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A massive tsunami about to engulf a tropical shoreA massive tsunami about to engulf a tropical shore When the epicentre of an earthquake is on the sea bed, a large submarine landslide can result. The sudden movement of the thousands of tonnes of water above it produces a series of fast moving waves, travelling at around 800 km/h (500 mph), called tsunamis (also sometimes known as tidal waves). The wave travels out in all directions at hundreds of kilometres an hour. In deep water, they are small, but as they approach the shallow coastal waters they slow down and build up in height. Some tsunamis are tens of metres high as they crash on to the shore. The waves may last for several hours.

Plate boundary

A block diagram showing tectonic plates in the PacificA block diagram showing tectonic plates in the Pacific
Tsunami is a Japanese word meaning “harbour wave”. Japan suffers more tsunamis than any other country—on average one every seven years. This is because the islands of Japan lie close to the boundary of two tectonic plates. Here, the Pacific plate is sliding down beneath the Eurasian plate. As it does so, one plate grinds against the other. Occasionally, the two plates "lock" together. Pressure builds up in the rocks underground until they suddenly give, causing an earthquake. If part of the sea bed drops rapidly as a result of the quake, a tsunami results. 

The 2004 Indian Ocean tsunami killed more than 283,000 people in countries bordering the Indian Ocean, from Sri Lanka to Indonesia.

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