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What is a tsunami?

A tsunami is a wave caused by earthquakes or undersea volcanic eruptions

The 1944 Pacific-wide tsunami near Japan

A large earthquake off the southeast coast of Kii Peninsula, Japan, caused a Pacific-wide tsunami in 1944. This illustration, called a Tsunami Time Travel Map, was produced by NOAA and the International Tsunami Information Center. View more Tsunami Time Travel Maps.

Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. Out in the depths of the ocean, tsunami waves do not dramatically increase in height. But as the waves travel inland, they build up to higher and higher heights as the depth of the ocean decreases. The speed of tsunami waves depends on ocean depth rather than the distance from the source of the wave. Tsunami waves may travel as fast as jet planes over deep waters, only slowing down when reaching shallow waters. While tsunamis are often referred to as tidal waves, this name is discouraged by oceanographers because tides have little to do with these giant waves.