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A tsunami (from Japanese 津波 meaning wave in port or "harbour wave") is one or a series of ocean surface waves that occur after a large earthquake (having a vertical component of movement), seaquake, volcanic activity, slumps, or meteorite impacts in or near the sea. Tsunamis are sometimes referred to as tidal waves, a reference to their often not looking like cresting waves when they reach shore, but instead like a tide that keeps rising, but the term is misleading as tsunamis are not caused by tides, and its use is being discouraged by oceanographers. While it has also been noted that tsunamis often have little to do with harbors (despite the Japanese translation of the word), the term "tsunami" is used exclusively to refer to water surges caused by physical displacement of water and is thus more correct.

It is probable that the Japanese term was first coined because surges would have the greatest impact, and be most widely witnessed in coastal population centres, which are often built around natural harbours, rather than in sparsely populated or unpopulated areas.

Evidence shows that megatsunamis, which are caused by significant chunks of an island collapsing into the ocean, are also possible.

Related to a tsunami is a seiche. Often large earthquakes produce both tsunamis and seiches at the same time. In addition there is evidence that some seiches have also been caused by tsunamis.

Physics

In deep water, the energy of a tsunami is constant, a function of its height and speed. Thus, as the wave approaches land, its height increases while its speed decreases. A tsunami has a very long wave length (in the order of 100 km), which makes it act as a shallow-water wave. Since the speed of a shallow-water wave is g d {\displaystyle {\sqrt {gd}}} , where g {\displaystyle g} is the gravitational acceleration and d {\displaystyle d} is the water depth, a tsunami in the open ocean can obtain a speed of about 700 km/h. While in deep water a person at the surface of the water would probably not notice, the wave can increase to a height of 30 m and more as it approaches the coastline. Tsunamis can cause severe destruction on coasts and islands.

Warning systems

Many cities around the Pacific, notably in Japan but also in Hawaii, have warning systems and evacuation procedures in the event of a serious tsunami. Tsunamis are predicted by various seismologic institutes around the world and their progress monitored by satellites. The first rudimentary system to alert communities of an impending tsunami was attempted in Hawaii in the 1920s. More advanced systems were developed in the wake of the April 1, 1946 and May 23, 1960 tsunamis which caused massive devastation in Hilo, Hawaii. The United States created the Pacific Tsunami Warning Center in 1949, and linked it to an international data and warning network in 1965.

One system for providing tsunami warning is the CREST Project (Consolidated Reporting of Earthquakes and Tsunamis) implemented on the West coast (Cascadia), Alaska, and Hawaii of the United States by the USGS, NOAA, the Pacific Northwest Seismograph Network, and three other university seismic networks.

Tsunami prediction remains an imperfect science. Although the epicenter of a large underwater quake and the probable tsunami arrival times can be quickly calculated, it is almost always impossible to know whether massive underwater ground shifts have occurred, resulting in tsunami waves. As a result, false alarms are usually the rule.

History

There are many ancient descriptions of sudden and catastrophic waves, particularly in and around the Mediterranean. Thousands of Portuguese who survived the great 1755 Lisbon earthquake were killed by a tsunami which followed a few moments later.

No system can protect against a sudden tsunami. A devastating tsunami occurred off the coast of Hokkaido in Japan as a result of an earthquake on July 12, 1993. As a result, 202 people on the small island of Okushiri lost their lives, and hundreds more were missing or injured. This tsunami struck just three to five minutes after the quake and most victims were caught while fleeing for higher ground and secure places after surviving the earthquake.

The most recent series of lethal tsunamis occurred on December 26, 2004 in the Indian Ocean, with fatalities running in the tens of thousands, ranging from those in the immediate vicinity of the quake in Indonesia and Thailand to people thousands of kilometres away in Bangladesh, India, Sri Lanka, the Maldives, and even Somalia in eastern Africa. As of 11:25 am EST on December 27, 2004, news reports estimate the death toll at 23,000. Unlike the Pacific Ocean there is no organised alert service covering the Indian Ocean, in part due to there having been no major tsunami there since 1840.

However, if there was a very large subduction zone earthquake (magnitude 9.0) off the west coast of the United States, for example, people in Japan would have up to 18 hours (and likely warnings from warning systems in Hawaii and elsewhere) before any tsunami arrived, giving them some time to get away to higher ground.

See also

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