Friday, 9 December 2011

Megaquake warning for the high Himalayas

Take a hotspot of potential conflict and add a magnitude 9 earthquake. It sounds like a nightmare scenario, but it could play out in the troubled Kashmir region of the Indian subcontinent, according to Roger Bilham of the University of Colorado at Boulder. Worse still, Bilham fears that such a major quake is likely to trigger landslides that could dam the Jhelum River, which drains from the Indian-controlled Kashmir Valley into Pakistan. That could put the Kashmir Valley under water within three months - and would also threaten disastrous flooding in Pakistan if the waters were released too quickly. The two nations should develop a co-operative plan to deal with the aftermath of a Kashmir megaquake, Bilham argues. Bilham's new estimate of the largest quake possible in Kashmir comes from 8 years of readings from GPS sensors placed in areas under both Indian and Pakistani control. The work was made possible only thanks to collaboration with both nations, which are in a tense military stand-off over the contested territory.

In this region, the Indian plate is slowly burrowing under the Tibetan plateau. Bilham was looking for where the relative movement of the Tibetan plateau was slowest, as this indicates where compression is building up, and a rupture is eventually likely to occur. He had expected this to be in the Pir Panjal Range, to the south of the Kashmir Valley, but instead it was in the Zanskar Range to the north. This means that the zone likely to rupture when a quake eventually happens could be 200 kilometres wide, rather than about 80 kilometres, as was previously thought. The zone would encompass the Kashmir Valley - including the city of Srinagar, home to some 1.5 million people. If slippage occurs over a length of 300 kilometres, as is possible, a megaquake of magnitude 9 is the likely result. Given building codes and population in the region, that could mean a death toll of 300,000 people. What Bilham can't predict from his GPS results is when such a disaster might happen. Bilham points out that seismologists have been caught out by recent megaquakes, including the magnitude 9 Tōhoku quake that hit Japan in March, by basing their estimates on historical patterns, rather than physical measurements. "I think you have to plan for the worst case," he says. Bilham revealed his findings at the American Geophysical Union meeting in San Francisco this week. - New Scientist.

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