Himalayan earthquakes are more common in winter because the weight of summer鈥檚 heavy monsoon rains suppresses tremors, a new study suggests.
Laurent Bollinger at the French Commission for Atomic Energy (CEA) and colleagues looked at a database of quakes that occurred in Nepal between 1995 and 2005 and found that there were 40% fewer tremors in the summer than in the winter.
The seasonal difference was even more pronounced when they eliminated very small tremors. Quakes of magnitude 4 or greater on the local scale were 63% less common in the summer months than during winter. 鈥淢agnitude 4 tremors can be felt as light vibrations 鈥� they might knock a door shut for instance,鈥� says Bollinger. The likelihood of seeing such seasonal variation by chance is less than 1%, he adds.
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Tremors in the region are caused by one section of the Earth鈥檚 crust sliding under another (subduction), so increasing weight on the upper plate may suppress such sliding, Bollinger suggests.
Snow effect
Previous research in Japan suggests the weight of snow on Japanese mountains could create seasonal variations in tremors. However, this mechanism would suppress winter tremors too, so Bollinger鈥檚 team ruled this out as an explanation for the seasonal variations found in Nepal.
Instead, the team believes that Nepali tremors are suppressed by the weight of water dumped on the south side of the Himalayan mountain chain during the summer monsoon.
These regions can receive up to 6 metres of rainfall per year 鈥� concentrated in the monsoon season 鈥� and the water accumulates in aquifers, or groundwater reservoirs. Moreover, the researchers found that the period when tremors were at their lowest each year corresponded to when rainfall was heaviest.
Slippery when wet
鈥淭here is another explanation which we cannot rule out,鈥� Bollinger told New 杏吧原创. 鈥淚t could be that the water from the monsoon is trickling through the fissures in the rock 鈥� slowly reaching the region about 10 kilometres down where two segments of the Earth鈥檚 crust overlap 鈥� and generating these tremors.鈥�
If it takes six months for the water to penetrate to 10 km, which Bollinger says 鈥渋s possible鈥�, then the water could lubricate the segments. This could facilitate the sliding and, as a result, increase the number of winter-time tremors (also see California鈥檚 cloudbursts pave the way for quakes).
鈥淥ur data does not allow us to distinguish between these two theories at the moment,鈥� he says. He adds, however, that the second possibility does not sit easily with him because it would be 鈥渧ery fortuitous鈥� if the trickling water always reached its destination deep in the crust exactly at the peak of the winter earthquake season.
Bollinger and his team will soon be publishing further research, in the journal Nature, which they say will strengthen their hypothesis that seasonal weight variations cause the anomaly.
Journal reference: Geophysical Research Letters (DOI: 10.1029/2006GL029192)
