Spectral characteristics of seismic ambient vibrations reveal subglacial hydraulic changes beneath Glacier de la Plaine Morte, Switzerland

Abstract. Glaciers have a complex hydraulic and dynamic behavior that needs to be investigated to improve our understanding of changes in the cryosphere. To tackle this issue, we employ various passive seismic analysis methods on continuous measurements from a temporary seismic array deployed on Glacier de la Plaine Morte in Switzerland. First, we asses the reliability of ambient noise Horizontal-to-Vertical spectral ratio (HVSR) measurements to the glacier's dynamic environment. The spatiotemporal variations in HVSR curves are predominantly attributed to changing nearby noise conditions influenced by hydraulic, drainage-related tremors, moulin resonances and anthropogenic sources. A careful 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. Only a few hours of HVSR measurements may lead to biases in the interpretation of the HVSR curve. Despite the influence of these external factors, with long time series of the HVSR measurements, we successfully detect a spatiotemporal trend in HVSR curves. Notably, an HVSR trough emerges following the drainage of Lac des Faverges, an ice-marginal lake that rapidly drains, causing water to flow through a channelized system beneath the glacier. This HVSR trough is indicative of a low-seismic velocity layer at the ice-bed interface. Seismic velocity changes derived by interferometry support the presence of a low-velocity layer at the ice-bedrock interface. Inversion and forward modelling reveal a probable thickness of this low-velocity layer of 10–30 m and a change in S-wave velocity up to 40 %. This layer has a local extend covering an estimated 4.5 to 27 % of the glacier, as indicated by the spatial variations in HVSR trough throughout the array and an independent water volume estimate. The changing seismic velocities are thus a manifestation of temporal water storage at the glacier bed in response to sudden injection of lake water. Our results highlight the value of long time series of HVSR measurements which show variations in the peak/trough structure that reflect hydraulic changes beneath the ice surface.