THE earthquake off the Indonesian coast that triggered the Boxing Day tsunami was the second-largest on record and 2½ times more powerful than originally thought.

The most comprehensive analysis yet of the quake has confirmed that its magnitude was initially underestimated: in fact it measured 9.3 rather than 9.0 on the Richter scale.

The increase raises the size of the earthquake by a factor of 2.5, as the Richter scale is logarithmic and each full point signifies a 30-fold increase in energy release. Only one more powerful earthquake has ever been recorded — a magnitude 9.5 event that struck Chile in 1960.

The December 26 quake tore a 740-mile (1,200km) rupture along the Sumatra-Andaman fault, lifting the ocean floor by more than 42ft (13m), according to the latest calculations. This produced tsunami waves reaching maximum heights of 98ft, which killed between 200,000 and 300,000 people in 11 countries around the Indian Ocean.

Two studies of the event, from Northwestern University in Illinois and the University of Science and Technology of China, published today in the journal Nature, provide the clearest picture yet of what happened.

The Northwestern research shows that the quake was of magnitude 9.3. The Chinese study confirmed that the rupture stretched 740 miles along almost the entire length of the Sumatra-Andaman fault.

The revised magnitude and extent of the rupture suggest that the north Indian Ocean is no longer at significant risk from future earthquakes along the Sumatra-Andaman fault, as pressure has been released along almost its entire extent.

The studies, which went to press before Monday’s magnitude 8.7 earthquake on the Sunda Trench fault, the southern extension of the Sumatra-Andaman fault, do indicate, however, an increased risk in that region because of extra stress created by the Boxing Day event.

The findings were published as the British team that forecast the Sunda Trench earthquake completed calculations showing that the risk of a third event on the nearby Sumatra fault, which runs beneath the devastated city of Banda Aceh, is not as great as was first feared.

John McCloskey, of the University of Ulster, said that the latest quake had not added to stress on the Sumatra fault.

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