The massive earthquake which unleashed deadly tsunamis in many Asian countries on Sunday has literally redrawn the map, moving some islands several meters, scientists say.

As humanitarian agencies are rushing aid for survivors, experts are piecing together the details of the seismic slip that sparked the fatal tsunamis following the world's biggest tremor in 40 years and fourth largest since 1900.

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Sunday's destruction, Nature magazine reported, was unleashed by a 'megathrust' -- a huge, sudden juddering movement beneath the ocean floor. A buildup of pressure caused the Indian Ocean floor to lurch some 15 metres towards Indonesia, burrowing under its tectonic plate and triggering the ferocious swells that smashed the surrounding shores.

The movement is likely to have altered the geography of neighbouring islands such as Simeulue, the Andaman and Nicobar islands and Sumatra itself, Bill McGuire, a geophysicist at University College of London, was quoted as saying.

"In terms of the specific position of Sumatra on the planet, it will have moved," he says. "Things have shifted literally within minutes."

The quake, the report said, follows almost two centuries of tension during which the Indian plate has been pressing against the Burma microplate, which carries Sumatra and the other islands. The two move against one another at an average rate of around 6 centimetres a year -- about the speed at which fingernails grow.

But this movement does not occur smoothly. There has not been a significant quake along this fault since 1833, hence the huge force of this one.

The Indian plate's jarring slide released the Burma microplate from its tension, causing it to spring violently upwards, the report said.

This bulge could have altered the elevations, as well as the positions, of the nearby islands, McGuire says. The Andaman and Nicobar islands may have been raised by the quake; meanwhile, slightly further from the fault line itself, water levels indicate that the Indonesian city of Banda Aceh has been left lower than before.

Californian seismologists, Nature said, are already planning an expedition to see exactly how the area's geography has changed. They will use the Global Positioning System to work out how the region's maps will need to be redrawn.

Quakes of this type, called subduction earthquakes, are commonplace throughout the world but rarely strike with such force, Roger Musson of the British Geological Survey in Edinburgh was quoted as saying.

"This is the largest earthquake I've seen in my career as a seismologist," he told Nature.com. "The length of the rupture was 1,200 kilometres -- I could hardly believe it."

The earthquake, measured at magnitude 9.0, actually consisted of three events within seconds of each other, Musson says. The initial slip, which occurred to the west of Sumatra's northern tip, triggered two further slips to the north.

The total force released was enough to jolt the entire planet, and has been estimated as equivalent to almost 200 million tonnes of TNT.

The sea floor bulge unleashed a wave that surged throughout the Indian Ocean, hitting Indonesia and Thailand within an hour, Sri Lanka and India within four hours, and ultimately causing deaths as far away as East Africa.

The swell would have been imperceptible in the open ocean, but as it entered shallower coastal waters it reared up to several metres in height, destroying seafront villages and resorts and washing through farms up to 5 kilometres inland.

Coastlines nearest to the quake's epicentre would have suffered the heaviest blow, Musson told Nature. As the wave spreads its energy is fanned out over a wider area, and as it moves it is gradually tamed by friction.

The aftershocks can be expected to last for several weeks, says McGuire, although Nature said experts are confident that there will not be another tsunami. But the disaster has highlighted the sheer unpredictability of earthquakes, despite huge research efforts in Japan and the United States.

"Still nobody has predicted an earthquake successfully in my opinion," says McGuire.