By Alexandra Witze and Devin Powell
SAN FRANCISCO — If you want to know the chance of an earthquake in the Himalayas or a volcanic eruption in Iceland, check the calendar. Seasonal patterns of rainfall and snowfall can affect how often quakes and volcanoes go off, scientists reported December 8 at a meeting of the American Geophysical Union.
In the Himalayas, the weight of water from monsoon rains helps dampen seismicity for a few months each year, says geophysicist Thomas Ader of Caltech. Seismic records collected from 1998 to 2004 show that the number of quakes drops in the rainy summer months and rebounds when things dry out.
Water from monsoon storms runs off the mountains and into India, where its weight bends the Earth’s crust slightly. From season to season, this bending causes the surface to move back and forth, as recorded by Global Positioning System stations, in time with the rise and fall in earthquake frequency.
Calculations by Ader and his colleagues suggest that the flexing eases the stress on the fault where two tectonic plates collide beneath the Himalayas. This change in stress is quite small — less than a hundred-thousandth of the stress caused by the motion of the plates. But it lasts for months, perhaps giving the earthquakes time to slowly incubate.
“It’s a very rare way of changing the stress on a fault,” Ader said at a news conference at the geophysics meeting.
In this case, though, what goes down must come up. When the drier months come, the land rebounds, freed of its watery burden. An uptick in the number of earthquakes ushers in the winter.
In Iceland, the opposite is true: Summer is the time of greater hazard, when the country’s massive ice caps slim down during the annual melt. Where that ice happens to sit atop active volcanoes, the changes can trigger eruptions below, says Fabien Albino, a geophysicist at the University of Iceland in Reykjavik.
Albino and his coworkers have created computer simulations of how stress changes within rock when overlying weight is removed. Above the Icelandic volcano Katla, for instance, the ice cap is 6 meters thinner in the summer than in winter. This change translates to a tiny but noticeable shift in rock stress, Albino says.
Katla erupted most recently in 1918, 1955, 1999 and possibly in 2011. Each time the eruption happened between May and November. More small earthquakes also occur around Katla in summer than winter, Albino says.
In the short term, the work suggests that magma will rise within Katla — and possibly erupt — during warmer months when the stress is unloaded from top, Albino says. In the longer term, a different phenomenon helps dictate how Katla responds: Iceland’s ice has been retreating for more than a century, and that long-term decline means that Katla has a little bit less ice covering it every year to suppress eruptions.
In related work, Albino has studied possible links between Katla and the neighboring volcano Eyjafjallajökull, whose 2010 eruption temporarily closed European airspace. Albino’s calculations suggest that the stress changes caused by Eyjafjallajökull’s eruption were not enough to affect future eruptions at Katla, as some scientists had suggested.
Other factors, such as how fluid flows through the rock, still connect the two volcanoes, says geophysicist Andrew Hooper of the Delft University of Technology in the Netherlands.
