Chile earthquake: What is a tsunami?

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The word tsunami comes from a combination of the Japanese words tsu or harbor, and nami or

wave.

They are created when an underwater fault ruptures, with one side rising and the other falling. The rising tectonic plate lifts massive quantities of water above sea level. As gravity forces the water to return to normal levels, huge waves can be created.

In deep water, the waves can range in size from a couple of inches to a foot or two, hardly noticeable as it passes a ship. It can travel as fast as a jetliner, about 620 mph, or 10 to 100 times as fast as a wind-driven wave. But even at that speed, it can take several hours for it to cross the ocean. As the wave reaches shallower water, it slows down, the energy it contains is compressed and the wave can get much higher. Tsunami waves as high as 30 to 40 feet have been reported in the past.

As the tsunami wave nears shore, water will be sucked out to sea, exposing coral and other submerged objects. Then the water will rush back in, covering land surfaces as much as a mile or more inland. Contrary to the name, tsunamis are not a single wave, but a series of waves that can be separated by as much as 20 minutes or more, and the series can last for hours.

A network of buoys spaced around the Pacific Ocean can monitor the progress of the wave, measuring the slight increases in surface height. By monitoring these buoys, the National Oceanic and Atmospheric Administration was able to conclude that the tsunami was traveling slightly faster than expected and would strike Hawaii about 11:09 a.m. local time rather than some 10 minutes later, as had been predicted earlier.

And it is not only the east-facing shores of the islands that will be affected. Tsunamis curl around the shore, creating nearly as much havoc on the opposite side of islands as they do on the facing shore.

Most tsunami warnings call for people to evacuate inland to get out of the path of the waves. But some recent research suggests that, if that is not possible, climbing to the upper stories of buildings near the shore can also provide protection.

-- Thomas H. Maugh II

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