Jan 25, 2006

Tsunami - an old term that has become new

A new term to many Indians that became familiar on Dec 26th, 2004. Tsunami is a Japanese word derived from Tsu (harbour) and nami (wave). It is a series of waves displaced on a massive scale either due to volcanic eruptions, earthquakes or meteorite impacts. The most common cause is an undersea earthquake.

Tsunamis can be generated when the sea floor displaces the overlying water vertically. Such displacement occurs at the plate boundaries when denser oceanic plates slip under the continental plates in a process known as subduction. Waves are formed as the displaced water mass moves under the influence of gravity to regain its equilibrium and radiates across the ocean like ripples on a pond.

A single tsunami event may involve a series of waves of varying heights; the set of waves is called a train. In open water, tsunamis have extremely long periods (the time for the next wave top to pass a point after the previous one), from minutes to hours, and long wavelengths of up to several hundred kilometres.

The wave travels across the ocean at speeds from 500 to 1,000 km/h. As the wave approaches land, the sea shallows and the wave no longer travels as quickly, so it begins to 'pile-up'; the wave-front becomes steeper and taller, and there is less distance between crests.

Tsunamis propagate outward from their source, so coasts in the "shadow" of affected land masses are usually fairly safe. However, tsunami waves can diffract around land masses. They also need not be symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography.

One of the early warnings of a Tsunami comes from nearby animals. Many animals sense danger and flee to higher ground before the water arrives. The Lisbon quake is the first documented case of such a phenomenon in Europe. The phenomenon was also noted in Sri Lanka in the 2004 Indian Ocean earthquake. Some scientists speculate that animals may have an ability to sense subsonic Rayleigh waves (a type of surface wave associated on the Earth with earthquakes and subterranean movement of magma) from an earthquake minutes or hours before a tsunami strikes shore.

Japan has implemented an extensive programme of building tsunami walls of up to 4.5m high in front of populated coastal areas. Environmentalists have suggested tree planting along stretches of sea coast which are prone to tsunami risks.

Tsunamis occur most frequently in the Pacific Ocean. The Pacific Tsunami Warning Center was established to track these killer waves and provide warning. Unlike in the Pacific Ocean, there is no organized alert service covering the Indian Ocean. This is due to the absence of major tsunami events. In light of the 2004 Indian Ocean tsunami, UNESCO and other world bodies have called for a global tsunami monitoring system.

Source of information : Wikipedia

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