Sizing Up The Tsunami: Why It Wasn't So Big

The earthquake in Chile on Saturday not only brought down buildings and killed hundreds of people — it also created a tsunami. The tsunami set off alarms around the Pacific basin. Eventually, the waves turned out to be pretty small, at least beyond Chile.

To understand why, let's start with how a tsunami is created. You can make one 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, the Nazca plate, thrust itself underneath an adjoining plate, the South American plate, which is what Chile sits on. That jacked up a stretch of sea floor about 400 miles long.

Geophysicist Brian Shiro from the Pacific Tsunami Warning Center describes how that creates a wave: "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."

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. The wave front stretches vertically from the ocean surface all the way to the bottom — with a lot of water following behind. The thrust creates two wave fronts moving in opposite directions away from the fault.

Now, the Chilean quake was the fifth-biggest in the past 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, with the U.S. Geological Survey, says there's some evidence that the floor didn't move that much. "We're not seeing a lot of coastal uplift from this earthquake," says Benz, "so it appears that there wasn't a large amount of displacement of the sea floor to cause a tsunami."

There was some 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.

"It was not in deep enough water to have enough water to actually excite and move," says Shiro, "to create the initial pulse of the tsunami. There's less mass to start the process off."

Less water above the fault means less comes crashing down after the quake.

Nonetheless, a special device lying on the sea floor near Chile — called a DART buoy (deep-ocean assessment and reporting of tsunamis) — did detect an underwater wave.

"This thing is sitting down there measuring the weight of the water over it," says Shiro, "and it's very sensitive, so as the wave comes over it, it measures it as a pressure increase."

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, one wave would propagate to the northwest, toward Hawaii and Japan.

After the first DART buoy picked up the signal, the Pacific alert went out. Then more DART buoys picked up the waves as they traveled across the Pacific; scientists have added more than 30 of the devices since the big quake and tsunami in the Indian Ocean in 2004. The buoys showed that the waves weren't that big.

But the final measure of a tsunami can't be taken until it hits shallow water near a shoreline. Say's Harley Benz of the USGS: "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."

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 3 feet in Hawaii. The tsunami reached Japan as well, but did little damage.

Benz says that with each tsunami, the computer models for tracking them get better. They'll need to; he says the Chilean fault that buckled over the weekend probably isn't finished moving yet.

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