SEISMIC ANISOTROPY IN THE FORE-ARC REGION OF THE HIKURANGI SUBDUCTIONZONE, NEW-ZEALAND

Shear waves from local earthquakes recorded on an array of nine broad- band seismograph stations have been used to investigate the anisotropi c structure within the crust of the overlying Australian plate, and wi thin and below the subducting slab underlying the southern North Islan d, New Zealand. Earthquakes near the plate interface were used to imag e the crustal anisotropy, whereas deeper, down-dip subduction zone eve nts were used to sample the subducting slab, and the mantle below. Apa rt from a small amount of variable near-surface anisotropy (top few ki lometres of the crust), the measured fast shear-wave directions of bot h the crust of the overlying plate (phi = 51 degrees +/- 18 degrees) a nd the mantle (phi = 41 degrees +/- 15 degrees) are similar, and are n early parallel to the strike of the subduction zone, and the dominant geological features of the region. SKS measurements, which mainly samp le the mantle beneath the subducting plate, give a value of phi = 28 d egrees +/- 11 degrees, and measurements made elsewhere in the New Zeal and region using SKS and ScS2 produce similar results. The measurement s of crustal anisotropy are similar to those found further south on th e Wellington Peninsula, and confirm the presence of near-surface aniso tropy. The maximum delay times for crustal earthquakes indicate a perv asive shear-wave velocity anisotropy of 4% in the crust of the overlyi ng plate, and the increase in the delay times with depth for the subdu ction zone earthquakes translates to a shear-wave velocity anisotropy of 1.4% in the mantle below the subducting slab. These results, when c ombined with the SKS measurements, suggest trench-parallel flow in the mantle below the subducted slab, and a coherence of the deformation p rocesses in the crust of the overlying plate and the mantle beneath th e subducting slab.