Three-dimensional distribution of seismic anisotropy in the Hikurangi subduction zone beneath the central North Island, New Zealand

We use earthquake arrival time data in the central North Island, New Zealand, in inversion for 3-D Vp azimuthal anisotropy. It is parameterized with an isotropic component and two azimuthal anisotropy parameters for each node, and local earthquake shear wave splitting observations define the initial anisotropy model. Our results suggest that mantle flow, either within the mantle wedge or below the subducted slab, is not the primary source of anisotropy in the Hikurangi subduction zone. The largest region of anisotropy that we image is within the subducted slab, where anisotropy is consistently trench-parallel and 5-9% magnitude from 32 to 185 km depth. Bending-induced yielding in the slab offshore and as it passes through the trench provides an explanation for this slab anisotropy. Beneath the Taupo Volcanic Zone (TVZ), there is 1-4% trench-normal anisotropy within 30 km of the surface of the slab, within a region entrained with motion of the slab. In contrast, only very weak (0-2%) anisotropy is imaged in the region of high-temperature partial melt arising from active corner flow in the mantle wedge. In the crust geologic structure is important, with the largest crustal anisotropy (14%) related to schist. We image significant anisotropy aligned with the margins of the TVZ, related to extensive fracturing on the margins as the TVZ actively extends. Within the heavily intruded and underplated TVZ lower crust, the orientation of anisotropy switches to align with the extension direction. This orientation is consistent with other geophysical data suggesting the presence of connected melt in this region.