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Soon California Institute of Technology's James Whitcomb, Jan Garmany and Don Anderson weighed in with more evidence. In a search of past records, they found a distinct drop in the speed of P waves 3½ years before the 1971 San Fernando quake (58 deaths), the largest in California in recent years. The P waves had returned to their normal velocity a few months before the tremor. Besides providing what amounted to a retroactive prediction of that powerful quake, the Caltech researchers demonstrated that it was primarily the velocity of the P waves, not the S waves, that changed. Their figures were significant for another reason: the P-wave velocity change was not caused by a quirk of geology in the Garm region or even in the Adirondacks, but was apparently a common symptom of the buildup of dangerous stresses in the earth.
In fact, dilatancy seems to explain virtually all the strange effects observed prior to earthquakes. As cracks open in rock, the rock's electrical resistance rises because air is not a good conductor of electricity. The cracks also increase the surface area of rock exposed to water; the water thus comes in contact with more radioactive material and absorbs more radon—a radioactive gas that the Soviet scientists had noticed in increased quantities in Garm-area wells. In addition, because the cracking of the rock increases its volume, dilatancy can account for the crustal uplift and tilting that precedes some quakes. The Japanese, for instance, noticed a 2-in. rise in the ground as long as five years before the major quake that rocked Niigata in 1964. Scientists are less certain about how dilatancy accounts for variations in the local magnetic field but think that the effect is related to changes in the rock's electrical resistance.
With their new knowledge, U.S. and Russian scientists cautiously began making private predictions of impending earthquakes. In 1973, after he had studied data from seven portable seismographs at the Blue Mountain Lake encampment, Columbia University's Aggarwal excitedly telephoned Lynn Sykes back at the laboratory. All signs, said Aggarwal, pointed to an imminent earthquake of magnitude 2.5 to 3. As Aggarwal was sitting down to dinner two days later, the earth rumbled under his feet. "I could feel the waves passing by," he recalls, "and I was jubilant." In November 1973, after observing changes in P-wave velocity, Caltech's Whitcomb predicted that there would be a shock near Riverside, Calif., within three months. Sure enough, a tremor did hit before his deadline—on Jan. 30. Whitcomb's successful prediction was particularly important. All previous forecasts had involved quakes along thrust faults, where rock on one side of a fault is pushing against rock on the other. The Riverside quake took place on a strike-slip fault, along which the adjoining sides are sliding past each other. Because most upheavals along the San Andreas Fault involve strike-slip quakes, Whitcomb's forecast raised hopes that seismologists could use their new techniques to predict the major earthquakes that are bound to occur along the San Andreas.