Did Darwin Help Predict Chilean Quake?

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Richard A. Kerr
2010-02-27 20:13
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Charles Darwin helped forecast today's magnitude-8.8 earthquake in Chile, which has, at press time, killed more than 200 people, caused extensive damage, and sent a modest-size tsunami around the Pacific. Seismologists are giving the famed naturalist credit for reporting telltale signs that helped later scientists forecast that the giant temblor—one of the 10 most powerful on record—was imminent in the South American country. "This was not a big surprise, though no one could tell when it would strike,” says seismologist Hiroo Kanamori of the California Institute of Technology in Pasadena.

Darwin himself visited central Chile a few years after a quake of approximate magnitude 8.5 struck in 1835. He was there as part of his voyage on the Beagle and noted how the quake had reshaped the land. Over the years, these observations have helped seismologists anticipate another magnitude-8 quake on this section of fault off central Chile, where the Nazca tectonic plate dives beneath South America. The principle is simple: A fault segment that has ruptured in a large quake in the past will—once enough strain has again accumulated—likely fail again in a similar-size earthquake. “This area is very well known as a seismic gap,” says geophysicist Christophe Vigny of the Ecole Normale Supérieure in Paris.

Since the Darwin quake, that segment of fault has been “quiet, silent” in terms of large earthquakes, says Vigny. The magnitude-9.4 quake of 1960—the world’s largest quake in a century or two—ruptured 1000 kilometers of fault to the south of Darwin’s 300-kilometer gap. In 1906, the Valparaiso quake broke a segment to the north. Assuming strain was building since 1835 in the gap between the two ruptures, something was going to give, seismologists reasoned, and probably sooner rather than later.

Last year, geophysicist Jean-Claude Ruegg of the Institut de Physique du Globe in Paris, Vigny, and colleagues—including several from the University of Chile in Santiago—reported GPS observations in Physics of the Earth and Planetary Interiors that put some numbers on the Darwin supposition. By measuring the slow rising and falling of the ground as the inexorable collision of the Nazca plate squeezed the coast, they could calculate how fast strain was actually building. Assuming it had been building that fast since Darwin’s day, the gap was “a likely spot for a major [magnitude-8.0 to 8.5] subduction earthquake in the coming decades,” they wrote.

Haiti and the now-filled Chilean seismic gap are hardly the only places where seismologists have forecast large quakes in the coming decades. And many of these big gaps are uncomfortably near dense populations: off Padang, Indonesia; just north of Delhi, India; and hard by Istanbul, Turkey, to name a few. Seismologists warn, however, that no one should wait for them to deliver more precise timing on when exactly these temblors will hit.

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