Monday, December 27 2004 22:42 Hrs (IST) - World Time -

New Delhi: Tsunami wave, which hit India for the first time today (Dec 26, 2004) wreaking havoc across the southern coastline, is a known phenomenon in the Pacific Ocean region stretching from Chile in Latin America to Japan in Far East Asia.

The waves, which were caused by a massive earthquake on the Indian Ocean and claimed thousand of lives in India and six other countries, are usually triggered by seismic disturbances -- coastal earthquakes, volcanic eruptions, or undersea landslides -- that jolt the ocean floor.

Often, tremors displace ground surface, sending an entire column of water in motion outward from an earthquake region -- often with disastrous results thousands of kilometres from their origin.

The difference between Tsunami waves and tidal waves is that while the former hit only the harbours and coastal areas even as the other parts of the sea remain calm, the latter make the sea rough.

The result is a deep wave that reaches from the sea's surface to the floor and travels horizontally at speeds up to 500 miles per hour and reaches heights between 50 and 100 feet.

The waves travel faster in deeper water and when it approaches the shore, they rise further. It is the sharp elevation of the ocean floor near the coast that slows the bottom of the wave while the top keeps moving at the original speed.

Vast quantity of water is then piled up into a vertical wall that finally crashes over the shore with amazing force.

In the open sea, Tsunamis are only about a meter high, but when they reach a shoreline, they can be taller than a house and weigh millions of tons.

The word "Tsunami" has come from Japanese language, describing very long, low seismic sea waves. It is usually found in the Pacific Ocean where there is significant movement of the earth's tectonic plates.

Japan is one of the two Asian countries -- the other being Indonesia -- which has been frequently hit by Tsunami waves.

The Japanese suffered the fury when hundreds died after a huge wave travelling at a speed of 750 km per hour smashed the country in 1960 following a series of quakes in Chile on the other side of Pacific Ocean.

Major Tsunamis occur in the Pacific Ocean region only about once per decade and the major ones -- the Prince William Sound, Alaska, in March 1964, and the Tsunami generated off the coast of Chile in 1960 -- have been devastating over large distances.

Tsunamis have been responsible for thousands of fatalities, especially in Japan and Indonesia.

In the Indian Ocean, which was hit by the quake today, the Indo-Australian plate is being subducted beneath the Eurasian plate at its east margin.

Therefore, most Tsunamis generated in this area are propagated toward the southwest shores of Java and Sumatra, rather than into the Indian Ocean.

The first sign of an approaching Tsunami is the sea tide draining away from the shore.