Their study, published in the British journal Nature, attempts to identify areas where a quake could erupt through "stress transfer," meaning the process by which a massive quake hikes pressure on adjacent or nearby faults.

The 7.9-magnitude May 12 quake ripped open a fault running from the southwest to the northeast at the boundary between the Longmen Shan highlands and the Sichuan basin. It caused land to slip by as much as nine metres (30 feet).

South of this boundary is a region that is a complex geological mosaic, with faults that can move horizontally, known as "strike-slip" faults, and ones that move up and down, known as "thrust" faults.

The scientists believe the big quake eased pressure on several faults around the northeastern end of the boundary, and also reduced stress on a thrust fault, known as the East Chengdu fault, that runs southeast of the city of Chengdu.

But southwest of Chengdu, in a region inhabited by millions of people, the picture is different.

Several faults, including one near the city of Ya'an and another at Xiong Po, have come under greater stress.

Lead researcher Tom Parsons of the US Geological Survey told AFP that the stress increases on these faults were at least sufficient to trigger a big aftershock, if evidence from other temblors worldwide were a guide.

But when this feared event will happen, and its magnitude, are unknown, he emphasised.

Sometimes a big aftershock occurs very soon after the initial quake.

In 1999, for instance, a 7.1 quake hit Duzce, northwestern Turkey, shortly after the 7.4-magnitude Izmit quake, while the 2004 9.3-magnitude Sumatra quake, which unleashed the Indian Ocean tsunami, was followed a few months later by a quake measuring 8.7.

Other aftershocks can occur years later, though.

"Globally, the period of concern can last for months, years and up to decades, with an average of 7-10 years," explained Parsons.

So far, there have been only a few large aftershocks in the Sichuan basin since May 12, with a notable lack of shocks in the six-magnitude range, he said.

This points to two possibilities, one good and the other bad, said Parsons: either regions surrounding the main shock are not very highly stressed, "or we may still expect a few more large events."

The other authors of the paper are Chen Ji of the University of California at Santa Barbara and Eric Kirby of Penn State University, Pennsylvania.

They hope the study will help the Chinese authorities seeking to strengthen earthquake protection, such as building standards and civilian awareness, in vulnerable areas.

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