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Tsunami

 


Tsunami, Japanese word meaning “harbor wave,” used as the scientific term for a class of abnormal sea wave that can cause catastrophic damage when it hits a coastline.

Tsunamis can be generated by an undersea earthquake, an undersea landslide, the eruption of an undersea volcano, or by the force of an asteroid crashing into the ocean. The most frequent cause of tsunamis is an undersea earthquake. When the ocean floor is uplifted or offset during an earthquake, a set of waves is created similar to the concentric waves generated by an object dropped into the water.

Most tsunamis originate along the Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km (24,000 mi) long, which encircles the Ocean Pacific. Since 1819, about 40 tsunamis have struck the Hawaiian Islands.

A tsunami can have wavelengths, or widths (the distance between one wave crest to the next), of 100 to 200 km (60 to 120 mi), and may travel hundreds of kilometers across the deep ocean, reaching speeds of about 725 to 800 km/h (about 450 to 500 mph). A tsunami is not one wave but a series of waves. In the deep ocean, the waves may be only about half a meter (a foot or two) high. People onboard a ship passing over it would not even notice the tsunami. Upon entering shallow coastal waters, however, the waves may suddenly grow rapidly in height. When the waves reach the shore, they may be 15 m (50 ft) high or more. Tsunamis can also take the form of a very fast tide or bore, depending on the shape of the sea floor.

Tsunamis have tremendous force because of the great volume of water affected and the speed at which they travel. Just a cubic yard of water, for example, weighs about one ton. Although the tsunami slows to a speed of about 48 km/h (30 mph) as it approaches a coastline, it has a destructive force equal to millions of tons. Tsunamis are capable of obliterating coastal settlements.

 

Adopted from: Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.


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