Tsunamis are dramatic waves usually generated when a submarine earthquake causes a sudden shift in the ocean floor along a fault line.
Map showing the hourly progression of a tsunami that originated just south of Alaska.
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 tsunami is a wave or series of waves that are generated in a body of water by a sudden disturbance that displaces water (tsunami is Japanese for 'harbor wave'). Found mainly in the Pacific, tsunami are typically caused by earthquakes and landslides in coastal regions. In mid-ocean, the wave height is usually under 1 m, the distance between succeeding crests being of the order of 200 km, and the velocity about 750 km/h (400 mph). Near the coast, friction with the sea bottom slows the wave, so that the distance between crests decreases, the wave height increasing to about 25 m or more.
Volcanic eruptions, nuclear explosions, and even impact of meteorites, asteroids, and comets from outer space can also generate tsunamis.