Monitoring Greenland calving volumes with glacial earthquakes

The Greenland ice sheet is losing mass as a result of both increased surface melting and runoff and increased ice discharge from marine-terminating outlet glaciers. Ice discharge at these glaciers includes both submarine ice melting and iceberg production. One can distinguish two types of calving -tabular and rolling or capsize icebergs- which have different implications for the glacier stability and the fresh water flux in the proglacial fjord and around Greenland.

At near-grounded glaciers calving typically occurs when buoyancy forces cause km-scale icebergs of the full-glacier thickness to slowly capsize against the terminus face. The generated contact force is responsible for glacial earthquakes (GEs) of magnitudes 5 that are recorded globally. A few studies tried to understand the link between the iceberg-to-terminus contact force magnitude and the iceberg volume and no one could find a simple relationship due to the complexity of the phenomenon.

With a numerical mechanical model of iceberg capsize, we compute a catalog of 30 000 contact forces run with different sets of icebergs attributs and dimensions. The force of 200 GEs were inverted from 20-100 s bandpass filtered Greenland seismic records. When compared to the bandpass filtered mechanical contact forces, the seismic force history allows to compute accurate calving volumes when one single quantity as the earthquake magnitude is limited for uniquely characterizing the iceberg size without a 1 km^3 uncertainty.

We calculated the calving volumes of 40% of all GEs that were referenced in Greenland between 1993 and 2013. Based on preferred distributions of iceberg sizes at individual glaciers, the total mass loss from these seismogenic calving events was estimated to be at least 450 Gt. Whilst the cumulated mass loss seems to be equally partitioned between East and West Greenland glaciers over the two investigated decades, the contribution of Western glaciers has been accelerating since 2010 along with an increasing number of calving events and iceberg sizes. Finally, the contribution of capsize icebergs was evaluated to be around 10% of the total mass that is discharged at individual glacier termini and over Greenland, with increasing contributions of rolling icebergs at North Western glaciers.