Contribution of crystal orientation and grain boundary compliance to low shear velocity observed near base of polar ice sheets

Abstract

SUMMARYSeismic shear wave velocity (S-velocity) shows a decrease towards the base of ice sheets in Antarctica and Greenland that is not accompanied by a corresponding decrease in compressional velocity (P-velocity). This decrease has been interpreted as arising from liquid water below the melting point (pre-melt water) at grain boundaries, but the lack of a corresponding decrease in P-velocity has not been explained. Representing grain boundaries as displacement discontinuities allows the change in P- and S-velocities to be written as functions of the normal and shear compliance of the grain boundaries. This allows the normal-to-shear compliance ratio of the grain boundaries to be constrained, and seismic anisotropy resulting from a partial orientation of grain boundaries to be estimated. This approach demonstrates that the observed reduction in S-velocity with no significant decrease in P-velocity near the base of ice sheets in Antarctica and Greenland can be explained by pre-melt water at small aperture grain boundaries. Such water may enable sliding along the grain boundaries and so may enhance creep of ice near the base of ice sheets. If stress state is anisotropic the aperture of water-containing grain boundaries may vary with azimuth, with the most open grain boundaries oriented with strikes perpendicular to least compressive stress. Microcracks and fractures may be treated also as displacement discontinuities and, together with oriented grain boundaries, may contribute to shear wave splitting as observed in West Antarctica in a fast-moving ice stream.