SKS splitting and the seismic anisotropy of the mantle beneath the Hikurangi subduction zone, New Zealand

The analysis of 12 SKS phases recorded on broadband stations above the Hikurangi subduction zone in New Zealand shows clear evidence of mantle anisotropy, with the fast direction (28 degrees +/- 5 degrees) almost parallel to the strike of subduction and the dominant geology of the region. The slow shear-wave delay times show a systematic change with the azimuth of the arrivals which, if hexagonal symmetry is assumed, indicates that either the axis of symmetry of the anisotropic volume beneath the subduction zone is not horizontal, or that more than one anisotropic layer is present. The magnitude of the delays (1.5 +/- 0.4s) suggests that the anisotropy is most probably confined to the top 300 km of the mantle.