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

Abstract

The major strike-slip faults in the greater Marlborough region, central New Zealand, are of both scientific and societal interest as they accommodate relative plate motion in the upper plate of an oblique subduction zone and pose a high seismic risk to central New Zealand. Studies in California suggest that some plate-bounding strike-slip faults are frictionally weak and that crustal anisotropy is controlled by the ambient stress. Whether these observations are more generally applicable to major strike-slip faults has yet to be determined. We have used inversions of focal mechanism and first motion data to calculate the principal stress directions in Marlborough and related them to the geometry of the major faults. The average angle between the axis of maximum horizontal compressive stress (SHmax) and the average strike of the major faults is 60°; this is substantially higher than the ∼30° expected for reactivation of a vertical strike-slip fault given Byerlee friction and hydrostatic fluid pressure. This geometry can be explained, however, by the faults having a moderately low friction coefficient (∼0.35), a moderately high fluid pressure (∼0.7 × lithostatic) or some combination of the two. This observation substantiates the hypothesis that the San Andreas fault is not unique in being frictionally weak. We have also conducted shear-wave splitting analysis on local S phases to determine the directions of crustal anisotropy and investigated their orientations with respect to the geological fabric and the principal stress directions. The anisotropy determined using shallow earthquakes reveals that the fast direction is 65 ± 50° and is generally aligned with the NE-CSW-striking faults, and we therefore conclude that crustal anisotropy in Marlborough is controlled more by the geological structures than by the prevailing stress field.