Oct. 16, 2008 -- On the west coast of Greenland, a glacier's crumbling edge is producing seismic groans.
As the Arctic warms, scientists are keeping a close eye the Jakobshavn glacier. Already one of the world's fastest moving ice streams, over the last decade scientists watched alarmed as it sped up further, sometimes sliding dozens of feet per day toward the Ilulissat Fjord.
The heightened activity is having strange side effects. In 2003, scientists first noticed the glacier producing earthquakes between magnitude 4.6 and 5.1 in strength. The quakes happened slowly, over a period of 30 minutes to several hours, and were undetectable by people even though they registered on seismometers around the globe.
Now a new study suggests the huge icebergs breaking off the edge of Jakobshavn are to blame.
Despite recent thinning, the glacier edge is still half a mile thick, and stretches along more than two miles of coastline. When icebergs break off, or calve, they splash into the fjord and grind against its bottom in a small cataclysm; the biggest chunks can stretch along the entire length of the glacier and be 1,500 feet deep.
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"You don't want to be close to one of these," said Jason Amundson of the University of Alaska, Fairbanks. "They're almost a kilometer tall, so picture something tall and skinny bobbing in the water. They're unstable, and they're going to roll 90 degrees."
When researchers first discovered glacial earthquakes they thought they were caused by the glacier sliding over bedrock. Some thought Jakobshavn might slip 30 feet in half an hour.
But after studying the glacier for the past two melt seasons, Amundson thinks that's unlikely. "We found that after a calving event, there's only a small acceleration in sliding in the main glacier, not enough to cause the seismic signal we see."
Instead, he noticed that the quakes coincided with large calving events. The glacier edge sits in the water, resting against the bottom of the fjord. So when a berg calves and starts to roll, it scrapes hard against solid ground.
"Picture a box against a wall at a 45-degree angle: As the box rolls, the corner scrapes along the floor," Amundson said.
Victor Tsai of Harvard University agrees that calving, rather than sliding of the main glacier, is the likely cause of the quakes. "They physically observed one of the icebergs rolling over and recorded seismic waves that look a lot like the glacial earthquakes we've seen," he said.
"This is important to understanding ice sheet stability," Amundson said. "Changes in calving affect how the glacier responds to climate," making understanding the strange quakes crucial to studying the effects of climate change on the world's glaciers.
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