Seismic anisotropy from local earthquakes in the transition region from a subduction to a strike-slip plate boundary, New Zealand

Shear wave splitting is used to investigate anisotropy in the crust and upper mantle in a subduction zone (lower half of the North Island of New Zealand), and its transition to oblique transform faulting (Marlborough area, northern South Island). In Marlborough, delay times show almost no increase with depth, and it is most likely that the higher frequency phases used in this study respond mainly to lithospheric anisotropy. In the central Marlborough Fault System (MFS), fast polarizations are subparallel to the faults. Anisotropy is attributed to the presence of metamorphosed schist (eclogite), of 30 +/- 10 km thickness and located 50-80 km beneath the MFS. On the edges of the MFS, fast polarizations are parallel to the maximum compressive stress direction, consistent with crack-induced anisotropy in the crust. The shear zone, which is as wide as the island in the mantle as inferred from SKS phases, seems to occur in a narrower zone in the crust. In the lower half of the North Island, fast polarizations from events at all depths are oriented parallel to the strike of the Hikurangi subduction zone as well as to the faults. Polarisations are similar to those of SKS phases, which mainly sample the mantle. This suggests that the lithosphere and the upper mantle asthenosphere deform in a coherent strike-slip shear. We calculate 1.2 +/- 0.3% velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth. In order to match the SKS delay times, this result requires the presence of anisotropic material down to 580 +/- 100-km depth, or a change in anisotropy with depth, or frequency dependent splitting.