Reverse glacier motion during iceberg calving and the cause of glacial earthquakes

Monday, 14 December 2015: 14:10
3009 (Moscone West)
Tavi Murray1, Meredith Nettles2, Nick Selmes1, Mac Cathles3, Justin C Burton4, Timothy James5, Stuart Edwards6, Ian Martin6, Tim O'Farrell7, Robin A Aspey7, Ian Charles Rutt8 and Tim Bauge9, (1)Glaciology Group, Swansea University, Swansea, United Kingdom, (2)Columbia University of New York, Palisades, NY, United States, (3)University of Michigan, Ann Arbor, MI, United States, (4)Emory University, Atlanta, GA, United States, (5)Swansea Univeristy, Geograpy, Swansea, United Kingdom, (6)Newcastle University, Newcastle Upon Tyne, United Kingdom, (7)University of Sheffield, Electronic and Electrical Engineering, Sheffield, United Kingdom, (8)Swansea University, Cardiff, United Kingdom, (9)Thales UK, Research and Development,, Reading, United Kingdom
About half Greenland’s mass loss results from iceberg calving, but the physical mechanisms of calving are poorly known and in situobservations are sparse. Glacial earthquakes, globally detectable seismic events, are associated with calving and are occurring at increasing numbers of outlet glaciers in Greenland and Antarctica. However, the processes causing them have not been clear.

We installed a wireless network of on-ice GPS sensors at the calving margin of Helheim Glacier for 55 days during summer 2013. The glacier is a major SE Greenland tidewater outlet and during our observations retreated ~1.5 km in a series of calving events. Our GPS sensors captured glacier motion with cm-level accuracy at locations very close to the calving front with a high temporal sampling rate. Calving causes a minutes-long reversal of the glacier’s horizontal flow and a downward deflection of its terminus seen on multiple GPS sensors. Each major calving event is associated with a glacial earthquake.

For example, a glacial earthquake / calving event on day 206 produced an iceberg of volume 0.36 km3and aspect ratio 0.23. A GPS sensor close to the front showed a pre-earthquake speed of 29 m/day. Immediately prior to the earthquake centroid time, the sensor reversed its direction and moved upglacier at ~40 m/day and downward 10 cm. The reversed motion was sustained for ~200 s and was followed by downglacier rebound and upward movement.

The reverse motion of the glacier results from the horizontal force caused by iceberg capsize and acceleration away from the front. We use analog laboratory experiments to demonstrate that the downward motion results from hydrodynamic pressure drop behind the capsizing berg, which also causes an upward force on the solid Earth. We show that these horizontal and vertical forces are the source of glacial earthquakes. Proper interpretation of the earthquake events should allow remote sensing of calving processes at the margins of Greenland and Antarctic glaciers.