ENVIRONMENT

Hearing the earth move

BOB LEVIN December 2 1985
ENVIRONMENT

Hearing the earth move

BOB LEVIN December 2 1985

Hearing the earth move

Warning signs were everywhere: one wall rising along a geological fault, foreshocks and tremors—even snakes slipping out of their lairs to die in the snow. To scientists in China it suddenly seemed clear that a major earthquake was about to strike the northeastern industrial city of Haicheng. As a result, the government evacuated the area on Peb. 4, 1975, and when a quake hit late that night it killed about 300 people—not the estimated 100,000 lives it might have claimed without the evacuation. Some scientists foresaw a new era in earthquake prediction. But that forecast was quickly dashed: 18 months later the Chinese city of Tangshan, 150 km southeast of Peking—unwarned and unready—was hit by a quake that killed some 700,000 people in one of the worst natural disasters in history.

The Haicheng incident remains the foremost success story in the annals of earthquake prediction. But while seismologists—who study earthquakes — and volcanologists —who monitor volcanos—are far from precise in their forecasts, they have improved immeasurably since the days when the Earth’s rumbling was attributed to the wrath of the gods. Now, scientists use the theory of plate tectonics, which holds that the Earth’s crust is broken into giant, slowly shifting plates floating independently on a semimolten interior. Usually the plates slide harm-

lessly past each other, but in some regions two lock together and then, under continued pressure, break violently apart. That occurred in Mexico last September, when the slow grinding of an oceanic plate under the North American plate resulted in a devastating earthquake that killed 20,000 people. A similar process caused the eruption of the Nevado del Ruiz volcano in Colombia two weeks ago, in which 22,000 people died.

In the case of earthquakes, the seismic gap theory is the key to forecasting. Developed by scientists at New York’s Columbia University during the early 1970s, it holds that the likeliest place for one to occur is the spot along a fault that has been quietest for the longest period of time—thus building up the most tension.

In Canada the federal department of energy, mines and resources monitors potential troublespots through a coastto-coast network of 97 seismographic stations which measure ground motion. Among the most closely watched areas: western Quebec, the mouth of the St. Lawrence River, parts of the Maritimes and, above all, British Columbia’s densely populated Lower Mainland. The question, says Dieter Weichert, head of seismology for the federal government’s Pacific Geoscience Center, near Victoria, is whether the oceanic and continental plates nearby are gliding smoothly past each

other or sticking and straining. That is similar to the question Mexican scientists were asking about their region before the Sept. 19 quake. “Well,” said Weichert, “now they know.”

For U.S. scientists the greatest geological worry is California’s San Andreas fault, which stretches northsouth for 1,000 km—almost the entire length of the state. In 1981 the U.S. federal emergency management agency predicted that there is a better than 50-per-cent chance another “great quake” will strike north of Los Angeles within 20 to 30 years. (A “great quake” measures more than 8 on the Richter scale, a system devised in 1935 by U.S. seismologist Charles Richter, and each succeeding whole number increases the strength of the tremor 10 times.) Last spring U.S. geologists made an even more specific prediction for the remote ranching town of Parkfield, which lies near the fault’s midway point about 100 km southwest of Fresno. There, they say, an earthquake of about 6 in magnitude has occurred about every 22 years—with the next one expected by 1993.

As a result, survey scientists have staked out Parkfield with all kinds of detection devices. Dilatometers monitor the strain on fluid-filled balloons placed in holes bored into the earth. Lasers beamed against mirrors measure minute changes in the Earth’s surface, while “creep meters” monitor the changing length of a wire stretched across the fault. Still, scientists privately admit that predicting earthquakes is a risky business because a false alarm could undermine their professional reputations or depress real estate values in more populated areas. At the same time, withholding a warning could be disastrous. As well, volcano monitoring has become more sophisticated since Mount St. Helen’s erupted in May, 1980, killing 60 people in the state of Washington and triggering a boom in volcanology. Now the geological survey group reports that the number of the world’s active volcanoes under close observation has doubled to 90. And at Mount St. Helen’s itself, scientists using such equipment as seismometers and lasers have predicted 16 of the past 19 minor eruptions of the still-active volcano. But they are still unable to forecast the force of those eruptions. Declared survey service geologist Daniel Dzurisin: “That will be very difficult to predict because in a manner of speaking the volcano doesn’t know either.” For volcanologists and seismologists alike, Mother Nature remains a formidable—and inscrutable—opponent.

BOB LEVIN in Toronto with correspondents’ reports