Surface icequakes and basal stick-slip events reveal daily grounding line migration and seawater intrusion at a marine-terminating glacier in East Antarctica

Abstract. As they reach the ocean, Antarctic outlet glaciers transition from grounded to floating at their so-called grounding lines (GL). This transition is known to be mechanically controlled by tides, which induce ice flexure visible at the surface from satellite and ground geodesy and often used as a proxy for grounding line position. Here, we use a dense seismic node array to study the spatial and temporal dynamics of surface, crevasse-induced icequake activity and basal, sliding-induced seismicity at the grounding zone of the Astrolabe Glacier, a fast-moving outlet glacier in East Antarctica. We observe that surface icequakes mimic the expected, tide-induced, ice flexure pattern, as they delineate the grounding line position inferred from previous geodetic studies, and migrate landward as tides rise. We show, however, that the mechanical grounded to floating transition is better evidenced by the spatial distribution of basal sliding-induced stick-slip events, occurring on a limited number of clusters and which depict a grounding line position that is offset inland compared to that identified from the surface. These basal events undergo tidally-driven cycles of activation and de-activation, consistent with sea water intrusion inland over at least 3 kilometers at high tides. Following these results, we propose that the monitoring of stick-slip events could be used as the most accurate means of tracking grounding line retreat over long timescales.