Upper mantle anisotropy in the New Zealand region

Shear-wave splitting parameters of fast polarization direction (Phi) and de lay time (delta t) are determined using data from the Southern Alps Passive Seismic Experiment (SAPSE), on the South Island of New Zealand and in the surrounding region. Our results clearly show that Phi are subparallel to tr ends of the Alpine and Marlborough Faults, and to the Pacific-Australian pl ate boundary. The delta t values range from 0.6-2.2 s with an average value of 1.6 s; the largest values are from the central South Island. The main s ource of the observed shear-wave splitting is an anisotropic region between 40-400 km. The width of the zone is approximately 200 km. We attribute the coincidence of surface structural trends with the measured Phi, and the la rge delta t values, to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upp er mantle.