Monsoons responsible for more Himalayan quakes in winters than summers

January 5th, 2008 - 1:28 pm ICT by admin  

Washington, Jan 5 (ANI): New analysis by seismologists has indicated that monsoons might be behind the increased frequency of earthquakes in the Himalayan region in the winters as compared to the summers.

According to a report in Live Science, the study was made by Philippe Avouac of Caltech (California Institute of Technology) and his colleagues.

For years, seismologists had observed that far more earthquakes shook the massive Asian mountain range in the winter months than in the summer, but they couldn’t pinpoint the cause of this seasonal change.

But, a new study of GPS and satellite data presented last month at a meeting of the American Geophysical Union, has connected the increase in earthquake activity in the winters to the monsoon season that drenches the region each summer.

Satellite measurements of water levels in the rivers of the Ganges basin showed a strong seasonal change a 4-meter rise began at the onset of the monsoon season in mid-May, reaching a maximum in September, followed by a slow decrease until the next monsoon began.

As the monsoon rains swell the rivers of the Ganges basin, they increase the pressure bearing down on the region. As the rains stop and the river water soaks through the ground, the built-up load eases outward toward the front of the Himalayan range.

This outward redistribution of stress leads to horizontal compression in the mountain range later in the year, that triggers the wintertime tremors.

While changes in water levels elsewhere (usually by tides) have been proposed to trigger earthquakes, the Himalayan mechanism seems to be unique, Avouac told Live Science.

“Seasonal variation has been reported in other places, but I don’t know any other place where it is so strong or where the cause of the signal is so obvious,” he added.

After analyzing a catalog of 10,000 Himalayan quakes, the research team had earlier found that there were twice as many during the winter months (December to February) as during the summer.

For example, for magnitude-3 quakes, there were up to 150 per month in the winter, but only 75 in the summer. For magnitude-4 tremors, the winter average was 16 per month, while the summer rate fell to eight per month.

The numbers for larger, more damaging earthquakes would follow a similar pattern, said Avouac. (ANI)

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