THE chain reaction that set off enormous, deadly tsunami waves started miles beneath the ocean floor, off the tip of the Indonesian island of Sumatra.

The waves which swamped coastlines across Asia were generated by the fifth largest earthquake since 1900.

Geological plates pressing against each other slipped violently, creating a bulge on the sea bottom that may be as high as 10 yards and up to 745miles long.

That impact caused the tsunami and triggered a series of aftershocks.

Roger Musson, a seismologist with the British Geological Survey in Edinburgh, said the disaster had happened because the whole of the Indian-Australian plate is moving north.

He said: "This will happen again, over the next several million years, and has been happening for several million years in the past."

David Booth, another seismologist with the British Geological Survey and doctor in geophysics, added that not all quakes would trigger a tsunami.

But when they did, he added, the results would be devastating.

He said: "First of all, the earthquake has got to curve underwater in the ground under the ocean and also the movement of the rock has got to reach the surface.

"Where there is a displacement of the ocean floor, it causes a movement on the surface and it spreads out from there at a speed of about hundreds of miles an hour.

"It's fast, but slow enough for warning to be given, if a sophisticated warning system is set up."

Dr Booth said an Australian scientist had suggested in September that an Indian Ocean warning system be set up, but it takes a year to create one.

He added that those living on the Indian Ocean were less likely than Pacific coastal dwellers to know the warning signs of a tidal wave about to hit ? water receding unusually fast and far from the shore.

As well as the main earthquake itself, there were at least half a dozen powerful aftershocks, one of magnitude 7.3.

DrBooth said waves created by the quakes need only be a few yards high, but as they reach shallower waters, the momentum builds up to such an extent that, by the time the waves reach shore they can be up to 50 yards high.

He went on: "The ocean floor is being pushed under the continent, and enormous stresses build up.

"Because the length of the fault is very long you have the potential for a very large earthquake as the whole fault moves as one."

Up until yesterday, there had only been one or two small tidal waves in the world this year - nothing compared to the scale of Asia's tsunami, the vibrations of which were recorded in the UK, around 5000 miles away.

In 1883, a tsunami following the eruption of Krakatoa volcano between the Java and Sumatra killed 36,000.

The passage of that tsunami was traced as far away as Panama.