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When quake centers are marked on a map of the world, it becomes clear that many earthquakes do indeed occur along plate boundaries. The earthquake-marked "ring of fire" around the Pacific Ocean, for example, neatly outlines the Pacific plate. But earthquakes can also occur well within a plate, possibly because the plate structure has been weakened in those places during periods of ancient volcanism. Charleston, S.C., for instance, is more than 1,000 miles away from the edge of the North American plate; yet it lies in a seismically active area (see map page 39) and was hit by a major quake that killed 27 people in 1886. New Madrid, Mo., near the middle of the plate, was the site of three huge quakes in 1811 and 1812. Wrote one resident of the then sparsely populated area: "The whole land was moved and waved like the waves of the sea. With the explosions and bursting of the ground, large fissures were formed, some of which closed immediately, while others were of varying widths, as much as 30 ft."
Long before the plate-tectonics theory was conceived, scientists were aware that rocks fracture only under extreme stress. As early as 1910, Johns Hopkins Geologist Harry Reid suggested that it should be possible to tell when and where quakes were likely to occur by keeping close tab on the buildup of stresses along a fault. But the knowledge, instruments and funds necessary to monitor many miles of fault line and interpret any findings simply did not exist. Earthquake prediction did not draw much attention until 1949, when a devastating quake struck the Garm region of Siberia, causing an avalanche that buried the village of Khait and killed 12,000 people. Stunned by the disaster, the Soviets organized a scientific expedition and sent it into the quake-prone area. Its mission: to discover any geologic changes—in effect, early warning signals—that might occur before future quakes. The expedition remained In Siberia far longer than anyone had expected. But it was time well spent. In 1971, at an international scientific meeting in Moscow, the Soviet scientists announced that they had achieved their goal: learned how to recognize the signs of impending quakes.
The most important signal, they said, was a change in the velocity of vibrations that pass through the earth's crust as a result of such disturbances as quakes, mining blasts or underground nuclear tests. Earth scientists have long known that tremors spread outward in two different types of seismic waves. P waves cause any rock in their path to compress and then expand in the same direction as the waves are traveling. S waves move the rock in a direction that is perpendicular to their path. Because P waves travel faster than S waves, they reach seismographs first. The Russian scientists found that the difference in the arrival times of P and S waves began to decrease markedly for days, weeks and even months before a quake. Then, shortly before the quake struck, the lead time mysteriously returned to normal. The Russians also learned that the longer the period of abnormal wave velocity before a quake, the larger the eventual tremor was likely to be.*