Constraining Ice Shelf Anisotropy Using Shear Wave Splitting Measurements from Active‐Source Borehole Seismics

Abstract

Observations of ice shelf anisotropy on borehole seismic data are presented. Hot-water-drilledboreholes were made by the Aotearoa New Zealand Ross Ice Shelf Programme through a grounding-line proximal site at Windless Bight and the central Ross Ice Shelf site HWD-2. The boreholes were used to freeze seismometers into the ice at different depths. Seismic observations of shear wave splitting were made on the borehole seismometers using active sources deployed at the surface. These shear wave splitting data were used to constrain anisotropic ice crystallographic preferred orientations (CPO) within the ice column. Forward models of seismic properties from different CPO geometries are compared to the observations and a best fitting CPO model is found to explain the seismic anisotropy at HWD-2. This model consists of a vertical girdle of ice c axes that constitute 80% of the CPO in combination with tight horizontal clusters, which contribute 20% of ice c axes. The origin of the modeled CPO is discussed with regard to calculated strain rates at the site and found to be indicative of the current shear kinematics with vertical shear plane and horizontal shear direction. At HWD-2 the 370 m thick ice shelf is calculated to consist of at least 197 m of anisotropic ice.

Crystallographic Preferred Orientation (CPO) Development Governs Strain Weakening in Ice: Insights From High‐Temperature Deformation Experiments

“…Terrestrial ice c ‐axis data are usually provided by microstructural analyses of ice cores (e.g., Azuma et al., 1999; Weikusat et al., 2017). However, recent developments in cryo‐seismology may provide a cost‐effective way of estimating ice c ‐axis orientations across a wide region (on the order of several square kilometres or more) via inversion of seismic anisotropy data (Lutz et al., 2020; Smith et al., 2017).…”

Microstructure and Crystallographic Preferred Orientations of an Azimuthally Oriented Ice Core from a Lateral Shear Margin: Priestley Glacier, Antarctica

“…Of these studies, even fewer describe CPOs fully oriented in a kinematic reference frame (Hudleston, 1977;Monz et al, 2021;Hellman et al, 2021). Interpreting geophysical and radio-echo sounding data to predict large scale CPOs within large polar ice streams (Bentley, 1972;Kohnen and Gow, 1979;Horgan et al, 2011;Harland et al, 2013;Smith et al, 2017;Jordan et al, 2020;Lutz et al, 2020;Hellmann et al, 2021b) requires an understanding of the structure and dynamics of shear margins, currently lacking due to this disparity in sample collection.…”

Mechanics and dynamics of pinning points on the Shirase Coast, West Antarctica

“…The Ice-sheet and Sea-level System Model (ISSM) is an open-source, finite-element ice flow model that solves the conservation equations for mass and momentum in combination with appropriate boundary conditions and the constitutive relationship for ice (Larour et al, 2012). The shallow shelf (or shelfy-stream) approximation (SSA) (Morland, 1987;MacAyeal, 1989) of the full-Stokes equations, appropriate for sliding over a very weak substrate such as water or water-saturated subglacial till, is used to simulate iceshelf and ice-sheet flow. The fast-flowing RIS ice streams draining the West Antarctic Ice Sheet are characterised by thawed bases and significant sliding over soft subglacial till (MacAyeal et al, 1995;Joughin et al, 2004), justifying the SSA for the present study.…”