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What might cause such a ripple to spread across a fault remains a mystery. Numerous ideas have been suggested. Brune believes sliding rock physically deforms like tires squealing on pavement. In this case, what greases the skid is an invisible air pad that prevents the two surfaces from establishing frictional contact. Just last week, in a paper published by the science journal Nature, a team of researchers from the U.S. Geological Survey in Menlo Park offered an alternative possibility. Groundwater, they theorized, trapped under high pressure, might also serve to pry faults apart, allowing them to slip with a minimum expenditure of energy.
Yet another mechanism capable of inducing fracture has been suggested by the Landers earthquake. Because the quake triggered scores of sympathetic vibrations in volcanic and geothermal regions, some scientists have speculated that the Landers event shook underground magma chambers as though they were big cans of soda. The gas that fizzed forth could, in turn, have forced open a gap that eased the slip of surrounding rock. Whatever the mechanism, experts agree, it has only hastened the fracture of a fault zone that was already stressed up and ready to go.
What scientists fear is that the southern San Andreas has reached a similarly critical threshold. "If the Landers earthquake put a little stress on the San Andreas," exclaims Allan Lindh, chief seismologist of the U.S. Geological Survey, "then what about the accumulated stress of 300 years of plate motion?" For Lindh and other experts, the Landers quake and its resulting tremors are all too reminiscent of the increased seismic activity that preceded the great San Francisco blowout of 1906. "I mean," says Lindh, with a dramatic pause, "how much more on the edge of our chairs can we be?"
FOOTNOTE: *All magnitudes here are given on the moment magnitude scale, a more precise measure of earthquake energy that has largely replaced the Richter scale.