Seismic anisotropy and lithospheric deformation of the plate-boundary zone in south island, new zealand: Inferences from local s-wave splitting

Abstract

Insights into plate-boundary deformation are gained from the shear-wave splitting of local S phases that originate within the lithosphere of the South Island, New Zealand. Analysis of the splitting parameters from land stations reveals changes in both delay times and fast azimuths with earthquake depth, earthquake-station back-azimuth and initial polarization azimuth, suggesting both laterally and depth varying anisotropy. When the average results are examined as a whole via tomographic inversion and spatial averaging, consistent patterns in delay times and fast azimuths exist. Spatially averaged fast azimuths reveal a localized high-strain zone in the southern central region of the South Island. Based on fast azimuths observed above 100 km depth, we suggest that the plate-boundary subparallel anisotropy that is produced by pervasive shear is mainly distributed within a zone extending ~130 km SE of the Alpine fault in southern South Island and is widely distributed in northern South Island. Average station delay times of ~0.1-0.4 s compared to 1.7 s SKS delay times from previous studies in South Island further suggests a deeper seated anisotropic zone in some areas, but in other regions frequency-dependent anisotropy could explain the observed splitting. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.