A Himalayan earthquake that could result in unprecedented loss of human life is overdue, according to geologists.

They have calculated that a swath of territory along a 600km line stretching from western Nepal into northern India could soon be hit by catastrophe.

Roger Bilham, professor of geological sciences at the University of Colorado at Boulder, in the United States, warned: "One or more great Himalayan earthquakes appear to be overdue.

"A grim seismic future will accompany the new millennium in northern India, Pakistan, Bangladesh and Nepal."

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Himalayan earthquakes are caused by the collision of the Indian tectonic plate with Asia, a process that began 50 million years ago.

One product of this is the Himalayan mountains. Another is a propensity for earthquakes.

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As India collides with Asia, at a speed of some 1mm a week, the subcontinent is being forced down into the mantle beneath Tibet.

The process is not smooth. It occurs in a series of violent jumps or slips that are felt on the surface as earthquakes. In the past 50 years, they have claimed the lives of more than 20,000 people in the region.

Bilham and colleagues have analysed British colonial records of Indian earthquakes dating back to 1803 to extract data on two centuries of such slips.

With American and Asian collaborators, they have used global positioning system receivers sited across India, Nepal and China to measure the minute movements of the plates over the past five years.

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By drawing together this data, the scientists have found a 600km central gap along the collision zone, which has not undergone a slip for several centuries.

They believe that this interval matches the average gap between significant Himalayan earthquakes.

Bilham predicts that an earthquake in the region today would probably cause a slip of more than 6m and possibly as much as 15m. This would produce an earthquake of a magnitude between 8 and 8.7.

Population growth and urbanisation across the Ganges plain could multiply casualty rates compared with previous tremors, possibly producing a death toll in excess of 1 million.

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