Stress and crustal anisotropy in Marlborough, New Zealand: evidence for low fault strength and structure-controlled anisotropy

Abstract

The major faults in Marlborough and Wellington are of both scientific and societal interest as they play a key role in accommodating relative plate motion in the upper plate of an oblique subduction zone, and are thought to pose a high seismic risk to central New Zealand. Studies in California suggest that some plate boundary strike-slip faults, similar to those in central New Zealand, are frictionally weak and that crustal anisotropy is controlled by the ambient stress. However, whether these observations are generally applicable to all major strike-slip faults is yet to be determined. In this project we have addressed three main objectives, which are: to investigate the frictional properties of the faults in Marlborough in terms of their geometric relationship to the regional stress field; to investigate the cause of crustal anisotropy using results from shear-wave splitting; and to explore to what extent the faults in Marlborough are mechanically similar to those in California. We have used inversions of focal mechanism and first motion data to calculate the stress tensor and relate it to the geometry of the major faults. We have also conducted shear-wave splitting analysis on local S phases to determine the directions of crustal anisotropy and investigate their relationship to the geological fabric and the principal stress directions. The observed angle between the maximum horizontal compressive stress direction and the average strike of the major faults is 60°; this is substantially higher than the ~30° optimal angle expected for a vertical strike-slip fault given Byerlee friction and hydrostatic fluid pressures. The geometry can be explained, however, if the fault's friction coefficient is moderately low (~0.35) or the fluid pressure is moderately high (~0.7xlithostatic). These end-member values are similar to those inferred for the San Andreas fault in southern California. The maximum compressive stress direction is markedly different from the average strike of the major faults, enabling us to distinguish between stress- and structure-controlled anisotropy. Anisotropy directions determined from earthquakes less than 50 km deep reveal that the fast directions are principally aligned with the NE-SW-striking faults, and we therefore conclude that the anisotropy is mainly controlled by the geological fabric. Fault-parallel fast directions have also been observed in California, but stress-related anisotropy is present to greater distances from the fault there than seen in our results from Marlborough. The observation that faulting occurs at high angles to the maximum horizontal compressive stress direction substantiates the hypothesis that the San Andreas fault is not unique in being a frictionally weak fault. The results from our shear-wave splitting calculations suggest that anisotropy in the crust varies spatially in regions of active faulting but that in Marlborough, at least, it is controlled more by the geological structures than by the prevailing stress field.