| Summary: | Glacier flow instabilities are mainly caused by changes at the ice-bed interface, where basal slip and sediment deformation drive basal glacier movement. Understanding basal processes and their response to external forcing remains difficult due to limited access to the glacier bed. In this study, we examine variations in subsurface and subglacial mechanical conditions of Kongsvegen glacier (Svalbard). This glacier shows strong indications of an imminent new rapid flow event. To investigate the potential causes of such an acceleration, we combine conventional measurements such as surface velocity (20 GNSS along the flow line) and basal water pressure (two boreholes) with seismic measurements (14 seismometers along the flow line and 6 seismometers in boreholes) and a unique measurement of basal drag applied to the sediments. We use our seismic measurements to study (1) subglacial hydrological conditions and (2) changes in ice structure using seismic interferometry. At seasonal time scales, we observe an inverse relationship between basal drag and surface runoff. At shorter time scales, the basal drag is initially very sensitive to meltwater input, but this sensitivity gradually decreases and almost disappears as the season progresses. The response of the glacier to meltwater inflow often reflects transient flow behaviour. In winter, we observe a doubling of the basal drag compared to the previous winter. These changes are concomitant with changes in seismic velocity suggesting a downward propagation of the surge. Using our multi-instrument approach, we discuss the different behaviours observed during two years of recordings to highlight the mechanisms of glacier destabilisation.
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