Global warming yields novel 'glacial earthquakes' in polar areas

Newfound temblors, most common in summer months, have proliferated in recent years

Seismologists at Harvard University and Columbia University have found an unexpected offshoot of global warming: "glacial earthquakes" in which Manhattan-sized glaciers lurch unexpectedly, yielding temblors up to magnitude 5.1 on the moment-magnitude scale, which is similar to the Richter scale. Glacial earthquakes in Greenland, the researchers found, are most common in July and August, and have more than doubled in number since 2002.

Scientists Göran Ekström and Victor C. Tsai at Harvard and Meredith Nettles at Columbia will report on Greenland's glacial earthquakes this week in the journal Science. Ekström, Nettles and colleagues first described glacial earthquakes in 2003, but that report did not recognize the seasonality or growing frequency of the phenomenon.

"People often think of glaciers as inert and slow-moving, but in fact they can also move rather quickly," says Ekström, professor of geology and geophysics in Harvard's Faculty of Arts and Sciences. "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 some 10 cubic kilometers in size to lurch down valleys known as "outlet glaciers," which funnel all of Greenland's glacial runoff toward the surrounding sea.

"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 Columbia's Lamont-Doherty Earth Observatory. "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."

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

Of the 136 earthquakes analyzed, more than a third occurred during the months of July (22 earthquakes) and August (24 earthquakes). By comparison, January and February each saw a total of only four earthquakes between 1993 and 2005. Non-glacial earthquakes in polar regions show no seasonal variability.

Greenland's overall number of glacial earthquakes also increased markedly between 1993 and 2005. Annual totals hovered between 6 and 15 through 2002, followed by sharp increases to 20 earthquakes in 2003, 24 in 2004, and 32 in the first 10 months of 2005. A single area of northwestern Greenland, where only one seismic episode was observed between 1993 and 1999, experienced more than two dozen glacial quakes between 2000 and 2005. Polar regions have not experienced increases in non-glacial earthquakes in recent years.

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

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Ekström, Nettles and Tsai's work was funded by the National Science Foundation.


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