Glacial earthquakes in Greenland, the researchers found, are most common in July and August, and have more than doubled in number since 2002.

Seismologists Goran Ekstrom and Victor C. Tsai at Harvard and Meredith Nettles at the Lamont-Doherty Earth Observatory, a member of The Earth Institute at Columbia University, will publish a report on Greenland's glacial earthquakes this week in the journal Science. Ekstrom, Nettles and colleagues first described glacial earthquakes in 2003, but that report did not recognize the seasonality or increasing frequency of the phenomenon.

"People often think of glaciers as inert and slow-moving, but in fact they can also move rather quickly," says Ekstrom, professor of geology and geophysics at Harvard who will be moving to Lamont-Doherty in the spring. "Some of Greenland's glaciers, as large as Manhattan and as tall as the Empire State Building, can move 10 meters in less than a minute, a jolt that is sufficient to generate moderate seismic waves."

As glaciers and the snow atop them gradually melt, water seeps downward. When enough water accumulates at a glacier's base, it can serve as a lubricant, causing blocks of ice 10 cubic kilometers in size to lurch down valleys known as "outlet glaciers," which funnel all of Greenland's glacial runoff toward the surrounding seas.

"Our results suggest that these major outlet glaciers can respond to changes in climate conditions much more quickly than we had thought," says Nettles, a postdoctoral researcher at Lamont-Doherty. "Greenland's glaciers deliver large quantities of fresh water to the oceans, so the implications for climate change are serious. We believe that further warming of the climate is likely to accelerate the behavior we've documented."

Although Greenland is not known as a hotbed of traditional seismic activity caused by the grinding of the Earth's tectonic plates, seismometers worldwide detected 182 earthquakes there between January 1993 and October 2005. Ekstrom, Nettles and Tsai examined the 136 best-documented of these seismic events, ranging in magnitude from 4.6 to 5.1. All were found to have originated at major valleys draining the Greenland Ice Sheet, implicating glacial activity in the seismic disturbances.

Moreover, of the earthquakes they analyzed, the researchers found more than one-third occurred during the months of July (22 earthquakes) and August (24). By comparison, January and February each saw a total of only such four glacial earthquakes between 1993 and 2005. In contrast, non-glacial earthquakes in northern latitudes show no such seasonal variability.

In addition, the number of glacial earthquakes in Greenland increased markedly between 1993 and 2005. Annual totals hovered between 6 and 15 through 2002, which was followed by sharp increases to 20 earthquakes in 2003, 24 in 2004 and 32 in the first ten months of 2005. A single area of northwestern Greenland, where only one seismic event was observed between 1993 and 1999, experienced more than two dozen glacial quakes between 2000 and 2005.

Although glacial earthquakes appear to be most common in Greenland, Ekstrom, Nettles and Tsai have also found evidence of glacial earthquakes originating from mountain glaciers in Alaska and at glaciers located in ice streams among the edges of Antarctica.

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