Seismic anisotropy beneath the lower half of the North Island, New Zealand

Teleseismic ScS and SKS events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the North Island, New Zealand. This area lies above the Hikurangi subduction zone, and the array provides ray paths which sample the mantle both above and below the slab. Shear wave splitting measurements give similar fast polarizations and delay times at each station. The average SKS fast polarization is approximately NE-SW, subparallel to the strike of subduction and the major geological features with an average SKS delay time of 1.6 +/- 0.1 s. This lack of variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle. The anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence. Any anisotropy in the slab is probably due to fossil mineral alignment. Anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow. The similar NE-SW fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench-parallel direction in both regions.