Adherents of the theory of plate tectonics, which revolutionized earth science during the 1960s, say that the world’s surface is a mosaic of rigid plates floating slowly around a semimolten interior. Some plates are giant slabs of ocean floor, others are a combination of continent and ocean floor. The constantly moving plates sometimes glide past each other with relative ease. But they occasionally grind together like ice floes, sometimes sticking together, with pressure building up, then breaking free with explosive force, causing an earthquake. The San Andreas Fault, where the Pacific Ocean plate rubs against the North American plate, is particularly susceptible to this phenomenon. Scientists say that the way the plates in that region sometimes grind against each other is responsible for the earthquakes that regularly erupt in California.
The main fault line, which stretches 800 miles almost along the entire length of California and reaches within one mile of San Francisco and 30 miles of Los Angeles, is the best understood of all fractures in the earth’s crust. The San Andreas Fault moves at a rate of 1.4
inches each year, occasionally producing substantial quakes. But that is where certainty stops. California actually has a spider’s web of smaller cracks—not all of them visible— branching off from the San Andreas Fault, all with different rates of movement. And two years ago, seismologists discovered a major hidden group of subterranean faults in the Los Angeles basin that constitute a whole new class of earthquake hazards. They have added a host of imponderables to the inexact science of earthquake prediction.
Foreshocks: Dieter Weichert, acting director of the Pacific Geoscience Centre near Victoria, said that the number of major earthquakes that scientists have been able to predict accurately is “negligible.” There are various signs that could signal a quake, he added, but none of them is definite. One sign is a series of smaller foreshocks in an area where that kind of activity is unusual. Foreshocks alerted scientists to the China quake of 1975, but those warning signals are easily overlooked in California, which experiences thousands of tremors every year. As for last week’s earthquake, seismologists said that there were one or two
shocks that in hindsight could have been interpreted as foreshocks, but they were not particularly unusual for the area and did not appear to be cause for alarm.
Severity: Weichert added that when the ground begins to rise—usually by a few inches—in the vicinity of a fault, it can be a sign that a quake is imminent. But that phenomenon, too, offers no certainty that an earthquake will occur, nor of the likely severity of an
upheaval. As a result, most scientists rely on the seismic gap theory, which was developed at New York City’s Columbia University during the 1970s. According to that theory, the likeli-
est place for an earthquake to occur is the spot along a fault that has been quietest for the longest time, building up the most tension.
Using that reasoning, the U.S. Geological Survey predicted in 1988 that there was a 60per-cent chance that another so-called great quake—which would dwarf last week’s upheaval—would strike southern California within 30 years. Clarence Allen, a seismologist at the California Institute of Technology in Pasadena, said that the highest probability of such a quake appears to be in the Coachella Valley, south of Palm Springs, Calif. That segment of the San Andreas Fault has been dormant for at least 196 years, far longer than most others.
Rupture: A “great quake” measures more than 8 on the open-ended Richter scale (a system devised in 1935 by American seismologist Charles Richter), with each succeeding number representing a tenfold increase in the strength of the tremor. California has had only two quakes of that magnitude in recent history: one in 1857 that produced a 180-mile-long rupture in the Mojave Desert, east of Los Angeles, and the San Francisco earthquake of 1906, which scientists estimate would have measured 8.3 on the Richter scale and which killed 700 people, many of them from a disastrous fire that followed the quake. Seismologists who have been studying the Mojave segment of the San Andreas Fault have determined that it can produce sizable earthquakes as often as every 50 years or as in-
DANGEROUS PRESSURES IN THE EARTH
by forces in the Earth’s interior, the Pacific and North American plates move almost imperceptibly in different directions. Sometimes, the plates lock together, then break free with explosive force. As a result, at least eight substantial earthquakes, and thousands of smaller ones, have shaken the region during the past century.
frequently as every 330 years. Last week’s earthquake, which registered 6.9 on the Richter scale, occurred in a segment of the San Andreas Fault that scientists have identified in recent years as a high-risk area. Said Allen: “We correctly identified it, but I won’t say we predicted it because prediction implies a time.” Another expert, Columbia’s geologist Christopher Scholz, said that the quake affected only about two-thirds of the high-risk area of the fault, which extends about 45 miles from San Juan Bautista to Palo Alto on the San Francisco peninsula. “There is still a region of about 20
km to the north which did not rupture,” said Scholz, adding that the region now has a higher risk of a quake in the future.
Cracks: Although last week’s rupture did not qualify as the Big One, it was the seventh substantial earthquake in California since the 1906 San Francisco quake. Two of them alerted geologists to faults that they had not known existed. It was only in 1987 that they discovered the reason: the faults are buried as deep as 10 miles below the earth’s surface, part of a network of subterranean cracks whose only indicator is a fold, or buckling, of the earth’s crust. Previously, scientists had assumed that the hills and mountains created by those folds were gradual formations produced over a long period of time, rather than by the sudden, violent changes of earthquakes.
But, while the folds have helped scientists to learn more about the mechanics of earthquakes, their recent discovery demonstrated how much more scientists have yet to learn. Said Robert Clayton, a Victoria-born geophysi-
cist at the California Institute of Technology: “In a sense, we’re making progress, but our appreciation of the problem is running faster than we are.” Scientists point out that southern California has up to 15,000 earthquakes a year. That makes it impossible to determine which are foreshocks and which are simply tremors that release pressure. Said Clayton: “It’s an immensely complicated geological place.” Massive: In Canada, the attention of seismologists has recently focused on the Juan de Fuca Fault, which runs beneath the Pacific Ocean from the centre of Vancouver Island to
Cape Blanco, Ore. John Adams, a seismologist at the Geological Survey of Canada in Ottawa, said research indicates that the Pacific plate was slipping beneath the edge of the North American plate along that fault fine. That could create a massive earthquake in British Columbia, Washington and Oregon within 50 to 100 years, he said. The federal department of energy, mines and resources is also monitoring western Quebec, the mouth of the St. Lawrence River and parts of the Maritimes that register periodic flurries of seismic activity.
One of Canada’s biggest earthquakes occurred on Nov. 25, 1988, in the Charlevoix region of Quebec. It registered 6 on the Richter scale, knocked out power and telephone service and was felt as far away as New York City, which experienced a 30-second tremor. John Ebel, assistant director of Boston College’s Weston Observatory in Weston, Mass., said that the Charlevoix area suffered quakes of similar magnitude in 1870 and 1925. Scientists say that they do not know why that particular
region is active. But they have detected several faults along the St. Lawrence valley in Quebec, as well as what they call a “major impact structure” from a meteor that struck tens of millions of years ago.
Faults in Eastern Canada and the northeastern United States differ from those along the continent’s West Coast. They are not between two plates, like the San Andreas or Juan de Fuca faults, but are in the middle of the North American plate, which extends all the way out to the middle of the Atlantic Ocean. Those intraplate faults, as they are called, are much
older than the California ones—hundreds of millions of years old as opposed to about a million—and are less geologically active.
Fraught: The three most devastating earthquakes in North America occurred midway between St. Louis, Mo., and Memphis, Term., in 1811 and 1812. They sent hills sliding into the Mississippi River, which altered its course. Another great quake destroyed much of Charleston, S.C., in 1886. As a result, while attention is now focused on California, a major earthquake could strike somewhere in eastern North America. But the time frame for such an occurrence is even more difficult to predict because the seismic cycle is so much greater on the East Coast than on the West. In a science fraught with so many imponderables, it is unlikely that experts in the near future will be able to give any more advance warning than they did for the latest catastrophe to shake California.
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