"It could happen any time, it could take another 20 to 50 years, or another 200 years," Phil Cummins, senior earthquake analyst at the Australian government's seismic monitoring organisation Geoscience Australia, told Reuters.

The area is part of the world's most active earthquake zone that runs along the Java-Sunda trench to the west and south of Indonesia and then curves to run across the top of New Guinea island, accounting for about one third of the world's earthquakes.

Cummins said the activity was caused by the movement of tectonic plates that make up the earth's surface. The Indo-Australian plate is pushing north and under the Eurasian plate by about five or six centimetres (2 inches) a year.

That annual movement might not be much. But when pressure builds up over one or two centuries, the result could be a powerful earthquake that results in the earth suddenly moving as much as 12 metres (39 feet) or more.

That's what happened off the west coast of Indonesia's Sumatra island on Dec. 26 last year when an estimated 1,200 km (750 miles) of fault line slipped about 20 metres (65 feet).

The huge rupture caused the biggest earthquake in 40 years, lifting the sea floor and causing a tsunami that swept across the Indian Ocean, hitting 13 nations and leaving about 230,000 people dead or missing.

Indonesia was the worst affected, with 166,000 people dead or missing in Aceh province on the northern tip of Sumatra.

AFTERSHOCKS CONTINUE

A second earthquake with a magnitude of 8.7 struck the island of Nias off Sumatra on March 28, leaving more than 900 dead, while a 6.9 magnitude earthquake southeast of Nias rocked the area on May 14.

Professor Ray Cas from Melbourne's Monash University, said the increased number of smaller earthquakes in the region made it less likely another major quake was on the way.

"The more small-scale earthquakes that occur in that region, the better. But that doesn't mean that somewhere else along that fault zone, you won't get a similar build up of pressure," Cas told Reuters.

"It's in zones where there are known active faults, and where there has been little significant earthquake activity for some time, that there is a need to be concerned."

However, Professor John McCloskey of the University of Ulster in Northern Ireland, who predicted the Nias earthquake, this month said the region was vulnerable to another major earthquake and tsunami further south, near Siberut island, also off Sumatra but south of Nias.

Professor Peter Mora, from the University of Queensland's Earth Systems Computational Centre, said he would not be surprised if another earthquake with a magnitude 7 or more, struck the Sumatra region soon.

He said the area just off Sumatra's west coast was a hot-spot for large earthquakes. Only earthquakes above about magnitude 8 would create dangerous tsunamis, he said.

However, he said a major earthquake was just as likely in the Pacific Ocean in the region above New Zealand. A tsunami generated from that area could hit New Zealand and the east coast of Australia, possibly within a decade.

1833 REPEATED?

Cummins published a paper last September on the dangers of a major tsunami in the Indian Ocean, based on an 1833 earthquake, which had a magnitude 9.2 and struck south of Nias and Siberut islands.

The 1833 quake also triggered a large tsunami that swept much of the Indian Ocean, mirroring the impact of last year's tsunami disaster, his computer models show.