Glacier acceleration, glacial earthquakes, and ice loss at Helheim Glacier, Greenland

Satellite observations during the last decade have shown dramatic changes in flow speed at Greenland's outlet glaciers, often accompanied by retreats of several km in calving-front location and increasing numbers of glacial earthquakes. Geodetic, seismological, and oceanographic data collected as part of a multidisciplinary field experiment at Helheim Glacier, East Greenland, over three summer seasons (2006--2008), together with satellite imagery, place new constraints on the processes responsible for these changes. Analysis of high-rate GPS data from 2007 reveals several large, sudden increases in flow speed at Helheim Glacier. These abrupt accelerations are detected along the length of the glacier (~20~km) spanned by the GPS network, and coincide in time with major calving events and teleseismically detected glacial earthquakes. The calving events are implicated in the earthquake source process. Further, our results link changes in glacier velocity directly to calving-front behavior at Greenland's large outlet glaciers, on timescales as short as minutes to hours. No large earthquakes occurred at Helheim Glacier during the 2006 field campaign, providing the opportunity for comparison between seismically active and quiescent modes of glacier behavior. Data recorded in 2008 include near-field broadband seismic recordings and time-lapse photography, allowing us to refine our understanding of both the glacial earthquake source process and the glacier response to major ice loss events.