Anisotropy and crystalline fabric of Whillans Ice Stream (West Antarctica) inferred from multicomponent seismic data

Crystal orientation fabric (COF) describes the intrinsic anisotropic nature of ice and is an important parameter for modeling glacier flow. We present the results of three-component active-source seismic observations from the Whillans Ice Stream (WIS), a fast-flowing ice stream in West Antarctica. Surface-wave dispersion analysis, ray tracing, and traveltime inversion of compressional (P) and shear (S) waves reveal the presence of transversely isotropic ice with a vertical axis of symmetry (VTI) beneath approximately 65m of isotropic firn. The ice stream is characterized by weak anisotropy, involving an average ice thickness of approximately 780m. The analysis indicates that about 95% of the ice mass is anisotropic, and the crystalline c axes span within an average broad cone angle of 73 10 degrees with respect to the vertical axis. Moreover, the mean temperature T (below the firn) estimated from seismic data is -15 5 degrees C. These data do not show evidence of englacial seismic reflectivity, which indicates the lack of abrupt changes in the COF. The presence of azimuthal anisotropy due to transversely compressive flow or fractures aligned along a preferential direction is also excluded. We suggest that the observed VTI ice structure is typical of large ice streams in regions where basal sliding and bed deformation dominate over internal glacial deformation.