Strongly depth-dependent ice fabric in a fast-flowing Antarctic ice stream revealed with icequake observations

The crystal orientation fabric of glacier ice impacts its strength and flow. Crystal fabric is therefore an important consideration when modelling ice flow. Here, we show that shear-wave splitting (SWS) measured with glacial microseismicity can be used to invert for seismic anisotropy and ice fabric, if represented in a statistical sense. Rutford Ice Stream (RIS) is a fast-flowing Antarctic ice stream, a setting crucial for informing large-scale ice sheet models. We present >200,000 SWS measurements from glacial microseismicity, registered at a 38-station seismic network located ∼40 km upstream of the grounding line. A representative subset of these data is inverted for ice fabric. Due to the character of SWS, which accumulates along the raypath, we include information on the depth structure from radar measurements. We find that the following three-layer configuration fits the data best: a broad vertical cone fabric near the base of RIS (500 m thick), a thick vertical girdle fabric, orientated perpendicular to flow, in the middle (1200 m thick) and a tilted cone fabric in the uppermost 400 m. Such a variation of fabric implies a depth-dependent strength profile of the ice with the middle layer being ∼3.5 times harder to deform along flow than across flow. At the same time, the middle layer is a factor ∼16 softer to shear than to compression or extension along flow. If such a configuration is representative for fast-flowing ice streams, it would call for a more complex integration of viscosity in ice sheet models.