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The last large-scale killers occurred in Asia. One, last December in northern Pakistan, ravaged nine towns and took nearly 5,000 lives. The other, a February tremor in China, is believed to have killed hundreds. Indeed, not a day passes without earth tremors somewhere on the globe. Some of those quakes are too weak to be felt by humans; they can be detected only by sensitive seismographs. Others are more violent but occur on the ocean floor or in remote areas and do no harm. Some add to the long catalogue of destruction. Last week, for example, a 4.7 earthquake rocked lightly populated Kodiak Island, off the coast of Alaska. In July, a 6.8 quake struck Pagan, Burma, destroying or damaging half of the city's historic temples. Within the past several weeks, strong earthquakes struck Oroville, Calif., Mindanao in the Philippines, the Kamchatka Peninsula in Siberia and the southwest Pacific island of Bougainville.
With good reason, many primitive peoples regarded the terrible quakes they could not understand as the acts of a vengeful deity. As late as 1750, Thomas Sherlock, the Bishop of London, told his flock that two recent earthquakes were warnings that Londoners should atone for their sins. John Wesley agreed. In a 1777 letter to a friend, he wrote:
"There is no divine visitation which is likely to have so general an influence upon sinners as an earthquake." The ancient Japanese believed that the hundreds of quakes that shook (and still shake) their islands every year were caused by the casual movements of a great spider that carried the earth on its back. Natives of Siberia's quake-prone Kamchatka Peninsula blamed the tremors on a giant dog named Kosei tossing snow off his fur. Pythagoras, the Greek philosopher and mathematician, believed that earthquakes were caused by the dead fighting among themselves. Another ancient Greek, Aristotle, had a more scientific explanation. He contended that the earth's rumblings were the result of hot air masses trying to escape from the earth's interior.
In the past decade, the development of a bold new geological theory called plate tectonics—which offers an elegant, comprehensive explanation for continental drift, mountain building and volcanism—seems finally to have clarified the underlying cause of earthquakes. It holds that the surface of the earth consists of about a dozen giant, 70-mile-thick rock plates. Floating on the earth's semimolten mantle and propelled by as yet undetermined forces, the plates are in constant motion. Where they meet, friction sometimes temporarily locks them in place, causing stresses to build up near their edges. Eventually the rock fractures, allowing the plates to resume their motion. It is that sudden release of pent-up energy that causes earthquakes. Off Japan, for instance, the Pacific plate is thrusting under the Eurasian plate, causing the deep-seated quakes characteristic of the Japanese archipelago. In California, along the San Andreas Fault, two great plates are sliding past each other. The sliver west of the fault, which is located on the Pacific plate, is moving toward the northwest. The rest of the state is resting on the North American plate, which is moving westward. The sudden movement of a portion of the fault that had been locked in place for many years is thought to have caused the great San Francisco earthquake of 1906.