"The implications of our work show very clearly that earthquake hazard in this area continues to be high," said John McCloskey of the University of Ulster.

"There is a folk understanding that lighting doesn't strike twice in one place -- but earthquakes do," he told Reuters.

Although it is difficult to predict when another might occur, previous coupled earthquakes in Japan happened within a few years of each other.

BIG SLIP

Quakes happen when the Earth's tectonic plates collide.

About 300,000 people perished in the tsunami that followed the magnitude 9 undersea earthquake on December 26 last year. It was one of the largest earthquakes since 1900.

An estimated 1,200 km (750 miles) of faultline slipped up to about 20 metres (65 ft) along the subduction zone where the India Plate dives under the Burma Plate, according to the researchers.

"It is absolutely phenomenal from a seismological point of view," said McCloskey, who reported the findings in the science journal Nature.

The two plates come together at an area called the Sunda trench. Previous earthquakes on the Sunda trench set off fatal tsunamis in 1833 and 1861.

Stress builds up because of the movement of plates. The displacement changes the stress values everywhere in the region for hundreds of kilometres (miles).

McCloskey and his colleagues calculated there is an increase of up to 5 bars (a unit of stress or pressure) in the 50 km of the Sunda trench next to the rupture zone and up to 9 bars for about 300 km on the Sumatra fault near the city of Banda Aceh.

"It is one of the biggest increases in stress I've seen in the years I have been working with this," said McCloskey.

The Izmit earthquake in Turkey that measured 7.4 was triggered by stress increases of less than 2 bars, according to the researchers.

Professor Peter Styles, president of The Geological Society in Britain, said the type of triggering described by McCloskey and his team has occurred on other plate boundaries.

"While we cannot know ahead of time whether this would be tsunamigenic, every effort should be made to ensure that appropriate monitoring technologies and communication protocols are put in place to monitor the Indian Ocean," he said.

McCloskey also emphasised the need for a warning system.