<mediar rid='m1'><?xm-replace_text {mediar}?></mediar>In the aftermath of the waves that devastated coastlines around the Indian Ocean, claiming more than 70,000 lives, experts are piecing together the details of the seismic slip that sparked the fatal tsunami. The quake, the world's biggest for more than 40 years and the fourth largest since 1900, has literally redrawn the map, moving some islands by several metres.

Sunday's destruction was unleashed by a 'megathrust' - a huge, sudden juddering movement beneath the ocean floor. Here, 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 Nicobar and Andaman islands, and Sumatra itself, says Bill McGuire, a geophysicist at University College London, UK. "In terms of the specific position of Sumatra on the planet, it will have moved," he says. "Things have shifted literally within minutes."

Tense time

The quake 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.

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 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.

Uncommon force

Quakes of this type, called subduction earthquakes, are commonplace throughout the world but rarely strike with such force, says Roger Musson of the British Geological Survey in Edinburgh. "This is the largest earthquake I've seen in my career as a seismologist," he told news@nature.com. "The length of the rupture was 1,200 kilometres - I could hardly believe it."

<mediar rid='m2'><?xm-replace_text {mediar}?></mediar>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 seafloor 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, says Musson. 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 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. 

British Geological Survey

University College London