Anisotropic structure of the Hikurangi subduction zone, New Zealand-integrated interpretation of surface-wave andbody-wave observations

Abstract

A model of subduction-zone anisotropy for south and central North Island, New Zealand is presented by integrating the observations of shear-wave anisotropy from anomalous surface-wave polarizations, local S phases at a range of hypocentral depths, and SKS splitting analyses. Anomalous particle motions of the equivalent to fundamental- mode Love waves have been measured for propagation paths approximately along the strike of the Hikurangi subduction zone in eastern North Island, New Zealand. These observations are modelled in terms of an anisotropic crust and subducted Pacific slab with a pseudo-hexagonal symmetry axis plunging at 30° to the vertical and a fast axis parallel to the strike of the margin. A shear-wave anisotropy of 4 per cent is assumed. Shear-wave splitting analyses of regional phases in central North Island show no increase in fast–slow delay time (0.21±0.03s) with increased hypocentral depth, unlike previous observations in southern North Island, which display an increase in delay time of about 0.5s over 250km depth. Fast polarization directions are again approximately parallel to the strike of subduction. Observed SKS splitting parameters, on the other hand, do not vary significantly between southern and central North Island, and over the mantle wedge in western North Island. A mean fast–slow delay time of 1.5±0.3s and a fast polarization direction of 25°±7°, subparallel to the strike of the subduction zone, were determined for the SKS observations. Both the surface-wave polarization and local shear-wave splitting observations were modelled by an anisotropic New Zealand crust with a pseudo-hexagonal symmetry axis plunging at 15°–30° and a horizontal fast axis parallel to the strike of the subduction. The variation of the splitting delay times with hypocentral depth between central and southern North Island implies a horizontal symmetry axis in the subducted slab in the south, but an axis plunging parallel to the plate interface in central North Island. In the mantle wedge above the slab in western North Island and the mantle beneath the slab in central and southern North Island, a horizontal anisotropic symmetry axis fits the observations. 4 per cent anisotropy was assumed throughout the models. Within the New Zealand crust, the plunge of the symmetry axis of the anisotropic structure is compatible with the structural trends of the regional geology and fault orientations. The variation in the plunge of the anisotropic symmetry axis within the subducted Pacific lithosphere from central to southern North Island is interpreted to reflect the change in intraplate stress southwards towards South Island, where the India–Australian and Pacific plates are locked. The orientation of olivine due to trench-parallel flow would explain the observations in the mantle wedge and Pacific mantle below the subducted plate.