Constraints on Terminus Dynamics at Greenland Glaciers From Small Glacial Earthquakes

Many large calving events at Greenland's marine-terminating glaciers generate globally detectable glacial earthquakes. We perform a cross-correlation analysis using regional seismic data to identify events below the teleseismic detection threshold, focusing on the 24 hr surrounding known glacial earthquakes at Greenland's three largest glaciers. We detect additional seismic events in the minutes prior to more than half of the glacial earthquakes we study and following one third of them. Waveform modeling shows source mechanisms like those of previously known glacial earthquakes, a result consistent with available imagery. The seismic events thus do not represent a failure of the high subaerial ice cliff like that expected to trigger large-scale calving and a marine ice-cliff instability but, rather, rotational, buoyancy-driven calving events, likely of the full glacier thickness. A limited investigation of the prevalence of smaller seismic events at times outside glacial-earthquake windows identifies several additional events. However, we find that calving at the three glaciers we study-Jakobshavn Isbrae, Helheim Glacier, and Kangerdlugssuaq Glacier-often occurs as sequences of discrete buoyancy-driven events in which multiple icebergs ranging in size over as much as three orders of magnitude are all lost within similar to 30min. We demonstrate a correlation between glacial-earthquake magnitude and iceberg size for events with well-constrained iceberg-area estimates. Our results suggest that at least 10-30% more dynamic mass loss occurs through buoyancy-driven calving at Greenland's glaciers than previously appreciated.