In recent years, India has witnessed a rash of major earthquakes, the most recent of which devastated Bhuj last January. But if the authors of a report in the current issue of the journal Science are correct, the worst is yet to come in a different part of the country. According to the new analysis, one or more massive earthquakes may be overdue in the Himalaya, threatening the 50 million people that live in that region.

Roger Bilham and Peter Molnar of the University of Colorado and Vinod K. Gaur of the Indian Institute for Astrophysics in Bangalore base their grim conclusions on several lines of evidence. Geophysical data, they note, demonstrate that India's basement rock flexes and slides beneath the Himalaya much like the earthquake-ridden subducting oceanic crust. Second, satellite-based measurements reveal that India and southern Tibet are colliding and converging at a rate of about two meters a century, and that a relatively small swathe of the Tibetan Plateau's southern edge is absorbing most of this convergence. Third, the team observes, in the Himalaya, potential slip accumulates as elastic, rather than inelastic, strain. As such, massive earthquakes may well offer the only release for the mounting convergence strain in this belt.

Historical records show that at least as far back as the early 1800s, sizeable quakes have erupted in this region every few decades. But since 1950, the Himalaya have remained quiet. Indeed, the researchers report that some areas may not have experienced major earthquakes in the past 500 years.

"The main driving engine in the system is the movement of the Indian plate, which winds up the Greater Himalaya like a giant spring compressed against the Himalayan plateau. Deep beneath Tibet, India slides northward with relative ease," Bilham explains. "We know the inevitable outcome. The lock holding the spring will break, propelling the Himalaya southward in a giant earthquake." He adds that "to have the Indian and Himalayan plates unzip to remove the geologic stress requires one or more giant earthquakes to occur."

Bilham and Molnar suggest that up to seven earthquakes in the 8.1 to 8.3 magnitude range are overdue in this region. Considering past human tolls from earthquakes and the population increases that have occurred since then, the authors estimate that 200,000 people could perish in a single quake. If a massive earthquake strikes near one of the heavily populated cities in the Ganges Plain, however, "such an estimate may be too low by an order of magnitude.

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