Stress Orientations in a Locked Subduction Zone at the Southern Hikurangi Margin, New Zealand

Abstract

AbstractWe analyze the orientation of the stress field in the southern Hikurangi subduction zone, New Zealand, using focal mechanism inversions, S wave splitting fast directions, and gravitational stresses. Here the oceanic Pacific plate is being obliquely subducted beneath the continental Australian plate in the New Zealand plate boundary zone. The study makes use of 399 earthquakes for focal mechanism inversion and 425 earthquakes for shear wave splitting analysis, located with a network of seismic stations spanning southern North Island. We distinguish between stresses in the Pacific plate (from focal mechanism inversion) and Australian plate (from S wave fast directions) and gravitational stresses, in three regions: Western, Central Basin, and Eastern. In the Western region, the principal axis of horizontal compression (SHmax) is oriented NE‐SW, parallel to the margin, in the upper Australian and lower Pacific plate. In the Central Basin, SHmax in the Australian plate is oriented NW‐SE, perpendicular to the margin; in the lower subducting Pacific plate SHmax is oriented NE‐SW. In the Eastern region, SHmax is oriented NE‐SW in the upper plate, while in the lower plate there is a change in orientation to NNW‐SSE. We interpret the stress orientations of the lower plate in the Western and Central Basin regions as a consequence of bending of the subducting plate. Sources of upper Australian plate stresses are likely to be bending stresses, gravitational stresses, and tectonic loading, with differing relative magnitudes across the study area.