Joint reconstruction of icequake source mechanisms and 3D glacier structure from dense seismic array data

Glacio-seismology, which investigates the dynamics and processes of the cryosphere using seismic observations and methods, has strongly grown in interest over the past two decades in the context of global warming. To study the Argentière Glacier (French Alps), a dense array of 98 3-component seismic sensors was deployed in spring 2018 for 35 days. This period coincided with a temperature increase, which enhances the glacier’s seismic activity. The recordings bear the imprint of several thousand icequakes associated with ice-fracturing phenomena such as crevassing. We build a catalog of icequakes and their location using Matched Field Processing (MFP), which is a beamforming based approach.

Then, we jointly reconstruct icequake source mechanisms and the 3D glacier structure by exploiting the full waveform of the recorded 3-component data. The reconstruction relies on elastic wave modelling through numerical solution of the 3D elastodynamic equations using the Spectral Element Method (SEM). Accounting for the glacier surface topography is essential in order to correctly model surface waves, which mainly dominate the icequake data. We apply an alternating optimisation strategy that iterates between two sub-problems: estimating source mechanisms and updating glacier model parameters. The estimation of an icequake mechanism is formulated as the solution of a bi-quadratic minimisation problem depending on the moment tensor and the source wavelet. The model-parameter update is based on the application of a classical elastic Full Waveform Inversion (FWI).

This joint inversion strategy is applied to a set of selected icequakes with a high signal-to-noise ratio. We are able to reconstruct average model heterogeneities that align with the orientation of surface crevasses in several areas of the glacier. Some heterogeneities are reconstructed down to 100 m below the surface, enabling us to estimate the depth of crevasse fields surrounding the sensor network. Finally, we note a clear improvement in the reconstruction of the SH-wave in the updated model compared to what is obtained in a homogeneous medium. In the homogeneous approximation, the Rayleigh wave is reconstructed accurately, whereas the SH-wave is less well recovered. This improvement suggests that the SH-wave is strongly impacted by surface heterogeneities, more than the Rayleigh wave, and mainly drives the reconstruction of the structure. The estimated icequake source mechanisms do not appear to change significantly between the homogeneous model and the updated model obtained during the alternating strategy. This suggests a relative decoupling between source parameters and model parameters in the joint reconstruction problem, mediated by the Rayleigh and SH- waves. Such a decoupling is generally not observed in classical seismology, and therefore seems to be rather specific to the glacial context.