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Of late, the two teams have begun to use their computer models to peer into the future. What they see in these high-tech crystal balls is unsettling. "To relieve the stress Landers placed on it," says geophysicist Ross Stein, "the southern San Andreas would have to produce a 6.5-magnitude earthquake of its own."
The type of stress that has increased on the southern San Andreas is known as shear stress. It runs parallel to the fault, enhancing its tendency to slip. There is, however, another kind of stress, clamping stress, which retards slippage. It runs perpendicular to the fault, pinning the sides together like an invisible row of staples. "The situation we worry about most," says UCLA geophysicist David Jackson, "is when the shear stress increases and the clamping stress decreases. This is precisely what we think has happened."
| The Landers and Big Bear earthquakes cut through faults that form two sides of a triangle. When these faults fractured, the huge block of earth contained within the triangle shifted about a meter to the north, unclamping the San Andreas at the triangle's base. In deference to the menace posed by this singular geometry, Jackson calls the area the "Bermuda Triangle."
What everyone who lives in Southern California wants to know, of course, is not whether the southern San Andreas is going to slip, but when. To their frustration, scientists cannot answer that. The most careful calculations of stress transfer are based on the assumption that faults separate large blocks of earth, which stretch and compact in predictable ways. But the Southern California crust is so crisscrossed with faults that the material between them may behave more like sand. "Squeeze a block of wood," muses UCLA'S Jackson, "and it will become longer. But sand will behave in unforeseen ways."
In addition, scientists can only guess how much total stress accumulated along the southern San Andreas prior to the Landers earthquake. Geophysicist Geoffrey King of the Institut de Physique du Globe in Strasbourg, France, compares the predicament to trying to push a car uphill while blindfolded. Will the car move or not? "One person won't accomplish much," he observes, "but 10 people might. Our problem is that we don't know how many other people are already pushing on the car."
Indeed, scientists still do not know how much stress is required to start an earthquake in the first place. In laboratory experiments, explains University of Nevada, Reno, seismologist James Brune, two blocks of granite forced past each other generate a tremendous frictional heat. But earthquakes apparently do not. "Nature," says Brune, "has figured out an easier way of moving things around." After all, when carpet installers try to move a rug, they do not attempt to drag it all at once. "Instead," says Tom Heaton of the U.S. Geological Survey in Pasadena, "they put a little ripple in it. As the ripple moves from one end of the rug to the other, the rug moves with it."