Combining seismic tremor and GPS observations to characterize the seasonal evolution of the Greenland basal hydrologic system and its relationship to ice dynamics

Abstract

The mass-loss rate of the Greenland Ice Sheet is accelerating due to increased surface melt and changes in ice-sheet dynamics; however, our understanding of how and when increased melt may lead to increased ice velocity is limited in part by the difficulty in characterizing the evolution of the basal meltwater system and its effects on ice sheet-bedrock coupling.  To monitor the evolution of the basal hydrologic system and its relationship to surface ice velocities, we conducted a field experiment from May to September 2022 near North Lake on the western margin of Greenland. We deployed seismic and geodetic instruments that captured the seasonal evolution of the basal hydrologic system, as well as the drainage of two nearby supraglacial lakes.  Our seismic network included three small-aperture dense arrays, which recorded continuous data generated by the evolving hydrologic systems. The seismic arrays are used to detect and locate seismic tremor sources at the ice-sheet bed, likely correlated with basal meltwater flow.  We locate tremor sources every 5 seconds and use their spatiotemporal distributions to monitor the evolution of the basal flow system over the melt season.  Our observations show tremor activity increases starting around day 175, preceding the increase in ice surface velocities relative to the winter velocity.  Tremor activity peaks in the days before the two rapid lake drainage events (day 195), likely associated with precursory surface-to-bed drainage through a moulin west of North Lake.  Immediately after lake drainage, tremor activity shuts down, though ice velocities remain elevated over winter velocities.  Finally, around day 213 tremor activity increases, becoming more episodic, while ice velocities decrease toward winter velocities.  These observations provide new constraints on the interconnected feedback processes between supraglacial and subglacial hydrologic systems and suggest that ice surface velocities may not be directly correlated to the activity of basal meltwater flow over the melt season.