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

Teleseismic ScS and SKS events recorded on nine broadband seismograph stati ons have been used to investigate seismic anisotropy beneath the lower half of the North Island, New Zealand. This area lies above the Hikurangi subdu ction zone, and the array provides ray paths which sample the mantle both a bove and below the slab. Shear wave splitting measurements give similar fas t polarizations and delay times at each station. The average SKS fast polar ization 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 f ast polarizations are found in both the mantle wedge and the subslab mantle . The anisotropy in the lithospheric portion of the mantle wedge is most li kely caused by the preferred orientation of olivine due to the shear deform ation associated with oblique convergence. Any anisotropy in the slab is pr obably due to fossil mineral alignment. Anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenosp heric 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.