Abstract
Abstract. Hydraulic processes impact viscous and brittle ice deformation. Water-driven fracturing as well as turbulent water flow within and beneath glaciers radiate seismic waves which provide insights into otherwise hard-to-access englacial and subglacial environments. In this study, we analyze glaciohydraulic tremors recorded by four seismic arrays installed in different parts of Glacier de la Plaine Morte, Switzerland. Data were recorded during the 2016 melt season including the sudden subglacial drainage of an ice-marginal lake. Together with our seismic data, discharge, lake level, and ice flow measurements provide constraints on glacier hydraulics. We find that the tremors are generated by subglacial water flow, in moulins, and by icequake bursts. The dominating process can vary on sub-kilometer and sub-daily scales. Consistent with field observations, continuous source tracking via matched-field processing suggests a gradual up-glacier progression of an efficient drainage system as the melt season progresses. The ice-marginal lake likely connects to this drainage system via hydrofracturing, which is indicated by sustained icequake signals emitted from the proximity of the lake basin and starting roughly 24 h prior to the lake drainage. To estimate the hydraulics associated with the drainage, we use tremor–discharge scaling relationships. Our analysis suggests a pressurization of the subglacial environment at the drainage onset, followed by an increase in the hydraulic radii of the conduits and a subsequent decrease in the subglacial water pressure as the capacity of the drainage system increases. The pressurization is in phase with the drop in the lake level, and its retrieved maximum coincides with ice uplift measured via GPS. Our results highlight the use of cryo-seismology for monitoring glacier hydraulics.
Highlights
On high-melt glaciers, meltwater produced at the surface is routed through moulins and crevasses to the glacier bed
We find that the nature of glaciohydraulic tremors is time dependent and shows spatial variability on the sub-kilometer scale
The tremors are generated by subglacial water flow, icequake bursts, or in moulins
Summary
On high-melt glaciers, meltwater produced at the surface is routed through moulins and crevasses to the glacier bed. Bartholomaus et al (2015) show that these tremors serve as a proxy for subglacial discharge and find that the tremors reveal decreasing transit times of the water through the glacier throughout the melt season Building on their river application, Gimbert et al (2016) establish a glacier framework which relates seismic power Prel to discharge Qrel (using an arbitrary reference scaling). Given that sufficient meltwater is available, hydrofracturing can extend existing fractures to the glacier bed (Van Der Veen, 1998) Evidence for such events in combination with resonances in water-filled cavities is reported in Helmstetter et al (2015), who analyzed the recordings of an accelerometer deployed on ice. In the case of high englacial water pressures exceeding the ice overburden pressure, hydraulic jacking of the ice can occur. We discuss our results in the light of tremor origin, time evolution of the drainage system, and drainage regime (Sect. 5) and draw our conclusions (Sect. 6)
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