable of producing a magnitude 7 quake," Wallace said, "but we haven't predicted that such a quake will occur."

Maximum Magnitudes

Further hints that such a quake may occur in the foreseeable future arises from the pattern of the quakes that have occurred since 1978, Ryall and Wallace said. Clusters of temblors, with maximum magnitudes of about 6.0, have occurred roughly every 1 1/2 years, they said. Most of the quakes have occurred on the perimeter of a rough circle, with a radius of about 60 miles, centered approximately in the Chalfant Valley.

This phenomenon is called a Mogi doughnut, after the Japanese seismologist who first recognized it. Mogi found that a series of quakes in such a circular region frequently preceded a larger quake in the center of the circle.

Such a premonitory series of quakes in Kern County was associated with the July 21, 1952, magnitude 7.7 quake centered at Tehachapi, although scientists did not recognize their significance until later.

Near the Center

Monday's quake was near the center of the Mogi doughnut, Ryall said, but was smaller than the magnitude 7 that Mogi's results might have predicted.

Other scientists think it less likely that the series of quakes represents a Mogi doughnut, however.

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