Himalayas may be due for big temblors.

A narrow region along the southern edge of the Tibetan Plateau could be the spawning ground for earthquakes that threaten millions of people in southern Asia in decades to come, scientists warn.

Global Positioning System measurements show that the Indian subcontinent is crashing into Asia at a rate of about 20 millimeters per year, or 2 meters per century. About 80 percent of the energy of that collision is absorbed by rocks and fault zones in a 50-kilometer-wide band that runs along the northeastern border of India through the Himalayas, the so-called Himalayan arc, says Roger Bilham, a geophysicist at the University of Colorado at Boulder. The area around the arc absorbs the other 20 percent.

Nearly all of the energy accumulating in and around the mountains will eventually be released in earthquakes, according to an analysis by Bilham and his colleagues.

In 1905, 1934, and 1950, temblors measuring more than 8.0 on the Richter scale rocked separate parts of the Himalayan arc. Each occurred along existing faults between 100 and 300 km long. In each quake, the sides of these faults slipped about 4 m relative to one another, releasing 2 centuries' pent-up energy.

On the other hand, those three great quakes relieved the stress along only one-fourth of the Himalayan arc, says Bilham. The remainder of the region hasn't experienced a major quake in more than 300 years. This means that the faults in those areas have stored enough energy to cause temblors at least as large as those suffered last century. Bilham is quick to note that he and his colleagues aren't forecasting when or where along the Himalayan arc a major earthquake might occur.

"You can't stop these earthquakes," says Bilham. "You can only hope to mitigate their effects." He and his colleagues report their findings in the Aug. 24 SCIENCE.

India's population has doubled since the Himalayan earthquake of 1950. About 50 million people--including residents of the capital cities of Bangladesh, Bhutan, India, Nepal, and Pakistan--are at risk from similar temblors today. Bilham's team estimates that if a magnitude-8.0-plus earthquake occurred near one of the larger cities in the area, it could kill around 2 million people.