Abstract
AbstractCrevasses 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.
Highlights
Crevasses are commonly observed on ice bodies and have far-reaching implications for glacier dynamics
Since we are interested in Rayleigh waves, we use a coarse grid of slowness values associated with the minimum and maximum phase velocities of 1.25 and 2.25 km s−1, which covers the typical range expected for Rayleigh waves on glaciers (Walter and others, 2015)
A histogram calculated from all pixel orientations (Fig. 12d) yields a dominant orientation of approximately 55°, which is in agreement with the surface strike of the crevasses as determined by visual inspection. Comparing this with the average anisotropy fits in the frequency range 14– 30 Hz, we find that the Rayleigh waves propagate fastest in the direction of the crevasse surface strike (Fig. 12d)
Summary
Crevasses are commonly observed on ice bodies and have far-reaching implications for glacier dynamics. At the bottom of a glacier, basal crevasses can extend the subglacial drainage system some tens of meters into the ice (Harper and others, 2010). The process of crevassing affects glacial hydrology which, in turn, is crucial for ice flow dynamics (e.g., Iken and Bindschadler, 1986; Flowers and Clarke, 2002). By providing water pathways, crevasses promote cryohydraulic warming, softening the ice and influencing ice flow (Phillips and others, 2010). Colgan and others (2016) provide a comprehensive discussion on the role of crevasses
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