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The earthquake which triggered last December's Asian tsunami caused a rupture in the ocean floor more than 1,000km long, a new study reveals.
The finding is based on data gathered from Asian research stations which used GPS to monitor ground movements.
Scientists say they were surprised that such a large quake could happen in south-east Asia.
They tell Nature magazine that further studies into the behaviour of Asian earthquake zones would be prudent.
High accuracy
Over the last few years Christophe Vigny, from the Ecole Normale Superieure (ENS), has been leading a European Union-funded project which aims to measure seismic activity and tectonic movement in south-east Asia.
This was the biggest earthquake since Chile in 1960
Prof Madiaraga, Ecole Normale Superieure
The idea is to establish research posts whose locations can be determined to within a few millimetres. By plotting their positions over a number of years, researchers can detect how tectonic plates are moving with high accuracy.
Each station in this "Seamerges" programme monitors several GPS (Global Positioning System) satellites continuously and updates its position every 30 seconds. The raw data is processed to compensate for any external influences, such as disturbances in the atmosphere which could distort the GPS signal.
Similar systems exist in South America, another earthquake-prone region.
Dr Vigny is on vacation and uncontactable for interview; but his colleague Professor Raul Madariaga told the BBC News website what stations in the Seamerges programme, and others in Asia, saw in the early hours of 25 December.
'Giant zip'
"It started around Banda Aceh in northern Sumatra, and spread northwards to the Nicobar Islands," he said.
"It all happened over a period of about five minutes - the quake travelled around 3km/s, the sea floor opening up something like a giant zip."
Everything can be traced to the collision of two tectonic plates(Image: Royal Navy UK)
More details
The biggest displacement was registered in Phuket, Thailand, where the Earth's crust moved by 27cm; stations thousands of kilometres away, at Kunming in China, Bangalore and Hyderabad on the Indian mainland, and even Diego Garcia in the Indian Ocean, showed displacements of several millimetres.
"This was the biggest earthquake since Chile in 1960," said Professor Madiaraga, "and we don't know why it was so big.
"People thought earthquakes like this could only happen in Chile and Alaska; now there's a lot of concern in Japan about much bigger quakes."
GPS has given seismologists a powerful new tool for investigating seismic and tectonic movements, and clearly the data generated in south-east Asia will be of special interest in the coming months - particularly as another Nature study, published last month, showed that the threat of a further serious earthquake and, therefore, a tsunami, remains very real.
http://news.bbc.co.uk/1/hi/sci/tech/4680471.stm
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