Crevasse-induced Rayleigh-wave azimuthal anisotropy on Glacier de la Plaine Morte, Switzerland

Abstract

Crevasses and englacial fracture networks route meltwater from a glacier's surface to the subglacial drainage system and thus influence glacial hydraulics. However, rapid fracture growth may also lead to sudden and potentially hazardous structural failure of unstable glaciers and ice dams, rifting of ice shelves, or iceberg calving. Here, we use passive seismic recordings to investigate the englacial fracture network on Glacier de la Plaine Morte, Switzerland. Glacier dynamics and the drainage of an ice-marginal lake give rise to numerous icequakes, the majority of which generate dispersed, high-frequency Rayleigh waves. A wide distribution of events allows us to study azimuthal anisotropy between 10 and 30 Hz in order to extract englacial seismic velocities in regions of preferentially oriented crevasses. Beamforming applied to a 100-m-aperture array reveals azimuthal anisotropy of Rayleigh-wave phase velocities reaching a strength of 8% at high frequencies. In addition, we find that the fast direction of wave propagation coincides with the observed surface strike of the narrow crevasses. Forward modeling and inversion of dispersion curves suggest that the azimuthal anisotropy is induced by a 40-m-thick crevassed layer at the surface of the glacier with 8% anisotropy in shear-wave velocity.

Glaciohydraulic seismic tremors on an Alpine glacier

“…(2), we use the local Rayleigh wave velocities which are 1600, 1800, 2200, and 1600 m s −1 for arrays A0, A1, A2, and A3, respectively (Lindner et al, 2019). In the spatial domain, we apply a grid search over the entire glacier surface and its surroundings (assuming a horizontal plane) to calculate the r j values in Eq.…”

“…In mid-July on days 202 to 204, we installed three additional arrays, each consisting of five stations with an aperture of 100 m. For each of these stations, we used a three-component 4.5 Hz geophone (PE-6/B manufactured by Sensor Nederland) connected to an Omnirecs DATA-CUBE3 logging ground velocity at 400 samples per second. The geophones were installed in the snowpack and later on ice (for details see Lindner et al, 2019) while the digitizers stayed at the surface to retain GPS capability. Power supply for these stations was achieved via alkaline batteries which needed to be replaced on a weekly basis.…”

“…To investigate the interplay of glacial hydraulics and icequake activity on Glacier de la Plaine Morte, we build on the results of Lindner et al (2019). This study focuses on ice-fracturing events (Walter et al, 2009) to investigate azimuthal anisotropy of seismic wave propagation, but the events are also useful to study fracturing associated with glacial hydraulics, e.g., during outburst floods (e.g., Roux et al, 2010).…”

“…This study focuses on ice-fracturing events (Walter et al, 2009) to investigate azimuthal anisotropy of seismic wave propagation, but the events are also useful to study fracturing associated with glacial hydraulics, e.g., during outburst floods (e.g., Roux et al, 2010). Lindner et al (2019) detect icequakes by applying a short-term average and long-term average (STA-LTA) trigger (Allen, 1978) on bandpass-filtered data (10-20 Hz for A1, A2, A3; 7-15 Hz for A0), require at least three stations of an array to trigger concurrently, and disable the trigger for 3 s after a detected event to avoid overlapping event windows (for parameter details see Lindner et al, 2019). From the event catalogs from each array, we calculate the icequake detection rate in events per hour.…”

“…For further insights into the glacial hydraulics, we consider the icequake source locations as determined from plane-wave beamforming (for details see Lindner et al, 2019). Planewave beamforming enables us to determine the back azimuth of an incident signal with an array of sensors.…”