Strong variations in seismic anisotropy across the Hikurangi subduction zone, North Island, New Zealand

Abstract

Shear wave splitting measurements from SKS, SKKS, S and ScS phases reveal the anisotropic structure of North Island, New Zealand. Waveforms were recorded on temporary and permanent broadband seismographs. Strong anisotropy varies rapidly across the Hikurangi subduction zone. These changes constrain the width of the deformation associated with the plate boundary and define three different regions of mantle flow. Stations in the fore-arc region, eastern central North Island record fast polarisations subparallel to the trench and moderate delay times of around 2.5 s, consistent with earlier studies of North Island. This is interpreted as caused by trench-parallel mantle flow beneath the subducted slab with a possible contribution of trench-parallel fossil anisotropy within the subducted slab. Delay times increase towards the extending Central Volcanic Region (CVR) with values up to 4.5 s, one of the largest delay times measured worldwide. Strong S-wave anisotropy (7.5%) through 80 km in the mantle wedge in addition to the sub-slab anisotropy can explain the observed high delay times. One mechanism to create such high anisotropy is melt segregation of partially molten rock. A frequency dependence of this mechanism could also explain persisting discrepancies between local and teleseismic shear wave splitting measurements. To the west, delay times decrease systematically. Measurements in the far back-arc region of western central North Island show no apparent splitting. This suggests that mantle wedge dynamics terminate beneath the western border of the CVR. The apparent isotropy beneath the far back-arc may be related to local small-scale mantle convection or vertical mantle flow, possibly caused by the detachment of lithosphere.