The quake and tsunami are likely within the next few decades, they said.

On December 26, 2004, a 9.3-magnitude earthquake off Sumatra triggered tsunamis which killed 220,000 people in countries around the Indian Ocean, including 168,000 in Aceh province on the northwestern tip of Sumatra, in one of the deadliest natural disasters ever.

The Boxing Day quake marked the largest in a series of big earthquakes as large sections of the active fault beneath the eastern Indian Ocean flank failed over the past eight years, the study in the US magazine Science says.

"One question of great humanitarian and scientific importance is which remaining un-ruptured sections" of the fault will fail next, says the international study led by Kerry Sieh, director of the Earth Observatory of Singapore at Nanyang Technological University.

Until late last year the largest remaining unbroken Sumatran section was a stretch known as the Mentawai patch, the study said. But in September 2007 an 8.4-magnitude quake and several large aftershocks hit Sumatra's west coast, off Padang city, and caused 23 fatalities.

Study of a 700-year-long history of Sumatran seismic cycles implies that the September quake "was the beginning of an episode of failure of the Mentawai patch", the researchers say.

The amount of potential slip not relieved last year is enough to generate an 8.8-magnitude quake in the area where more than one million people live along the coast, the scientists said.

Such an event "would undoubtedly produce great damage in Padang and neighbouring cities and villages" while scientific modelling shows that a tsunami would hit the Padang and Bengkulu areas, it added.

"These results imply that losses of life and property could equal or exceed those in Aceh province in 2004," the study said.

The scientists made their conclusions by examining records of sea level change in coral around the Mentawai islands off Sumatra's west coast. The examination implies that the 700-kilometre (434-mile) section of the fault "has generated broadly similar sequences of great earthquakes about every two centuries for at least the past 700 years," the study said.

If previous cycles are a reliable guide, Sieh said, "we can expect one or more very large West Sumatran earthquakes as the rest of the Mentawai patch fails within the next few decades."

Another researcher, Danny Natawidjaja from the Research Centre for Geotechnology at the Indonesian Institute of Sciences, said he hoped the findings would encourage governments and humanitarian agencies to accelerate preparations.

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