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Chile quake 2010: Tsunami warning system worked as intended

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Tracking difficult

Tsunamis can be tough to track to anyone bobbing on the ocean's surface. They may represent an increase of eight to 20 inches in the average sea-level height as they pass. That brief change can get lost on a surface already rolling with ranks of large, storm-generated swells stretching as far as the eye can see, or wind-driven waves in a storm.

The answer: pressure-sensitive buoys as deep as 20,000 feet on the ocean floor. Since an earthquake such as Saturday's displaces water from the sea-floor up, these buoys sense the changes in bottom pressure tsunamis trigger as they propagate from their source.

As tsunamis sweep past chains of undersea volcanoes or similar natural sea-floor barriers, they either can intensify or weaken, depending on the conditions they encounter.

But the true forecasting challenge comes as the tsunami interacts with the near-shore bottom, Dr. Murty explains.

Valpariso, Chile, roughly 200 miles north of the quake's epicenter, experienced a tsunami topping 8 feet, while Robinson Crusoe Island, 400 miles offshore, was struck by a wave that reportedly killed four people. Eleven others remain missing.

By the time tsunamis began arriving in Hawaii, Hilo experienced tsunamis less that three feet tall.

Size isn't everything

Less than three feet may be puny by James Michener standards. But Eddie Bernard, a tsunami specialist and director of the National Oceanic and Atmospheric Administration's Pacific Marine Environment Laboratory in Seattle, notes that even small tsunamis can inflict damage, particularly in harbors. The repeated surges can smash boats against each other or against docks. And once a tsunami enters a harbor, it can in effect reverberate within the harbor for hours, much as sound waves from a bell reverberate.

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