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As a plate plunges down into Earth's interior, it moves in fits and starts -- sticking for awhile, then slipping. When it's stuck against the edge of a continental plate, stresses build up. When the locked zone gives way, parts of the seafloor may snap upward like a diver's springboard as the tension is released; other areas may sink downward. In the instant after the quake, the shape of the sea surface mirrors the contours of the seafloor below. But, just as quickly, gravity acts to return the sea surface to its original shape. As the rumpled sea flattens out, ripples race outward. A tsunami is born. (See Tsunami Attack animation, 17K. You will need the free Flash plug-in to view this animation.) On the open ocean, tsunami waves approach speeds of 500 mph, almost fast enough to keep pace with a jetliner. But gazing out the window of a 747, you wouldn't be able to pick it out from the wind-driven swells. In deep water, the waves spread out and hunch down, with hundreds of miles between crests that may be just a few feet high. A passenger on a passing ship would scarcely detect their passing. But in fact the tsunami crest is just the very tip of a vast mass of water in motion. Though wind-driven waves and swells are confined to a shallow layer near the ocean surface, a tsunami extends thousands of feet deep into the ocean. Because the momentum of the waves is so great, a tsunami can travel great distances with little loss of energy. The 1960 earthquake off the coast of Chile generated a tsunami that had enough force to kill 150 people in Japan after a journey of 22 hours and 10,000 miles. The waves from a trans-Pacific tsunami can reverberate back and forth across the ocean for days, making it jiggle like a planetary-scale pan of Jell-O. |
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