Seismic anisotropy in the fore-arc region of the Hikurangi subduction zone, New Zealand

Abstract

Shear waves from local earthquakes recorded on an array of nine broad-band seismograph stations have been used to investigate the anisotropic structure within the crust of the overlying Australian plate, and within and below the subducting slab underlying the southern North Island, New Zealand. Earthquakes near the plate interface were used to image the crustal anisotropy, whereas deeper, down-dip subduction zone events were used to sample the subducting slab, and the mantle below. Apart from a small amount of variable near-surface anisotropy (top few kilometres of the crust), the measured fast shear-wave directions of both the crust of the overlying plate (φ = 51° ± 18°) and the mantle (φ = 41° ± 15°) are similar, and are nearly parallel to the strike of the subduction zone, and the dominant geological features of the region. SKS measurements, which mainly sample the mantle beneath the subducting plate, give a value of φ = 28° ± 11°, and measurements made elsewhere in the New Zealand region using SKS and ScS2 produce similar results. The measurements of crustal anisotropy are similar to those found further south on the Wellington Peninsula, and confirm the presence of near-surface anisotropy. The maximum delay times for crustal earthquakes indicate a pervasive shear-wave velocity anisotropy of 4% in the crust of the overlying plate, and the increase in the delay times with depth for the subduction zone earthquakes translates to a shear-wave velocity anisotropy of 1.4% in the mantle below the subducting slab. These results, when combined with the SKS measurements, suggest trench-parallel flow in the mantle below the subducted slab, and a coherence of the deformation processes in the crust of the overlying plate and the mantle beneath the subducting slab.