Anisotropy of schists: Contribution of crustal anisotropy to active sourceseismic experiments and shear wave splitting observations

We have made sets of five independent compressional and shear wave velocity measurements, which with density, allow us to completely characterize the transverse isotropy of samples from five metamorphic belts: the Haast schis t terrane (South Island, New Zealand), Poultney slate, Chugach phyllite, Co ldfoot schist, and Pelona schist (United States). These velocity measuremen ts include compressional wave velocities for propagation parallel, perpendi cular, and at 45 degrees to the symmetry axis,shear wave velocity for propa gation and particle motion perpendicular to the symmetry axis, and shear wa ve velocity for propagation parallel to the symmetry axis. Velocity measure ments were made up to pressures of 1 GPa (similar to 35-km depth) where mic rocracks are closed and anisotropy is due to preferred mineral orientation. Our samples exhibit compressional wave anisotropy of 9-20% as well as sign ificant shear wave splitting. Metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments. Our data suggest that a significant thickness (10-20 km) of appropriately oriented (steeply dipping foliation) schist in the cru st could contribute as much as 45% of observed shear wave splitting. Our da ta set can also be used to model the effects of crustal anisotropy for acti ve source seismic experiments in order to determine if the anisotropy of th e terrane is significant and needs to be taken into account during processi ng and modeling of the data.