Seismic Mapping of Subglacial Hydrology Reveals Previously Undetected Pressurization Event

Understanding the dynamic response of glaciers to climate change is vital for assessing water resources and hazards, and subglacial hydrology is a key player in glacier systems. Traditional observations of subglacial hydrology are spatially and temporally limited, but recent seismic deployments on and around glaciers show the potential for comprehensive observation of glacial hydrologic systems. We present results from a high‐density seismic deployment spanning the surface of Lemon Creek Glacier, Alaska. Our study coincided with a marginal lake drainage event, which served as a natural experiment for seismic detection of changes in subglacial hydrology. We observed glaciohydraulic tremor across the surface of the glacier that was generated by the subglacial hydrologic system. During the lake drainage, the relative changes in seismic tremor power and water flux are consistent with pressurization of the subglacial system of only the upper part of the glacier. This event was not accompanied by a significant increase in glacier velocity; either some threshold necessary for rapid basal motion was not attained, or, plausibly, the geometry of Lemon Creek Glacier inhibited speedup. This pressurization event would have likely gone undetected without seismic observations, demonstrating the power of cryoseismology in testing assumptions about and mapping the spatial extent of subglacial pressurization. Plain Language Summary It is important to understand how glaciers are affected by climate change because glaciers contribute to fresh water resources, flood hazards, and sea levels. We want to understand how liquid water moves underneath solid glacier ice, because that water affects how glaciers move, melt, and crack. We put seismometers (the same sensors that detect earthquakes) on an Alaska glacier to detect tiny vibrations from the flowing water below. By observing the changing strength of the vibrations over time and comparing them to the amount of water that goes into and out of the glacier, we mapped where and when the water below the ice was under pressure. When a lake on top of the glacier drained into the water channels below the glacier, we used the seismic data to see that the water became pressurized under half of the glacier. We did not see other obvious signs of pressurization, such as the glacier speeding up dramatically, so we likely would not have known about the pressurization without the seismometers. To get the best understanding of glaciers, scient