What is a Tsunami

Tsunami ('soo-nar-me') is a Japanese word; 'tsu' meaning harbour and 'nami' meaning wave. Tsunamis are sometimes incorrectly called tidal waves but have nothing to do with tides. Tsunamis can travel up to 950 kilometres per hour - as fast as a passenger jet! Tsunami waves move outwards, away from their source. One or more waves can be created per event. Successive peaks can be anywhere from five to ninety minutes apart. The wave train that reaches the coast can range from 30 metre high breakers to barely noticed ripples - but in the open ocean, tsunamis have relatively small heights.

A tsunami is different from normal waves on the ocean. Wind-made ocean waves cause the water to move down to about 150 metres at most. In contrast, the passage of a tsunami involves the movement of water all the way to the seafloor. This means that the speed of a tsunami is controlled by water depth - as the wave approaches land it reaches increasingly shallow water and slows down. Compared to the front of the wave, the rear is still in slightly deeper water (so it is going slightly faster) and catches up. The result is that the wave quickly 'bunches up' and becomes much higher. The highest tsunami occur if they encounter a long and gradual shallowing of the water, because this allows enough time for the wave to interact with its surroundings and cause extensive damage to low-lying areas.