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 cean waves carry tremendous amounts of energy, enough to heave rocks and carve coastlines. But their power is minuscule compared to that of a tsunami. Tsunamis destroy towns and villages -- and they kill. The tsunami that struck the Saundaun province of northwestern Papua New Guinea on July 17, 1998, for example, completely obliterated two seaside villages and destroyed most of two others. An estimated 3,000 people died. The killer wave was the world's worst in over two decades -- but it was hardly the most destructive in history. That dubious honor belongs to a tsunami that struck Awa, Japan in 1703 and killed 100,000 people.
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| In 1964, an earthquake in Alaska generated a devastating tsunami. About 75 km from the epicenter, a demolished truck and displaced ship attest to the force of the waves. |
Tsunamis are frequently called tidal waves, but they have nothing to do with the tides. Rather, they are generated by geologic upheaval: earthquakes, landslides, volcanic eruptions, or even impacts by asteroids or comets. Like a pebble dropped in a pond, these events unleash a series of waves that propagate through the water in ever-widening circles. Unlike the tiny ripples created by that pebble, however, tsunami waves -- also called seismic sea waves -- are gargantuan. In the open ocean, a single wave may be more than 100 miles long and travel at speeds approaching 500 miles an hour. Because the waves are barely a few feet high, they pass undetected beneath ships. Yet once the tsunami waves reach shore, there's no missing them. Friction with the sea floor slows down the wave, beginning at the wave's front. The energy contained in the wave -- which had been distributed over the full water column in the deep ocean -- is thus compressed both vertically and horizontally. What that means is that in shallow water, the tsunami gets very, very high. In the New Guinea tsunami, waves more than 40 feet high surged over the shore at an estimated speed of at least 22 miles per hour.
Earthquakes and volcanic eruptions are most common at Earth's subduction zones -- regions where one tectonic plate dips beneath another, such as off the coasts of Japan, Chile, and Indonesia -- and there, too, tsunamis are most frequent. Because their country is so often hit by tsunamis, Japanese researchers are continually developing new ways to predict the waves' arrival. The newest system to be tested involves a fleet of buoys moored out in the ocean. They are linked to the satellite Global Positioning System, which monitors changes in the vertical position of the buoys of just a fraction of an inch. They work because tsunami waves have a very distinctive pattern: flat, with very long distances from crest to crest. Analyzing the changes in sea surface height can thus differentiate between ocean swells, tsunamis, and the rise and fall of the tides.
-- By Kathy Svitil
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