Glacial earthquakes help to track ice loss
Calving icebergs found as source of seismic events seen in glaciers
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 different dynamics than standard earthquakes, which occur suddenly after stresses building in the ground finally release. Glacial earthquakes, by contrast, can take several minutes to play out.
Finding data
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.
“Nearly half of Greenland’s mass loss occurs through iceberg calving, but the physical mechanisms operating during calving are poorly known and in situ observations are sparse,” the authors wrote.
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.
Simulated fjords
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
“Nearly half of Greenland’s mass loss occurs through iceberg calving, but the physical mechanisms operating during calving are poorly known and in situ observations are sparse.”
Science report
the movement of a calving iceberg and measure the forces it generated in the water.
Unpredictable events
The scientists found that as the calving ice fell, it would often flip backward and push the glacier back, causing it to briefly reverse direction, and triggering the earthquakes’ horizontal motion. Then, when it 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 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 authors wrote.
Calving events are sporadic and unpredictable, so catching them in action is no easy proposition. But by monitoring a network of quake detectors, scientists could keep track of exactly how much calving ice is being lost from glaciers around the world.