Deformation in Rutford Ice Stream, West Antarctica: measuring shear-wave anisotropy from icequakes
Ice streams provide major drainage pathways for the Antarctic ice sheet. The stress
distribution and style of flow in such ice streams produce elastic and rheological anisotropy, which
informs ice-flow modelling as to how ice masses respond to external changes such as global warming.
Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shearwave
splitting from three-component icequake seismograms to characterize ice deformation via crystalpreferred
orientation. Over 110 high-quality measurements are made on 41 events recorded at five
stations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this is
the first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitude
of the splitting ranges from 2 to 80ms and suggests a maximum of 6% shear-wave splitting. The fast
shear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider three
mechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdle
model (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model).
Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy – an HTI component
is needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment of
cracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination of
mechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropy
in an ice stream, as predicted by ice fabric models.