Quieting of hydraulic tremor: sudden changes in frictional conditions in subglacial channels

Abstract

The state and evolution of subglacial channels strongly impact glacier motion and as a result the mass balance of flowing ice bodies. Yet, the subglacial environment is difficult to access and thus often poorly constrained over significant temporal and spatial scales. This limits our understanding of complex subglacial hydraulic processes and consequently ice dynamics. Seismology can help overcome these observational constraints, providing new insights into fundamental processes in the cryosphere, such as frictional sliding and subglacial water flow. However, different seismogenic processes of the cryosphere often overlap in both time and space. Differentiating between them and interpreting associated seismic signals require appropriate methodological and instrumental approaches. Here, we investigate subglacial channel dynamics at the Rhone glacier (Switzerland) over one month in the summer of 2020, focusing on periods coinciding with glacier sliding episodes. To this end, we leverage the sensitivity of near-bed borehole geophones combined with seismic interferometry and beamforming techniques. We show that the hydraulic tremor, generated by turbulent water flow and resulting pressure variations acting against the subglacial channel bed and walls, acts as a dominant, stable, and coherent noise source. Beamforming analysis reveals the directional stability of the hydraulic tremor and points toward the junction of two subglacial hydraulic channels from which stick-slip asperities originate. The analysis also reveals instances of sudden hydraulic tremor quieting, in agreement with previous observations before and after seismogenic sliding episodes. We explain this quieting as sudden changes in frictional conditions within the subglacial channel corresponding to a rapid transition between a fully and partially filled channel. We discuss channel properties (geometry and bed conditions) that are needed to satisfy the physical conditions for the frictional quieting mechanism. Our analysis offers new insights into the complex mechanical interactions between ice, water, and bed properties and the hydraulic control of glacier sliding.