In the aftermath of the tsunami that devastated coastlines around the Indian Ocean, experts are piecing together details of the seismic slip that sparked the waves. The earthquake, the world's biggest for more than 40 years and the fourth largest since 1900, has literally redrawn the map, moving some islands by up to 20 metres.

The destruction, which claimed as many as 150,000 lives, was unleashed by a ‘megathrust’ — a sudden juddering movement beneath the sea floor. A build-up of pressure caused the floor of the Indian Ocean to lurch some 15 metres towards Indonesia, burrowing under a tectonic plate and triggering the ferocious swells that smashed into surrounding shores.

The earthquake followed almost two centuries of tension during which the India plate pressed against the Burma microplate, which carries the tip of Sumatra as well as the Andaman and Nicobar Islands. The plates move against one another at an average rate of about 6 centimetres a year, but this movement does not occur smoothly. There has not been a very large quake along this fault since 1833 — a fact that may have contributed to the huge force of this one. The India plate's jarring slide released the tension on the Burma microplate, causing it to spring violently upwards.

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 says. “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 that occurred within seconds of each other, Musson explains. The initial slip, which happened 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.

The seafloor bulge unleashed a wave that surged through the Indian Ocean. Initially, the energy of such a wave is distributed throughout the water column, and surface perturbation is small. Only when the water grows shallow, near the coast, does the wave emerge on the surface as a tsunami — the name is Japanese for ‘harbour wave’. In this case, the wave hit Indonesia and Thailand within an hour, and then Sri Lanka and India, ultimately reaching as far as eastern Africa.