Anatomy of an Earthquake

By Natalie Angier

The 300-mile ribbon of Mexican Pacific coastline that stretches from Manzanillo to Acapulco has long been considered one of the world's beautiful places, home to a sprinkling of fishing hamlets and resorts. Yet beneath the indigo waves and silky white beaches lies a jagged fault line that could be one of the deadliest in the Western Hemisphere. It was this fault that erupted under the Pacific last week, causing the earthquake that measured 7.8 on the Richter scale,* rocked coastal towns and brought disaster to Mexico City.

To scientists, the great quake and its aftershocks were not surprising. Karen McNally, a geophysicist at the University of California, Santa Cruz, had warned in 1981 that substantial seismic activity was likely in the area. "Everything we had seen," she says, "could not allow us to exclude the possibility of a major earthquake."

In a seeming paradox, the location of last week's quake was thought to be endangered because it had been calm for so long. The epicenter of the quake, in the ocean about 150 miles up the coast from Acapulco, lay within a kind of geological DMZ known as a seismic gap: a region that had not experienced a major earthquake for many years, but where bottled-up stress caused by tectonic-plate activity had reached the bursting point.

Solid though it appears, the earth's crust is composed of a dozen large plates and several smaller ones, ranging in thickness from 20 to 150 miles. The plates are in constant motion, riding on the molten mantle below and normally traveling at the pace of a millimeter a week, equivalent to the growth rate of a fingernail. Geophysicist Bill Spence of the U.S. Geological Survey in Colorado says, "They're just like a mobile jigsaw puzzle." The plates' travels result in continental drift, the formation of mountains, volcanoes--and earthquakes.

If plates carrying two continental masses collide, for example, the crust buckles, creating craggy mountain ranges like the Himalayas. If they grind past each other, as the Pacific and North American plates do under California's San Andreas fault, friction locks them together. Every so often, abrupt slippages occur and the earth around them shudders in what geologists call strike-slip quakes. Still another kind of tectonic phenomenon, the meeting of an oceanic and a continental plate, is responsible for the Mexican disaster.

With irresistible force, the Cocos plate, which forms part of the Pacific floor off Mexico, is pushing northeastward at a rate of 2 to 4 1/2 in. a year against the North American plate, which is creeping westward. As the Cocos plate dips

under the continental crust, the oceanic mass sticks in certain places, its motion halted by friction. But the force propelling Cocos forward remains unrelenting, building up strain in the rock of both plates. When the frictional forces are overcome, the "stuck" section of the Cocos plate lurches forward (at least 10 ft. last week), generating the shock waves of a "thrust" quake.

In a kind of seismic party line, one earthquake may signal that another could occur; sites that lie between past gaps hit by recent tremors are the areas most likely to rupture next, rather the way buttons popping on a shirt put greater pressure on the buttons still intact. Noting that earthquakes in the 20th century have periodically shaken surrounding regions, geologists knew that Mexico's Michoacan gap--quiescent for many decades--could not hold out forever. "Wherever stress builds up for a long time in a seismic gap," says David Simpson of Columbia University's Lamont-Doherty Geological Observatory, "something's got to give."

Ironically, coastal towns such as Zihuatanejo and Ixtapa, only 50 miles from the epicenter, suffered less damage than Mexico City, 200 miles away. That is because the shoreline is made of solid rock and thus shakes less violently. Mexico's capital, however, was built on an alluvial lake bed. As a result, the seismic waves, though diminished in intensity on their trip from the coast, were amplified in the city's sediment foundation. Many tall buildings in the densely populated metropolis may not have been built to rigid quake- resistant standards. Indeed, some turned out to be just the right height to vibrate or resonate sympathetically with the frequency of the seismic waves, thus shaking with greater vigor than other buildings.

If geologists are correct, more major earthquakes are in store--and soon --for the Pacific coastal areas of Mexico and neighboring Guatemala. McNally believes the region could be hit by as many as five earthquakes in the 8.0 Richter range during the next five years. Precisely when the temblors will occur is another matter. Geologists are still restricted to long-term predictions, parceled out by the year or decade rather than the month or day. But by closely monitoring quake zones, they hope to find subtle clues that will lead to more precise and reliable forecasts. Keiiti Aki, a geophysicist at the University of Southern California, has designed a detailed computer model that combines such varied earthquake signposts as seismic anomalies, strange animal behavior, changes in the water table and peculiar bulges along the terrain.

Last week's disaster may lead to more insights. Scientists had earlier set up sophisticated seismological instruments in and around the Michoacan gap, and the devices were working when the spasm occurred. Says Seismologist James Brune of the Scripps Institution of Oceanography: "It will be the best-recorded major quake ever."

FOOTNOTE: *Each unit on the Richter scale indicates about a 30-fold jump in energy released. A quake of magnitude 2 is hardly perceptible; a 5-pointer can shatter dishes and windows; the great San Francisco quake of 1906 is estimated to have been an 8.3; and the most powerful quake ever recorded, off the Chilean coast in May 1960, reached 9.5.

With reporting by Carol A. Johmann/New York and Charles Pelton/San Francisco