Greenland glacial quakes becoming more common

The number of earthquakes that occur beneath surging glaciers in Greenland has doubled in the past 4 years, another possible effect of the melting ice sheet there.

Just 2 years ago, scientists reported a newly recognized phenomenon: earthquakes occurring beneath glaciers, probably from sudden slips of those ice masses (SN: 1/3/04, p. 14: Available to subscribers at Earth sometimes shivers beneath thick blankets of ice). The magnitudes of those quakes, which aren’t associated with known faults, measure between 4.6 and 5.1, says Göran Ekström, a geophysicist at Harvard University.

Seismic data gathered from 1993 to late 2005 identify 136 quakes along the southeastern and southwestern coasts of Greenland, all of them originating beneath glaciers that flow at the fast pace of at least 2 kilometers each year, says Ian Joughin, a glaciologist at the University of Washington in Seattle.

About three-quarters of the temblors originated beneath the island’s largest glaciers: Kangerdlugssuac, Jakobshavn Isbrae, and Helheim. Recent field studies have shown that all three glaciers have thinned and accelerated during the past 2 years (SN: 12/17/05, p. 387: Available to subscribers at Glacial Change: Greenland’s ice loss doubled in 2005).

Greenland’s glacial quakes are five times as common in the summer as they are in winter, which hints that melting promotes the events. Before 2003, the island never suffered more than 15 such earthquakes per year. Since then, the temblors have steadily become more frequent. Between January and October 2005, more than 30 glacial quakes occurred in Greenland, Ekström reports in the March 24 Science.