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

Abstract

Seismic anisotropy beneath the lower half of the North Island, New Zealand, has been investigated using teleseismic ScS and SKS events recorded on nine broad-band seismograph stations. The study 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 were performed, giving similar fast polarisations and delay times at each station. The average SKS fast polarisation is approximately NE-SW, sub-parallel to the strike of subduction and the major geological features, with an average SKS delay time of around 1.6 s. The lack of variation in splitting parameters obtained at the western and eastern stations suggests that similar fast polarisations are found in the mantle both above and below the slab. 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 polarisations found in the asthenosphere both above and below the slab suggests that the mantle flow is in a trench-parallel direction in both regions, suggesting that the slab is acting as a passive rigid barrier to mantle flow. The ScS delay times are around half the SKS delay times, as was found for the permanent station SNZO. Further investigation revealed an apparent increase in delay time with period at SNZO, which may explain the discrepancy in delay times. The most likely cause of frequency-dependent anisotropy is fine-scale aligned heterogeneities.