Glaciers might move slowly, but they have their dramatic moments. Scientists tracking glacial earthquakes in Greenland have managed to crack open the mysterious dynamics of calving icebergs.
The results, published in the journal Science, could help scientists track the loss of the Greenland Ice Sheet, which is shrinking even faster than Antarctica.
Glacial earthquakes are caused by the calving of glacier ice – when a massive shard cracks like a gunshot and sloughs off the frozen wall.
Breathtaking as these events are, they're caused by very different dynamics than your standard earthquakes, which occur suddenly after stresses building in the ground finally release. Glacial earthquakes, by contrast, can take minutes to play out, and do so gradually (and often almost imperceptibly).
"You cannot feel it," said study co-author Meredith Nettles, a seismologist at Columbia University's Lamont-Doherty Earth Observatory. Nettles and colleagues first discovered glacial earthquakes in 2003, and has experienced a few of these icy temblors in person.
"If you're lifted up for 60 seconds and gradually put back down, you won't notice it," she explained. "It's a little bit like if you ride in an elevator -- what you notice is when the elevator stops and starts, you don't really feel the motion in between."
These events also have significant consequences for the loss of glacial ice that's happening in the context of rising sea levels and climate change.
"The Greenland Ice Sheet is losing mass very quickly right now; about half of it is from calving processes," Nettles said. "Calving is actually not as well understood as one would hope, and in order to make better models of how the ice sheet works and understand better what the contributions to sea level change are likely to be in the future, [it] really requires a better physical understanding of those processes."
The international team of scientists suspected that glacial earthquakes were linked to these calving events, but there was little data to support that theory. So they placed a wireless network of GPS sensors around the calving margin of Helheim Glacier, which is a major outlet of the Greenland Ice Sheet, and monitored the network for 55 days.
They also placed two cameras in front of the calving border, which took pictures on an hourly basis. These cameras acted like two eyes, allowing the researchers to reconstruct a three-dimensional model of the glacier's front and the calved icebergs.
Back in the lab, the researchers ran simulations in a cylindrical tank mimicking the fjords, which are thin fingers of seawater that extend into land (and where the water meets the Helheim Glacier wall). They used a rectangular box with the same density as the ice in order to mimic the movement of a calving iceberg and measure the forces it generated in the water.
The scientists found that as the calving ice fell, it would often flip backward and push the glacier so hard that it compresses the front of it like a spring, causing it to briefly reverse direction. For the Helheim Glacier, that's quite a surprise, Nettles said.
"It's typically moving in the forward direction something like a hundred feet per day. It's one of the fastest glaciers in the world," Nettles said. "So part of the reason it's so impressive to see it moving backwards is that normally it's moving forward so fast."
Then, when the iceberg plunged into the water, the water pressure behind it would plummet, causing the main glacier to go down 10 centimeters while pulling the earth upward. This generated the vertical forces observed in glacial earthquakes, the scientists found.
"These forces are the source of glacial earthquakes, globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in Greenland and Antarctica," the scientists wrote in the paper.
Calving events are sporadic and unpredictable, so catching them in action is no easy proposition. But by using such quake detectors to monitor these events, scientists could keep better track of how much calving ice is being lost from glaciers around the world.