Close-up: What causes tsunamis

Last updated at 08:51 29 March 2005


Under-sea earthquakes are the most common cause of tsunami but the giant "harbour wave" which the Japanese word depicts can also be sparked by sub-marine landslides or volcanic eruptions and, very rarely, by a large meteorite slamming into the ocean.

Tsunamis are very rare. There are roughly six major tsunamis each century, with about 90 per cent of all tsunamis occurring in the Pacific Ocean.

An earthquake has to be over about magnitude 6.75 on the Richter scale for it to cause a tsunami.

A local earthquake, such as the 8.7 registered off Sumatra yesterday, can generate devastating tsunami within minutes - and because the Indian Ocean has yet to install a tsunami warning system there will no advanced notice of the giant waves.

The Indian Ocean around Sumatra and Indonesia is among the most geologically active in the world and has been hit by many tsunami.

Tectonic plates grind together

The quakes are generated by the 13 tectonic plates which make up the Earth's surface "floating" on the semi-molten rock which lies beneath.

The plates grind together and cause friction as

they meet. The energy released as they jerk apart causes the sometimes catastrophic movements in the Earth's crust.

But instead of the shaking of the ground normally

seen in inland earthquakes, sub-marine earthquakes cause the sea floor to be lifted and then dropped, pushing the entire water column up and down.

The movement - sometimes up to 40ft in a split second - displaces billions of tons of water.

Because of the depth of the sea bed the rise in water levels may only appear slight but the volume of water that has been shifted - and the energy created - will be immense.

That energy is then transferred to the horizontal movement of the tsunami wave.

Waves travel in different directions

Within several minutes of a quake, the initial tsunami splits into any number of waves travelling in opposite directions.

Out at sea even the biggest tsunami will be just a few metres high but the crests of the resulting waves are spread out over tens or hundreds of miles or even hours apart.

In deep water tsunamis move very fast, up to 600

mph in deep ocean, but as the depth becomes shallower and there is less room for its energy the tidal force slows down but grows rapidly in height, piling the waves on top of themselves.

They can reach truly disastrous peaks of 100ft or more.

In addition to the large tsunami waves that crash onto shore, the waves push a large amount of water - run-up - above the regular sea level. Run-up can cause tremendous damage on land and is much more common than the huge, thundering tsunami waves.

Most tsunami hit the shore like a fast tide, propelled by the momentum of the mass of water behind it, which provides the power to carry on for many miles after overrunning shorelines or defences.

After the incoming wave - which is not normally the largest - comes the trough, when the huge volume of water dumped on the shore floods back out to compensate for the drop in sea level.

It is then that most of the worst damage is done as people and property are sucked back out to sea by the irresistible body of water or are injured by the debris crashing around them.

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