Crustal anisotropy in the vicinity of the Alpine Fault Zone, South Island, New Zealand

Petrophysical measurements of rock samples collected within the Haast, Torlesse, and Alpine Fault Zone terranes of the South Island of New Zealand indicate significant seismic P-wave velocity anisotropy at pressures representing depths of up to 30 km. The percentage of anisotropy increases with increasing metamorphic grade and thus decreases with structural distance from the Alpine Fault. A maximum anisotropy of 17.3% was obtained from a drillcore sample located within the garnet-oligoclase zone schist, immediately adjacent to the Alpine Fault. Shear-wave splitting is another important property of the schists. For propagation parallel to foliation, split shear waves show velocity differences up to 1 km/s. At elevated pressures, the measured seismic velocity anisotropy is caused by preferred mineral orientation and is not due to the presence of cracks. The pronounced velocity anisotropy will significantly affect propagating seismic waves collected during both natural and active source seismic experiments; this effect must be incorporated into the analyses of such seismic data.