Ambient H/V sensitivity to the dynamics of glaciers and ice sheets

Climate change is causing major shifts in the dynamics of the cryosphere, leading to sea-level rise, glacier break-off events, flooding, and landslides. Geological, thermodynamic and hydraulic processes at the base of an ice mass play a central role in ice flow dynamics, and understanding these is imperative for predicting ice body behavior in a changing climate. To this end, sustained ambient vibrations in glaciated environments can be used to monitor subglacial conditions over significant spatial extent with relatively low-cost acquisition.

In earthquake seismology, a well-established methodology to investigate subsurface properties is the horizontal-to-vertical spectral ratio (H/V) of ambient seismic ground unrest. In cryoseismology, the H/V approach is already used to invert for velocity profiles of ice or firn, to obtain bedrock topography and to identify the presence of basal sediments. To date, only a few hours of seismic vibration records are typically used. Yet in such short time records, biases may arise because of the dynamic character of the glacier. Seismic resonances within the soft ice layer and resulting H/V ratios are expected to vary with changes in subglacial hydraulic conditions.

We propose to leverage temporal variations in H/V spectra to investigate subglacial processes. As a case study, we first focus on the Glacier de la Plaine Morte (Switzerland), where a seismic array was deployed for four months in summer of 2016. During this time, an ice-marginal lake formed and suddenly drained through and under the glacier, making this seismic record ideal for our purposes. This drainage event is well recorded and strongly influences the H/V in terms of amplitude and resonance frequency. We next present ambient H/V measurements of the Sermeq Kujalleq in Kangia (also known as Jakobshavn Isbræ), one of Greenland’s largest outlet glaciers. Here, the H/V spectra show multiple resonances over time, whose origin we discuss in more detail. For both our study cases, separating variations in source and medium properties is pivotal. Tackling this challenge provides glaciologists with a valuable tool to investigate the poorly accessible subglacial environment, which holds the key to our understanding of ice flow and eustatic sea level rise.