Glacial earthquakes are moderate (roughly magnitude 5) seismic events that occur when ice flowing to the ocean from large glaciers, or ice sheets, breaks off (calves). They were discovered in 2003 by Göran Ekström and coworkers as an anomalous signal that was teased out of the records of the Global Seismographic Network (GSN), which consists of more than 150 stations for real-time monitoring of all earth oscillations, great and small. See also: Earthquake; Glacial geology and landforms; Glaciology; Iceberg; Seismographic instrumentation; Seismology
In contrast to tectonic earthquakes, glacial earthquakes are characteristically longer in duration: An M5 earthquake might last about 5 seconds, while an M5 glacial earthquake would last 30 seconds or longer. About 90 percent of all glacial earthquakes occur in Greenland; the rest have been detected in Antarctica and Alaska. Glacial earthquakes mostly occur during summer months, probably because meltwater reaching the glacier bed speeds up the ice flow. See also: Antarctica; Greenland
In Greenland, large icebergs can weigh a billion metric tons or more and can be over 4 km (2.5 mi) in length, which explains in part why their formation leaves a seismic signature. The exact mechanism that generates the distinctive signal has been unclear until recently, however. Reporting in the journal Science (July 2015), a team of scientists from the United Kingdom and the United States attributed the seismicity to the movement of the ice sheet and the earth during a calving event.
As an iceberg topples over into the sea, it generates forces that push its parent glacier backward (inland) and downward for up to a couple of minutes, followed by a rapid rebound. This movement of ice and earth is what causes the seismic signal.
By monitoring the seismic record, scientists can track from afar the calving of large icebergs into the ocean and the implied effects on the world’s ice sheets. In Greenland, calving is responsible for about half of the ice loss, with meltwater representing the other half.