In a study published on 17 March, a British team said there was "potential" for a massive early temblor in the ocean floor west of Sumatra as a result of tensions stoked up there by the 26 December quake.
  
They said the quake could be as powerful as 8.5, and possibly generate a tsunami.
  
"There is no doubt - our calculations show a very significant increase on stress on two major active faults in the Sumatra region," seismologist John McCloskey at Britain's University of Ulster said in an interview on 15 March.
  
"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."
  
He added: "When it comes to earthquakes, lightning does strike twice."
 
Uncertain science

McCloskey said that the indicators of an early, powerful quake in the Sumatra region were very strong, but admitted that no one could tell exactly when such an event would occur.
  
But his warning was exceptional. It was as close as it gets to a prediction in a science whose practitioners are notoriously cautious about false alarms.
  
McCloskey explained 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.
  
Fault information

Energy released by the 26 December 9.0-magnitude quake had boosted stress in adjoining parts of two dangerous faults, he said.
  
One fault runs under land to the east of the  quake region and crosses the northwestern tip of Sumatra, running under the city of Banda Aceh, the capital of the Indonesian province that was badly hit by that event.
  

An Acehnese man walks through
debris of wave-damaged houses

Banda Aceh was in fact rocked by a 5.9-magnitude quake last Friday.
  
The other fault, known as the Sunda Trench, runs under the sea to the south, parallel to the Sumatra coast, where two fatal tsunamis occurred in 1833 and 1861.
  
The 26 December movement ruptured 250,000sq km on a stretch of the Burma microplate, a narrow tongue of the Earth's crust that is jostled by the neighbouring Indian, Australian and Sunda plates.
  
That seabed plunge, by as much as 20 metres, triggered the tsunami, killing more than 273,000 people in 11 nations on the northern rim of the Indian Ocean.
  
Part of the energy released by the quake was transferred to the contiguous fault sections. It distorted, compressed and deformed the rock, adding to the burden at known stress points and creating new ones. 
  
Quake histories

Several known episodes in seismic history point to the danger of an imminent follow-on earthquake in subduction zones when the interplay between two vast forces, of sliding and vertical stresses, is right.
  
Just a very small increase in pressure on these tensed parts of the Earth's crust can trigger a catastrophic rupture.
  
In part of the Nankai Trough, southeast of Japan, five of the seven large earthquakes of the past 1500 years unleashed earthquakes in the fault's next section within the following five years, the study says.
  
Another example is what happened in Turkey in 1999. A 7.4-magnitude earthquake in Izmit, southeast of Istanbul, was caused by the stresses of previous temblors on the Anatolian fault.

In turn, this placed stress on the adjoining section of the same fault, unleashing a 7.1-magnitude quake at Duzce three months later.
  
The study by McCloskey's team was published on 17 March in the British weekly science journal Nature.