Spectral characteristics of seismic ambient vibrations reveal changes in the subglacial environment of Glacier de la Plaine Morte, Switzerland

Glaciers exhibit complex hydraulic and dynamic behaviour that needs to be studied to enhance our understanding of cryospheric changes. To address this, we apply a range of passive seismic analysis techniques to continuous data from a temporary seismic array deployed on Glacier de la Plaine Morte, Switzerland. First, we assess the reliability of ambient noise horizontal-to-vertical spectral ratio (HVSR) measurements and demonstrate that the variations in HVSR curves are predominantly attributed to changing nearby noise conditions influenced by hydraulic, drainage-related tremors, moulin resonances, and anthropogenic sources. We find that short time series (e.g. hours-long) may lead to biases in the interpretation of the HVSR curve. Hence, we perform an analysis of the local noise source variations related to glacier dynamic behaviour in order to distinguish between source and medium changes reflected in the HVSR measurements. In 130 d long time series of measurements we are able to detect a spatio-temporal trend and find that an HVSR trough emerges following the sudden drainage of an ice-marginal lake. This HVSR trough is indicative of a low-seismic-velocity layer at the ice-bed interface. Seismic velocity changes derived by interferometry support our findings of a velocity drop in the glacier after the drainage. Subsequently, inversion and forward modelling of the empirical dispersion and ellipticity curves reveal a probable thickness of this low-velocity layer of 10-30 m and a change in shear-wave velocity up to 40 %. This layer has a local extent covering an estimated 4.5 % to 27 % of the glacier, as indicated by the spatial variations in the HVSR trough throughout the array and an independent water volume estimate. Our findings suggest that the changing seismic velocities are a manifestation of temporal subglacial water storage in response to the sudden injection of lake water. Our results highlight the value of long time series of HVSR measurements which show variations in the peak and trough structure that reflect hydrological changes in the subglacial environment.