SOUTH Asia鈥檚 monsoons dump so much rain on the Himalayas that the water鈥檚 sheer weight seems to suppress seismic activity along the mountain range.
This is one of two theories put forward by Laurent Bollinger of the French Atomic Energy Commission (CEA) to explain why earthquakes of magnitude 4 or greater in Nepal (based on a local scale) were 63 per cent less frequent during the monsoons than during winter. Bollinger and his colleagues discovered this by looking at records of quakes that shook Nepal between 1995 and 2005. The likelihood of this variation occurring by chance is less than 1 per cent, he says. The team also found that each year, the tremors were fewest when the rainfall was heaviest.
The tremors are caused by Earth鈥檚 tectonic plates sliding under one another. The extra weight of the rainfall on the upper plate could suppress such sliding, says Bollinger. (Geophysical Research Letters, DOI: 10.1029/2006GL029192).
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The alternative theory is that the rains trickle down through fissures in the rock and reach the subduction zone about 10 kilometres below the surface. This could lubricate the plates, increasing the chances of quakes about six months after the monsoons. 鈥淥ur data does not allow us to distinguish between these two theories at the moment,鈥� Bollinger says.
