Stress and crustal anisotropy in Marlborough and Wellington

Abstract

We have investigated the mechanics of faulting and strain accommodation in the greater Marlborough region. This is a tectonically complex area containing several large faults known to have generated large earthquakes in the recent past. This research focussed on the frictional strength of those faults, and on differentiating between two hypothesized controls on the mechanical properties of the adjacent crust.

The major faults in Marlborough and Wellington play a key role in accommodating the oblique motion between the Australian and Pacific plates, and pose a high seismic risk to central New Zealand. Studies of the plate-bounding faults of the San Andreas fault system, which are geometrically similar to those in Marlborough and Wellington, suggest that the San Andreas faults are weak, and that crustal anisotropy is controlled by the ambient stress. However, whether these observations are more generally applicable to major strike-slip faults is yet to be determined. To address this concern, we have calculated the principal stress directions in Marlborough and related these results to the frictional strength of the major faults. We have also determined the directions of crustal anisotropy in Marlborough, and investigated their geometric relationships to the geological fabric and the principal stress directions.

We find that the faults are weak; they slip when shear stress is lower than expected for a typical friction coefficient. This suggests that the faults have either a moderately low friction coefficient or moderately high fluid pressure. These end-member values are similar to those inferred for the San Andreas fault in southern California. This substantiates the hypothesis that the San Andreas fault is not unique in being frictionally weak. In the crust in Marlborough, seismic anisotropy is controlled more by the geological structures than by the prevailing stress field, so that the cause of anisotropy varies from that near the San Andreas Fault.

This research complements work in California and elsewhere into the mechanics of major faults and helps provide a mechanical framework to interpret future geological and geophysical studies of New Zealand tectonics.

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Technical 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 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 these geometric relationship to the regional stress field; to investigate the cause of crust 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 60o; this is substantially higher than the ~30o 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 (~0.7 x lithostatic). The 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-related 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, however 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 by that in Marlborough, at least, it is controlled more by the geological structures than by the prevailing stress field.