Last December's earthquake off the coast of Sumatra tore more than 1,000 kilometres of the Earth's crust, say scientists writing in Nature today (14 July).

Their findings show that observation stations up to 3,000 kilometres away from the earthquake's origin shifted as a result of it. Only stations more than 4,000 kilometres away, including ones in Uzbekistan and Australia, were unaffected.

The rupture of the Earth's crust extended from the northern part of Sumatra to India's Andaman islands (see map below).

The team also determined that the rupture travelled at more than 13,000 kilometres per hour initially, then at more than 7,000 kilometres per hour along a second segment.

To characterise the earthquake, the team of Dutch, French, Indonesian, Malaysian, Thai and US researchers used data from approximately 60 global positioning system sites in South-East Asia.

They compared the positions of these sites before and after the earthquake. Sites more than 3,000 kilometres away, in southern China for instance, had moved by up to 10 millimetres in the direction of the earthquake's origin.

The closest sites, about 400 kilometres away, moved by several dozens of centimetres. The team found, for instance, that a site in Phuket, Thailand, shifted by 27 centimetres.

Having collected this data, the team set about finding out what sort of a rupture would have caused exactly these shifts.

To do this, they created computer models and changed the length and type of rupture until the computer-generated shifts corresponded as closely as possible to the shifts they had measured using the global positioning system stations.

This suggested a 1,000 kilometre-long break.

The researchers say that the Earth's crust will continue to move for "a long period of time" as a result of December's earthquake. They note that by 50 days after the earthquake, the island of Phuket had shifted another seven centimetres on top of the initial post-earthquake shift.

The risk of further "large" earthquakes in this region in the near future is "very high", write the team, adding that monitoring structural changes in the Earth's surface for the years to come is crucial.

Read more about tsunamis in SciDev.Net's Tsunami update.

The earthquake ripped through 1,000km of the Earth's crust. The pink boxes indicate the two sections the researchers analysed. Credit: http://www.demis.nl

Link to full Nature article

Reference: Nature 436, 201 (2005)