Constraining Anisotropy under the South Island of New Zealand
Abstract
Due to its well-understood tectonic history, New Zealand is an ideal location to investigate strain at depth through seismic anisotropy studies; however, previous investigations have resulted in conflicting interpretations. Central regions of the South Island, as well as its northern end, exhibit fast axis orientations nearly parallel to the Alpine Fault with magnitudes ranging between one and two seconds, independent of distance from the fault. The nearly fault-parallel splitting directions were considered to either represent lithospheric shearing or asthenospheric flow. In order to discriminate between these two proposed mechanisms for anisotropic generation, it is necessary to expand the current array of seismic stations. We report on investigations of shear-wave splitting measurements of waveforms obtained from ocean bottom seismometers (OBS) deployed off the South Island with approximately 100 km spacing. Despite obvious disadvantages of installing seismometers on the ocean floor, the installation of the OBS allowed us to study the extent of anisotropy across a distance approximately five times greater than the width of the South Island. Preliminary results reveal one event with clear SKS phases and another with SKKS phases on a number of stations. While a few of these yield null splitting results, fast orientations off the east coast tend to be near perpendicular to the Alpine fault, whereas the east coast exhibits fast orientations more parallel to the fault. If similar splitting measurements are made for more events, these results suggest that asthenospheric flow cannot account for the generation of anisotropy in the mantle under the South Island.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMDI41A2066Z
- Keywords:
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- 7208 SEISMOLOGY / Mantle;
- 7299 SEISMOLOGY / General or miscellaneous