New Zealand Journal of Geology and Geophysics abstracts

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

DAVID OKAYA

Department of Earth Sciences
University of Southern California
Los Angeles, CA 90089-0740, U.S.A.

NIKOLAS CHRISTENSEN
DARRELL STANLEY

Department of Earth & Atmospheric Sciences
Purdue University
West Lafayette, IN 47907, U.S.A.

TIM STERN

Research School of Earth Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand

and South Island Geophysical Transect (SIGHT)

Working Group

Abstract  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 drill-core 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.

Keywords  petrophysical measurements; Alpine Fault Zone; seismic anisotropy; shear-wave splitting

New Zealand Journal of Geology and Geophysics, 1995, Vol. 38: 579-583

0028-8306/95/3804-0579 $2.50/0 (c) The Royal Society of New Zealand 1995

PDF file of entire paper: medium quality (500K); (scanned from paper original: notes about this process)

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