Tsunami (soo-NAH-mee) is a Japanese word meaning harbor wave.
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, usually in an ocean. Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded. Owing to the immense volumes of water and the high energy involved, tsunamis can devastate coastal regions as has been witnessed in Sendi Island.
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth’s crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position.
More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, owing to the vertical component of movement involved. Movement on normal faults will also cause displacement of the seabed, but the size of the largest of such events is normally too small to give rise to a significant tsunami.
Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimetres above the normal sea surface. They grow in height when they reach shallower water.
Tsunamis cause damage by two mechanisms: the smashing force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying all with it.
About 80% of tsunamis occur in the Pacific Ocean, but are possible wherever there are large bodies of water, including lakes. They are caused by earthquakes, landslides and volcanic explosions.
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