HR: 14:00h
AN: T33E-02    [Abstracts]
TI: Stress and Crustal Anisotropy in Marlborough, New Zealand: Evidence for Low Fault Strength and Structure-Controlled Anisotropy
AU: * Balfour, N
EM: balfounata@student.vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, 6015 New Zealand
AU: Savage, M K
EM: martha.savage@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, 6015 New Zealand
AU: Townend, J
EM: john.townend@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, 6015 New Zealand
AB: The major faults in Marlborough and Wellington 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 is yet to be determined. We have used inversions of focal mechanism and first motion data to calculate the principal stress directions and relate them 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 investigated their relationship to the geological fabric and the principal stress directions. The average angle between the axis of maximum horizontal compressive stress ($S_{Hmax}$) and the average strike of the major faults is $60\deg$; this is substantially higher than the $\sim 30\deg$ expected for an Andersonian strike-slip fault. This geometry can be explained, however, by the faults having a moderately low friction coefficient ($\sim$0.35) or moderately high fluid pressure ($\sim$0.7$\times$lithostatic). The anisotropy directions determined using shallow earthquakes reveal that the fast directions are aligned with the NE--SW-striking faults, and we therefore conclude that the anisotropy is mainly controlled by the geological fabric. The observation that faulting occurs at high angles to $S_{Hmax}$ substantiates the hypothesis that the San Andreas fault is not unique in being frictionally weak. 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 the prevailing stress field.
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting