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Drowning in Asteroids

23 November 1999 7:00 pm
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A tsunami spreads outward from the simulated impact of a 200-meter-wide asteroid into the ocean. A new study of the hazards posed by impact-generated waves suggests that coastlines face substantial risks of being slammed by tsunamis from asteroid splashdowns within the next 1000 years.

Finally, some news to soothe the millennial jitters. Not. The odds are higher than you might expect that before Y3K, an asteroid will plunge into the ocean and trigger a powerful surge of water called a tsunami that inundates shorelines somewhere, researchers say.

The odds are vanishingly small that an asteroid will strike a particular spot on Earth. However, after any impact, collateral damage can range far and wide due to fires, dust clouds, and tsunamis, which can travel thousands of kilometers from impacts in the sea. Tsunamis are composed of a series of broad waves, causing repeated floods far inland, so even a 2-meter-tall tsunami, a mere pup, can damage low-lying areas far more extensively than normal storm waves. Researchers have used computer models to show that tsunamis from rare kilometer-sized asteroids could wipe out entire coasts, but no one had studied the hazards posed by smaller impacts.

A new analysis by geophysicist Steven Ward and planetary scientist Erik Asphaug of the University of California, Santa Cruz, concludes that the biggest tsunami hazard arises from asteroids between 30 and a few hundred meters across, which may strike the ocean every 1000 to 100,000 years. The team assessed many factors that determine risk, from impact rates to wave size and how energetic a wave remains after traversing the ocean. Their results, to appear in the journal Icarus, show that a typical coastal site facing a broad expanse of ocean has a 1 in 14 chance of experiencing a 2-meter-tall tsunami in the next 1000 years. The chances fall to 1 in 35 for 5-meter tsunamis and 1 in 345 for devastating 25-meter waves. Ward and Asphaug also assessed the specific hazards for six major cities. For instance, Tokyo has a 1 in 24 chance of a 5-meter tsunami in the next millennium, while New York City faces 1 in 47 odds of similar waves.

"This is a more realistic assessment than any I have seen," says planetary scientist Alan Hildebrand of the University of Calgary in Alberta, Canada. He notes that certain coastal and seafloor shapes can amplify tsunamis, increasing the hazard at some sites. However, planetary scientist David Crawford of Sandia National Laboratories in Albuquerque, New Mexico, cautions that his own supercomputer calculations of ocean impacts produce tsunamis up to 10 times smaller than those in Ward and Asphaug's analysis. "We've agreed to disagree," Crawford says.

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