Watching the Andes Grow

Satellites have revealed, for the first time, exactly how Earth's surface buckles when a slab of ocean floor dives beneath a continent. A 2-year study of the Andes and the Pacific coast of South America tracked the motions that give rise to the region's volcanoes and great earthquakes. The technique, presented in today's issue of Science, offers a promising way to improve predictions of earthquake severity along continental edges and other faults.

In the 1970s, the Defense Department developed the 24-satellite Global Positioning System (GPS) network to locate planes, ships, and soldiers to within 100 meters of their actual positions. During the last decade, geophysicists refined the now-declassified technology to yield measurements accurate to a centimeter or less. At such detail, researchers can monitor the motions of Earth's crustal plates, which grind past each other at a few centimeters per year.

In 1994, an international team launched a project to probe the complex collision of the Nazca oceanic plate with the west coast of South America. By tracking the movement of 43 GPS receivers in Bolivia, Peru, and offshore, the researchers could see more precisely the contortions at the boundary. Much of the Nazca plate appears to slide smoothly under the continent, accounting for about 44% of the converging plates' total motion of 8 centimeters per year. Another 12% of the movement crumples South America's western margin, raising the Andes' soaring plateaus and icy peaks. The remaining 44% could not be found, but the team speculates that it is locked up between the grinding plates, a tension that unleashes massive quakes every few decades. "It's exciting to see that plate motions during a 2-year period are similar to what the geologic record shows over millions of years," says geologist Seth Stein of Northwestern University in Evanston, Illinois.

Space geodesy is a potent new seismological tool, says geophysicist Sean Solomon of the Carnegie Institution of Washington in Washington, D.C. The frontier for geophysicists "is learning how faults take up [plate] motions at the boundary zones," he says. GPS will let researchers gauge the proportion of gentle slippage between plates versus strain accumulation at faults, Solomon believes--an equation that could help refine predictions of how frequently large quakes may strike.

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