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What is a tsunami?

Tsunami (pron: 'soo-nar-me') is a Japanese word; 'tsu' meaning harbour and 'nami' meaning wave. The phenomenon usually is associated with earthquakes, landslides or volcanic eruptions in, or adjacent to oceans and results in sudden movement of the water column. Until recently tsunami were called tidal waves, even though the event has nothing to do with tides.

A tsunami is different from a wind generated surface wave on the ocean. The passage of a tsunami involves the movement of water from the surface to the seafloor which means its speed is controlled by water depth. Consequently, as the wave approaches land and reaches increasingly shallow water it slows. However, the water column still in deeper water is moving slightly faster and catches up, resulting in the wave bunching up and becoming much higher. A tsunami often is a series of waves and the first may not necessarily be the largest.

The highest tsunami occur when they encounter a long and gradual shallowing of the water. which allows time for the wave to build and interact with its surroundings. Successive peaks can be anywhere from five to 90 minutes apart. The wave train which arrives at the coast can reach heights of up to 30 metres above sea level or create barely noticeable ripples. In the open ocean, even the largest tsunami are relatively small with wave heights of less than one metre.