Water-Related Seismic Sources in Glaciers and Ice Sheets (Invited)

Liquid water can have a profound impact on the flow of glaciers and ice sheets. Acceleration of ice flow via enhanced basal motion, hydro-fracturing and cryo-hydrologic warming are just three possible mechanisms that can drastically alter ice dynamics. At the same time, subsurface water flow is difficult to measure as the englacial and subglacial drainage systems are highly inaccessible. Although tracer experiments, speleological methods, radar measurements and deep drilling provide some information about water flow and changes thereof, more data on hydraulic processes are needed for the development and testing of numerical ice flow models. Recent studies have suggested that passive seismic techniques can be used to monitor englacial and subglacial water flow. This inter-disciplinary approach is motivated by the analogy between fluid-induced seismic sources in glaciers and volcanoes, which was first proposed in the late 70's. As seismological analysis is a valuable tool to monitor hydro-thermal activity in volcanic regions, it may consequently also reveal transient or sudden changes in a glacier's water drainage system. Here, we present results from continuous and event-based seismic monitoring exercises on Swiss mountain glaciers and the ablation zone of the Greenland ice sheet. We examine 'icequakes', sustained tremors and seismic background noise, whose sources are closely connected to the presence or movement of water. Analyzing icequake moment tensors and signal characteristics, spectrograms, noise source locations and simple models of resonating cracks, we can monitor the development and evolution of water passages below the glacier surface. Accordingly, our seismic measurements elucidate an area of the glacier, which has been difficult to investigate with traditional glaciological techniques.