RENEE MONTAGNE, Host:
The earthquake, as we know, in Chile this weekend brought down buildings and killed hundreds of people. And there was another threat, the massive quake trigged a tsunami, setting off alarms across the Pacific basin. NPR's Christopher Joyce tells us how scientists tracked the waves and why the big waves never materialized.
CHRISTOPHER JOYCE: Here's geophysicist Brian Shiro from the Pacific Tsunami Warning Center describing how that creates a wave.
BRIAN SHIRO: A tsunami is basically the ocean, all the way to the sea floor, kind of being picked up and dropped. That's what the earthquake does - it lifts the sea floor a little bit and then drops it back down.
JOYCE: Now, the Chilean quake was the fifth-biggest in the last century. But the biggest displacement was deep under the sea-floor, about 22 miles down. It's the sea floor movement that creates the tsunami. Geophysicist Harley Benz is with the U.S. Geological Survey.
HARLEY BENZ: We're not seeing a lot of coastal uplift from this earthquake, and so it appears that there wasn't a large amount of displacement of the sea floor to cause a tsunami.
JOYCE: There was some sea floor lift though - enough to create a tsunami. But one thing that may have kept it small was that the quake hit fairly near shore, in relatively shallow water. Here's Brian Shiro again.
SHIRO: It was not in deep enough water to have enough water to actually excite and move, to create the initial pulse of the tsunami. There's a less mass to start the process off.
JOYCE: Less water above the fault means less comes crashing down after the quake. Nonetheless, a special device lying on the sea floor near Chile - it's called a DART buoy - did detect an underwater wave.
SHIRO: This thing is sitting down there measuring the weight of the water on top of it. Ok? And it's very sensitive, and so as a wave comes over, it measures that as a pressure increase.
JOYCE: More DART buoys picked up the waves as they traveled across the Pacific; scientists have added over 30 of the devices since the big quake and tsunami in the Indian Ocean. The buoys showed that the waves were not that big. But the final measure of a tsunami can not be taken until it hits shallow water near a shoreline. Here's Harley Benz.
BENZ: You have this big pulse traveling at 500 miles an hour, and then when it gets to the other end it starts piling up because it's slowing down.
JOYCE: Christopher Joyce, NPR News.
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