|Icequake! Glacier calving makes the earth tremble.|
Where glacier meets sea in Icy Bay, Alaska, a chunk of ice the size of a house crashes into the water with a thunderous roar. A new study finds that such calving events are the main cause of "icequakes"—seismic:
tremblings of the earth that can be felt up to 200 kilometers away. The findings may help scientists better monitor the impact of climate change on the world's glaciers.
To study calving and other glacier activity, scientists have traditionally needed a camera, good weather, and good light to take a series of now-you-see-it, now-you-don’t pictures. In recent years, they've added GPS, satellites, and seismometers to the toolkit. But no one really appreciated the utility of seismometers until 2003, when the instruments picked up magnitude-4 and -5 quakes from the Greenland ice sheet from halfway around the world, says glaciologist Chris Larsen of the Geophysical Institute at the University of Alaska, Fairbanks. Suddenly, he says, glaciologists became very interested in what seismometers could tell them about why glaciers shake the earth.
Larsen and colleague Shad O’Neel of the U.S. Geological Survey in Anchorage got an opportunity to answer that question a few years later when the Alaska Earthquake Information Center (AEIC) added 21 new seismometer stations to the southeastern coast of Alaska. Piggy-backing on the earthquake-monitoring network, the duo set up long-term seismic observations of icequakes in the nearby Saint Elias Mountains. “Those stations were basically surrounded by glaciers,” says Larsen. “We were lucky that through happenstance, these great instruments got deployed.”
With the help of seismologists at AEIC, O’Neel and Larsen designed a method to automatically sift through the data that came into the seismometers and pick out the signals that came from the glaciers. They started by going through a month’s worth of data by hand to find clear icequake signals—a magnitude-1 or less quake located on an ice field with a gentler vibration pattern than an earthquake’s. Then they developed a computer program to pick out events with these features and let it run for a year and a half.
The result, which will be published in an upcoming issue of the Journal of Geophysical Research, is the biggest record of icequakes for such a broad region. The data reveal that 85% of these quakes originate at the ends of glaciers. Some seismologists had predicted that the quakes begin in the glacier's interior, as the massive slab of ice slides across the ground. But given the location of the icequakes and the high-rate calving observed on the glaciers with the most seismic noise, the team suspects that the vast majority of icequakes are caused by calving events.
Remote observations of icequakes can serve as an early-warning system for changes in calving patterns, says Larsen. And that could eventually tell glaciologists just how much ice a glacier is losing—a concern with a warming climate and the threat of sea-level rise.
“It’s a great paper,” says glaciologist Gordon Hamilton of the University of Maine in Orono. “They’ve done a nice job of actually developing a technique that you could apply to any network of seismometers for glacial earthquake data.” Hamilton says calving as the source of seismic activity agrees with work done in Greenland, but he was “astonished” by the frequency of the Alaskan icequakes, averaging 37 per day. “The glaciers, I’m sure, have always been doing that except that we’ve never had the means of knowing [before now].”
Glaciologist Jason Amundson of University of Chicago in Illinois is excited about what comes next for seismic monitoring. “It’s one thing to count events. It’s another to try to give a real physical meaning to what those events mean,” he says. While calving may be the overarching process, the details of which ice movement is producing each variation of the seismic signal still need to be resolved.
To answer that question, Larsen and O’Neel have already begun synchronizing their seismograms with high-def video of calving icebergs.