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26 December 2004: What happened

News

BOXING DAY'S disastrous tsunami was triggered by a classic tsunami earthquake - a relatively shallow event located 10km below the seabed on a subduction fault line that runs as little as 200km off the coasts of Indonesia, Malaysia and Burma.

The Indian tectonic plate is constantly trying to force its way under the Asian tectonic plate - the definition of a subduction fault. This is significantly different to the better known San Andreas Fault close to San Francisco.

On 26 December, strain energy, accumulated over decades of slow motion collision, was suddenly released. An area of seabed estimated to be 100km wide and 1,000km long was seriously distorted, over a characteristically long period of 15 minutes or more.

Most of it rose by 5m or more. Some of it, in particular close to Sumatra, actually sank. Horizontal displacement was as much as 11m. These movements churned up the deep ocean water, causing a complex pattern of troughs at the surface of the sea above.

Waves up to several metres high were generated in response, and troughs and waves raced outwards at speeds directly proportional to the square root of sea depth. In the deep ocean the waves reached 800km/h, with periods - intervals between crests - measured in many minutes.

The number and pattern of the waves that smashed into the surrounding coastlines was affected by the geometry of the area of distorted sea floor, the time it took for the distortion to happen, and by reflections and refractions off the many islands close by.

As the waves approached the coastline and the seabed rose, the waves slowed.

Kinetic energy was converted to potential energy, and the waves grew taller. The longer and shallower the approach to the beach the taller the waves.

Few of the waves broke before they hit the shore. In most cases the effect was a sudden violent increase in water level, more like a freakishly high tide than a storm wave. In many areas the tsunami waves were preceded by long period troughs, which drained water away from beaches before the actual wave was in sight. Waves also ran long distances up rivers and drainage channels.

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