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People run from an approaching tsunami in Hilo, Hawai'i, on 1 April 1946; note the wave just left of the man's head in right center of image. Credit: The Pacific Tsunami Museum in Hilo, Hawai'i
A Japanese word meaning "harbor wave." A tsunami is a wave or series of waves that are generated in a body of water by a sudden disturbance that displaces water. Found mainly in the Pacific, tsunami are typically caused by earthquakes and landslides in coastal regions. In mid-ocean, teh wave height is usually under 1 meter, the distance between succeeding crests being of the order of 200 km, and the velocity avout 750 km/h. Near the coast, friction with the sea bottom slows the wave, so that the distance between crests decreases, the wave height in creasing to about 25 meters or more.

Volcanic eruptions, nuclear explosions, and even impact of meteorites, asteroids, and comets from outer space can also generate tsunamis.

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Source: U.S. Geological Survey