A colossal earthquake may hit Indonesia's Sumatra island within 30 years, triggering a tsunami and making last month's deadly temblor look tiny by comparison, a geologist has warned.

Kerry Sieh, director of the Earth Observatory of Singapore, said the next big quake would last more than six times as long as the 7.6 magnitude quake which struck western Sumatra on September 30, leveling the city of Padang. "We expect it will be about a magnitude 8.8, plus or minus say 0.1," Sieh, an American professor, said at a presentation late Wednesday at the Nanyang Technological University, where the observatory is located.

He said last month's Sumatra quake lasted about 45 seconds.

"This one'll last about five minutes," Sieh said."This 7.6 is very, very small, minuscule compared to the great earthquakes."

The official death toll reached 1,115 on Wednesday but many more are feared dead after villages were turned into mass graves. Around 100,000 houses are estimated to have been destroyed, leaving about half a million people homeless.

Based on historical earthquake trends from geological analysis of coral specimens from the region, last month's quake was just a precursor, Sieh said.

Likening the pressures under the affected fault to a coiled spring, Sieh said the recent quake "had really very little effect in terms of relieving the spring" which will unleash pent-up energy possibly within the next 30 years.

"If you're a parent who has a child, you have to expect that child's going to experience that earthquake and the tsunami," he added.

A massive tsunami hit Indonesia and other countries in the Indian Ocean rim in 2004, killing about 220,000 people, most of them in Aceh province in northern Sumatra.