Link to Surge in Temblors Is Studied

Just minutes after the magnitude 7.4 Landers earthquake last Sunday, an unexpected “surge of seismic activity,” mainly very small earthquakes, began 250 miles away in the Long Valley Caldera near Mammoth Lakes, according to a report by the U.S. Geological Survey.

The increase in what scientists often call “sympathetic” minor quakes continued all the way up the eastern side of the Sierra Nevada and into the Cascade range around Lassen Peak and Mt. Shasta. Mt. Shasta is 530 miles northwest of the Mojave Desert region where the Landers temblor occurred.

“The observation of a sudden surge of triggered seismic activity along the eastern front of the Sierra Nevada has surprised scientists here and stimulated a great deal of discussion about its possible cause,” said Paul A. Reasenberg, writing for the Geological Survey from its regional office in Menlo Park.

Reasenberg said that seismic waves or shaking from the Landers earthquake--the state’s strongest in 40 years--could have imposed dynamic stresses on distant faults that caused them to move slightly.

“The fact that all of these areas have become active simultaneously and just after the Landers earthquake is of great interest from a scientific standpoint,” the report said. “It is not believed, however, that the increase in (seismic activity) . . . represents an increase in either earthquake hazard or volcanic hazard.”

About 260 seismic events, mainly earthquakes too weak to be felt by humans, occurred in the Long Valley Caldera in the three days after the Landers qake. Sixty-three were stronger than magnitude 1.5 and the strongest measured 3.7.

The Long Valley Caldera is a 10-mile-by-19-mile oval-shaped crater formed about 700,000 years ago by an eruption that ejected an estimated 250 times more ash and molten rock than Washington state’s Mt. St. Helens did when it erupted on May 18, 1980.

Other nearby volcanoes erupted in the Mammoth Lakes area between 500 and 600 years ago, although with nowhere near the ferocity of the prehistoric blasts in the caldera.

In May, 1982, after a series of sizable earthquakes in the area, the Geological Survey issued a volcanic watch, saying there were indications that molten rock was rising closer to the surface two miles east of Mammoth Lakes near California 203 and U. S. 395.

However, nothing happened and in mid-1984 the agency lifted the warning. Many citizens in Mammoth Lakes still complain that the Geological Survey’s warning made tourists and would-be property buyers unnecessarily wary.

The latest Geological Survey report said that earthquake activity within the caldera “has maintained persistent but relatively low levels through the first half of 1992, averaging fewer than 10 events per day. . . . The abrupt rate increase immediately following the Landers earthquake stands out clearly in both the magnitude and cumulative number.”

It took between one and two minutes for various kinds of waves from the Landers quake to reach the Mammoth area, Reasenberg wrote. Quakes began shaking beneath the area about six minutes later, his report said.

The Geological Survey report also said there have been sympathetic distant temblors after the April 25-26 Humboldt County earthquakes, where the main shock was magnitude 7.1, but they were no farther than 62 miles from the epicenters.

“The (present) activity at Lassen (Peak) and in the Mt. Shasta area is remarkable because of the great distance of these areas from the epicenter of Sunday morning’s earthquake in Southern California,” Reasenberg wrote.

Besides examining the theory that seismic waves may have caused the small earthquakes near Mammoth and farther north, the report also said that scientists are developing computer models that seek to determine the lasting effect the Landers fault slip may have had on stresses on other faults throughout the state.

“Such models were generally successful in explaining many of the changes in seismicity observed in central California after the Loma Prieta earthquake” of Oct. 17, 1989, Reasenberg said.

“However, the much greater distances involved in this case distinguishes this week’s observation from those after the Loma Prieta earthquake,” he said.

At magnitude 7.4, the Landers earthquake was nearly three times stronger than the Loma Prieta quake or the Humboldt County main shock.