ANOTHER worrying sign that Greenland鈥檚 ice may be vanishing fast comes from an increase in the number of glacial 鈥渆arthquakes鈥� recorded on the island.
G枚ran Ekstr枚m, a seismologist at Harvard University, says the rapid melting causes water to collect under the ice faster than it can drain away. This makes the glaciers slip faster into the sea, which causes more quakes.
Ekstr枚m discovered glacial quakes three years ago after noticing that some signals picked up by seismic monitors contained fewer high-frequency waves than is normal for an earthquake. He traced their source to slips within ice sheets. Most were in Greenland, but they also occur in Alaska and Antarctica.
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These slippages can be substantial. If an ice slab the size of Manhattan and as tall as the Empire State Building slips by 10 metres, the result is a quake measuring 5 on the Richter scale.
Ekstr枚m found a striking increase in the frequency of glacial quakes in Greenland. The numbers ranged from six to 15 a year between 1993 and 2002, but jumped to 30 in 2003, 23 in 2004 and 32 in the first 10 months of 2005 (Science, vol 311, p 1756). The increase matches a rise in Greenland鈥檚 temperatures over the same period.
