Hurricanes and tornadoes have seasons, but do earthquakes? They do in the Himalayas, and it's during the winter.

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.

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 to the monsoon season that drenches the region each summer.

When it rains, it shakes

The Himalayas are a highly quake-prone region because of the stresses building up between the Indian and Eurasian plates as India continues to drive into Asia.

Philippe Avouac of Caltech and his colleagues analyzed a catalog of 10,000 Himalayan quakes and found 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. (Quakes this small are often not even felt.) For magnitude-4 temblors (sometimes felt), 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, Avouac said.

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 temblors.

Unique mechanism

GPS instruments installed across the Himalayan front in 1994 yielded measurements of horizontal displacements that showed that the motion of the range was continuous along the front, but reached a maximum speed just before the earthquakes started to multiply.

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

"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 said.