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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: You can create a tsunami in your bathtub. Put your hand, palm open, under the water, parallel to the surface. Then quickly push down. You'll create an underwater wave that will push water to the end of the tub and up the side. That's what a thrust earthquake on the sea floor did in Chile. A big tectonic plate thrust itself underneath an adjoining plate. That jacked up a stretch of sea floor about 400 miles long.

Here's geophysicist Brian Shiro from the Pacific Tsunami Warning Center describing how that creates a wave.

Mr. BRIAN SHIRO (Geophysicist, Pacific Tsunami Warning Center): 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: When you lift the sea floor, you lift the water column above it. Then gravity smacks the water back down and creates the underwater wave.

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.

Dr. HARLEY BENZ: (Geophysicist, U.S. Geological Survey): 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.

Mr. 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.

Mr. 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: That's when computer models at the tsunami warning center in Hawaii started to calculate how powerful the tsunami waves were and where they were headed. Since the original fault line lay northeast to southwest, Hawaii and Japan on the northwest were likely targets, so the alert went out.

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.

Dr. 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: The shallow water allows the back end of the wave to catch up and pile onto the front end. Since the shape of every coastline is different, it's hard to predict how even a small tsunami will behave near shore.

In this case, the piling up amounted to only about three feet in Hawaii. The tsunami reached Japan as well, but did little damage. Benz says with each tsunami, the computer models for tracking them get better. They'll need to. He says the Chilean fault that buckled last weekend probably isn't finished moving yet.

Christopher Joyce, NPR News.

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