The Indonesian city of Bandar Aceh, which was already badly hit by the killer wave, could be at risk from a quake measuring up to 7.5 on the Richter scale and there is a potential for a tsunami-making 8.5 quake offshore, they warn.

"There is no doubt -- our calculations show a very significant increase on stress on two major active faults in the Sumatra region" since December 26, seismologist John McCloskey at Britain's University of Ulster told AFP.

McCloskey noted that in so-called subduction zones, an earthquake can be swiftly followed by another one if certain geological conditions are met.

"There is a very well established link between these stresses and following earthquakes," he said.

Energy released by the December 26 quake has boosted stress in adjoining parts of two dangerous faults, he said.

One fault runs under land to the east of the December 26 quake and crosses the northwestern tip of Sumatra. The other fault, known as the Sunda Trench, runs under the sea to the south, parallel to the coast, where two fatal tsunamis occurred in 1833 and 1861.

"We are not trying to cry wolf," said McCloskey. "We can point to many other quakes where the stresses like the one we've measured have resulted in a following earthquake, and we are suggesting there is a significantly increased risk.