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The implication of that information was not lost on visiting Westerners. As soon as he returned home from Moscow, Lynn Sykes, head of the seismology group of Columbia University's Lamont-Doherty Geological Observatory, urged one of his students, a young Indian doctoral candidate named Yash Aggarwal, to look for similar velocity shifts in records from Lamont-Doherty's network of seismographs in the Blue Mountain Lake region of the Adirondacks, in upper New York State, where tiny tremors occur frequently
As it happens, a swarm of small earthquakes had taken place at approximately the time of the Moscow meeting. Aggarwal's subsequent analysis bore out the Russian claims: before each quake, there had been a distinct drop in the lead time of the P waves.
As significant as those changes seemed, U.S. seismologists felt that they could not be really dependable as a quake-prediction signal without a more fundamental understanding of what was causing them. That explanation was already available. In the 1960s, while studying the reaction of materials to great mechanical strains, a team of researchers under M.I.T. Geologist William Brace had discovered that as rock approaches its breaking point, there are unexpected changes in its properties. For one thing, its resistance to electricity increases; for another, the seismic waves passing through it slow down.
Both effects seemed related to a phenomenon called dilatancy—the opening of a myriad of tiny, often microscopic cracks in rock subjected to great pressure. Brace even suggested at the time that the physical changes associated with dilatancy might provide warning of an impending earthquake, but neither he nor anyone else was quite sure how to proceed with his proposal. Dilatancy was, in effect, put on the shelf.
The Russian discoveries reawakened interest in the subject. Geophysicist Christopher Scholz of Lamont-Doherty and Amos Nur at Stanford, both of whom had studied under Brace at M.I.T., independently published papers that used dilatancy to explain the Russian findings. Both reports pointed out an apparent paradox: when the cracks first open in the crustal rock, its strength increases. Temporarily, the rock resists fracturing and the quake is delayed. At the same time, seismic waves slow down because they do not travel as fast through the open spaces as they do through solid rock. Eventually ground water begins to seep into the new openings in the dilated rock. Then the seismic-wave velocity quickly returns to normal. The water also has another effect: it weakens the rock until it suddenly gives way, causing the quake.