A Shock to Seismologists

Jolting as it was for Californians, last week's earthquake was even more of a shock to scientists. In a typical year, the Golden State is the site of 300 noticeable quakes, and seismologists have long predicted that a major quake is overdue. Yet the San Fernando quake struck an area that has been seismically inactive at least since the end of the last Ice Age--about 10,000 years ago. The region was still trembling when scores of scientists arrived with their portable instruments, anxious to find out why they had been caught so completely by surprise.

That will not be easy. California is perched on the so-called "Ring of Fire," --an earthquake-and volcano-prone region that circles the Pacific Basin. It reaches as far south as New Zealand on the west, north through Japan, across the Aleutians and down the coast of the Americas on the east. Only recently have geophysicists begun to understand what stokes the ring's "fires." The seismic activity, they think, is the result of slow, creeping movements of the Pacific Ocean floor against the continental margins that surround it. In California, these movements have produced a distinctive, local effect: a 600-mile fissure in the earth called the San Andreas fault, which begins in the Gulf of California, runs through most of the state, and then bends into the Pacific north of San Francisco. Furthermore, the sliver of land west of the fault, which includes Los Angeles, tends to move inexorably to the northwest, at a molasses-like average pace of an inch or so a year. Slow as that movement is, friction between the land masses causes them to stick together, and strains gradually build up. When the accumulating strain finally reaches the breaking point, the pent-up energy is suddenly released and results in violent shifting of land.

Wobbling Earth. Strangely enough, last week's tremors did not occur along the San Andreas fault or any of the active faults that are associated with it. They originated some distance away, along a swath roughly 20 miles long, running at approximately right angles to the big fault. It is in this area that the San Gabriel Mountains, north of Los Angeles, meet the San Fernando Valley. Judging by ruptures in this surface and readings from their instruments, scientists concluded that the mountains had either pushed a few feet over the valley, or that the valley had thrust underneath the mountains. However they occurred, the sudden, complex movements led to a significant quake--strong enough to tumble walls and knock down highway bridges.

Some scientists speculated that the gravitational forces exerted by the direct alignment of the sun, moon and earth during last week's lunar eclipse may have been sufficiently strong to trigger the release of the forces slowly stored up in a long inactive fault zone. Others thought that the quake might be connected with the slight eccentric movements of the spinning earth known as Chandler's Wobble. Such wobbling can displace the earth's axis of rotation at the poles by as much as 70 ft. during a year. In addition, there is a seven-year cycle of daily motion, which reaches a peak this year. Indeed, Dr. Charles Whitten of the National Oceanic and Atmospheric Administration, had presented data last summer suggesting that there might be an increase in major earthquake activity this year as a result of an increase in the amount of wobble. Other scientists were skeptical. Says Caltech Geophysicist Don Anderson: "We don't know if quakes cause the wobble or the wobble the quakes."

About one thing, however, scientists were agreed. The slow, steady buildup of strain that has long been taking place along the San Andreas fault was not released by the quake. In fact, land on opposite sides of the fault remained ominously still during the tremors. Seismologist Jerry Eaton of the National Center for Earthquake Research at Menlo Park, Calif., for one, thinks that the stress along the fault--which is already considerable--may even have been increased. In any case, most scientists are convinced that the stored-up energy must eventually be freed. That would cause a far more powerful quake that University of Michigan Physicist Peter Franken gloomily predicts could--if it struck in a populated area like Los Angeles or San Francisco--injure and kill tens of thousands of people. When will that day of seismic reckoning come? "Tomorrow or in a hundred years," says Caltech Geophysicist Clarence Allen. "I don't know."

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