Calving events at Helheim springs the glacier backwards and moves it downward, causing earthquake

UNAVCO has a fascinating article on how new GPS sensors capture glacial earthquakes after calving events at Helheim Glacier in Greenland. They note that when the calved “iceberg rotates and rolls off sideways, the glacier springs backwards and moves downwards”. This action causes a glacial earthquake. These earthquakes have been very hard to study in the past due to a multitude of reasons , but the new sensors should allow scientists to study the phenomenon from anywhere around the world.

Greenland and Antarctica lose massive amounts of ice from their extensive ice sheets through ice calving, particularly at the coastal margins. About 30 to 50 percent of Greenland’s ice loss is through calving. As the earth warms, the rate of calving has increased. Glacial earthquakes of magnitude 5 have been observed globally and are related to calving of large glaciers near their marine termini. The earthquakes are different from tectonic events because their duration is longer than a minute and the seismic data suggest a nearly horizontal force is acting on the glacier. Several possible explanations have been put forward to explain glacial earthquakes

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The GPS sensors show that as the calving chunk of ice moves down and away from the front of the glacier, the glacier moves backward (i.e., reverses its normal horizontal flow toward the coastal margin) because of the force of the ice loss and the acceleration away from the front. The sensors near the glacier’s terminus move downward because there is a hydrodynamic pressure drop that pulls water out from under the glacier. These forces explain the seismic signals recorded at far away seismic stations.

The study titled Reverse glacier motion during iceberg calving and the cause of glacial earthquakes can be viewed at American Association for the Advancement of Science magazine.

Tavi Murray and colleagues used cameras, GPS sensors, and the global seismographic network to closely monitor the Helheim Glacier -- a major outlet of the Greenland Ice Sheet -- for 55 days in 2013. The glacier retreated about a mile (1.5 kilometers) during that time, and the researchers were able to capture details of ten, large-scale calving events. Their findings illustrate how calving icebergs fall away from the glacier and temporarily reverse the glacier's course, causing glacial earthquakes that register about magnitude five on the Richter Scale.

New research on the Greenland Ice Sheet provides updated estimates of past and present ice loss. Here, video captures a huge calving event at Helheim glacier, southeast Greenland. Credit: Nicolaj Krog Larsen