Local subglacial hydrologic conditions mapped with glaciohydraulic tremor during an outburst flood at Lemon Creek Glacier, Alaska

Subglacial water flow significantly influences the dynamics of glaciers, affecting behavior such as basal sliding, sediment transport, and fracturing. However, the inability to directly observe most subglacial hydrologic systems is a significant obstacle in advancing understanding of glaciers. A promising technique to fill this observational gap is monitoring glaciohydraulic tremor, the continuous seismic signals created by moving liquid water and sediment within the glacier. Water turbulence and sediment saltation in flowing water have been modeled to generate high-frequency (1.5-20 Hz) ground motion, and measurements near glaciers of tremor in this range correlate with subglacial water outflow. Furthermore, temporal variations in subglacial water pressure, conduit size, and sediment flux can be inferred by investigating hysteresis between hydrologic metrics and different frequencies of the glaciohydraulic tremor. In this study, we monitor seismic tremor at Lemon Creek Glacier, Alaska using a high-density ( 300 m spacing) array of vertical-component seismometers deployed for a two-week period over the majority of the 1.5 km by 5.5 km glacier's surface. In particular, we observe the 36-hour drainage of a glacier-dammed lake directly into the subglacial hydrologic system. By comparing observed seismic tremor power across our dense array with proglacially-recorded water discharge, we identify two regions with distinct subglacial hydrologic response to this sudden influx of water. In the upper region of the glacier, temporal variations in the seismic data indicate an increased pressure gradient within the subglacial conduits. In contrast, the lower region of the glacier shows no such pressurization. This pattern is consistent with the observed glacier structure; the thinner, steeper, and more deeply-fractured lower region is less conducive to a pressurized subglacial hydrologic system than the upper region, even under increased water throughput. As the lake emptied and discharge decreased, both regions' seismic power decreased at a rate consistent with unpressurized conduits. In pursuit of improving observations of the subglacial environment, dense seismometer arrays like this one open the door to time-continuous mapping of subglacial hydrologic conditions.