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Himalaya mega-quakes likely every 1000 years

* 13:08 09 November 2006
* NewScientist.com news service
* Catherine Brahic

A colossal reservoir of energy stored up under the Tibetan plateau has been discovered – and it can only be fully released by mega-earthquakes striking about every 1000 years, researchers have found.

The study suggests that earthquakes in the past 200 years in the central Himalaya, while catastrophic, are small in comparison to what the region has seen in the past - and will see again.

The energy builds up as the result of the collision between the Indian subcontinent and Asia, and the movement of the two continental plates was tracked using GPS technology.

The reservoir of energy is so large because of the nature of the two plates. They are both continental, and therefore made of relatively low density rocks. This means that, rather than one heavier, denser plate plunging deep under a lighter plate, as happens when dense oceanic crust plunges under a continental plate, they both strive to float near the surface of the planet. This generate a lot of friction, causing a huge amount of energy to be stored underneath Tibet (see map).
Draining the reservoir

Large earthquakes registering between magnitude 7.5 and 8.2 tap into this reservoir but do not deplete it, the researchers have determined. Instead, every 1000 years or so, a mega-earthquake of magnitude 8.4 to 8.6 "is essential to drain the reservoir", says lead researcher Roger Bilham of the University of Colorado, US.

Historical records show that three such mega-quakes have devastated the Himalaya in the Middle Ages. In 1100 Eastern Nepal was hit. Bilham and his colleague Nicole Feldl's prediction suggests the region has a higher than thought risk of another mega-quake in the future, though it cannot be predicted when.

In fact, an 8.2 event killed 13,000 people in 1934. But Bilham says this was not enough to empty the reservoir, implying the region is not safe from another, far deadlier event.
False sense of security

"I think this study has to be taken seriously," says Roger Musson, a seismologist at the British Geological Society. He points to the authors' suggestion that one large earthquake in a region does not mean it is unlikely to be followed by another even bigger one.

"Conventional wisdom is that once you've had a big earthquake that's it. That's a safe place to live," says Bilham. Instead, his research suggests that in the Himalaya if the event is not big enough to drain the Tibetan plateau of its energy, then an area that has been hit by anything under magnitude 8.0 can expect another larger event.

This, say the authors, may explain why a 7.8 earthquake in Nepal in 1833 was followed by another bigger one just 101 years later.

"It also implies that the 1905 Kangra earthquake region, previously considered a region reprieved from an imminent damaging earthquake, could today host on or larger magnitude," Bilham and Feldl write in Nature.

Musson says those planning for disaster relief should take this finding into consideration, and not take regions that have recently had earthquakes to be safe from renewed disaster.

Journal reference: Nature (vol 444, p 165)