Stress Characterization Above and Below a Locked Subduction Interface

The southern portion of North Island, New Zealand is part of the Hikurangi margin where the Pacific plate is subducting beneath the Australian plate. We use microseismicity recorded from Nov. 2009 - Mar. 2010 using the temporary SAHKE seismic network, with 18 permanent GeoNet seismic stations to investigate the stress field at high resolution in both the overlying and subducting plate. Orientations of the principal stresses in the subducted plate were determined from microseismicity located with the NonLinLoc method, using a New Zealand specific 3D velocity model. For those earthquakes located within the study region and shallower than 50km, focal mechanisms were calculated using a Bayesian method and P-wave first motions. 340 high quality focal mechanisms were calculated and then grouped to determine 20 stress inversions. Orientations of the principal stresses in the overlying plate were determined from analysis of shear-wave splitting fast-directions of microseismicity. Origin times from 425 earthquakes, detected by GeoNet, were used to cut out events in the SAHKE catalog. The P-wave picks were made automatically using the APK function and the S-wave picks were made automatically using the SpickerC program. Fast directions were calculated using the MFAST (Multiple Filter Automatic shear-wave Splitting Technique) program. The subducted plate analysis yielded similar results to those for overlapping regions of previous studies. The maximum horizontal stress (SHmax) directions from the stress inversion are parallel to the strike of the subduction zone interface contours. The extensive normal faulting and SHmax direction in the subducted plate indicates that deformation is dominated by bending stresses. The overlying plate analysis showed three distinct regions of splitting: west of the Wairarapa fault (West), between the Wairarapa and the Dry Creek fault (Basin), and east of the Dry Creek fault (East). Fast directions for the West and East regions trend NE-SW, this is broadly parallel to the strike of the Wairarapa and Dry Creek fault and also to the main topographic trends. The similar trends of the fast directions in the West and East regions from the splitting results and the SHmax directions in the subducted plate below suggest that the bending stresses could be influencing the upper plate stress regime.