Science: Predicting Quakes: a Shaky Art

Why seismologists usually cannot tell when shocks mil strike

On a bitterly cold day in February 1975, the Communist Party chiefs in Haicheng, a city in northeastern China, convened in emergency session. They had just learned from seismologists that various signs, including changes in the water level of wells, pointed to an imminent earthquake. Convinced the threat was real, they issued an order to evacuate all homes and hospitals, close down shops and factories and turn off gas mains and furnaces.

That night, while thousands huddled in open fields or watched special film showings in the city square, the earth began to rumble. A quake measuring 7.3 on the Richter scale devastated Haicheng, collapsing buildings, turning bridges into grotesquely twisted heaps and tearing up roads. The death toll could easily have run into six figures. Thanks to the forewarning, it was probably fewer than 300.

Scientists hailed that prediction as one more indication that quake forecasting could become as reliable as weather forecasting. Such optimism has proved premature. True, other quakes have been predicted in China and elsewhere. But more often than not they have struck without specific warning, as in Italy last week, and in Algeria last October. Only a year and a half after the Haicheng temblor, an 8.2 quake near Tangshan, 90 miles southeast of Peking, caught seismologists by surprise and killed as many as 650,000. Says Polish-born Volcanologist Haroun Tazieff: "At the present level of research, nature almost always surprises man."

Even so, the earth's behavior is becoming much less mysterious. As late as 1750, the Bishop of London told his flock that two recent quakes were warnings from an angry deity. Today, scientists prefer another explanation, an all-encompassing view of the earth known as the theory of plate tectonics. It holds that the planet's surface consists of a dozen or so restless plates, each about 70 miles thick. Their movements explain volcanoes, the rise of mountains and the drift of continents. They account for quakes as well, most of which seem to occur where the great plates meet—at the so-called Ring of Fire, for instance, the tremor-and volcano-prone region that rims the Pacific Ocean and outlines the Pacific plate.

Driven by as yet unfathomed forces deep within the earth, the plates move no more than a few inches a year, bumping, jostling and grinding against each other with incredible force. These movements can have enormous consequences. Opposing plates often lock, so that great stresses begin to build up. When the pressures become so large that they exceed even the strength of the rock, the earth fractures, frequently along "faults" where earlier breaks have occurred. Like a wound spring suddenly uncoiling, the earth releases its stored energy in shocks that may be felt far beyond the fracture. This is an earthquake.