Tsunami

Tsunami, Japanese word meaning “harbour wave” and used as the scientific term for a seismic sea wave, a large ocean wave generated by an undersea earthquake. A tsunami is thought to be triggered when the ocean floor is tilted or offset during the quake, creating a set of waves similar to the concentric waves generated by an object dropped into water. Another possible cause is an undersea landslide or volcanic eruption. Most tsunamis originate along the so-called Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km (24,000 mi) long, that encircles the Pacific Ocean. Since 1819, for example, more than 40 tsunamis have struck the Hawaiian Islands.

A tsunami may travel hundreds of kilometres across the deep ocean, reaching speeds of about 725 to 800 km/h (450 to 500 mph). On entering shallow coastal waters, the wave, which may have been only about half a metre (a foot or two) high out at sea, suddenly begins growing rapidly. By the time it reaches the shore, it may become a towering wall of water 15 m (50 ft) high or more, capable of destroying entire coastal settlements.

The most deadly series of tsunamis on record occurred in the Indian Ocean on December 26, 2004. An undersea earthquake measuring 9.1 on the Richter scale took place 160 km (100 mi) off the western coast of northern Sumatra in Indonesia. It killed an estimated 283,000 people in coastal regions around the Indian Ocean. The previous most destructive tsunami was at Awa in Japan, which killed an estimated 100,000 people in 1703.

Tsunamis have erroneously been called tidal waves, but they have nothing to do with the diurnal pattern of high and low tides. Such waves, however, in combination with meteorological phenomena, can also sometimes be destructive. Storm surges (domes of water that rise underneath hurricanes or cyclones) can cause extensive coastal flooding when the storms reach land and are particularly devastating if they occur at high tide.