Dynamics of iceberg detachment and mélange motion during the August 21, 2009, calving event at Jakobshavn Isbræ

Despite its important role in the mass balance and dynamics of ice sheets and tidewater glaciers, iceberg calving is still poorly understood. In this investigation we present a detailed study of a well-observed calving event from Jakobshavn Isbræ on August 21, 2009, during which the glacier lost about 0.5 km3 of ice. This southern Greenland outlet glacier drains approximately 7% of the entire ice sheet and has retreated significantly in the past decade. Using seismic broadband data and time-lapse photography we investigate the detachment and capsizing of individual icebergs during the calving event and concurrent motion of the fjord debris cover (‘mélange’). The latter likely influences the calving process by providing backpressure to the ice cliff. The calving seismograms of broadband seismometers at 4-550 km distances show two distinct arrivals of low-frequency surface waves, also known as glacial earthquakes. Full waveform modeling of Rayleigh and Love waves constrains the forces acting during iceberg capsizing. On the other hand, high-frequency seismic bursts are related to englacial fracturing and mélange motion, which we compare to velocities derived from photogrammetric analysis of the time-lapse imagery. This combined seismic and photogrammetric analysis clearly associates different seismic signals with physical processes that are active during major calving events. These insights will help to better understand the physics of glacier calving, as they will help interpret past and future seismic records of individual calving events.