HR: 0830h
AN: S31C-0783    [PDF]
TI: Variations in S-Wave Splitting Across an Array in the Fore-arc Region of the Hikurangi Subduction Zone, New Zealand
AU: * Marson-Pidgeon, K
EM: katrina.marson-pidgeon@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, 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, Box 600, Wellington, 6015 New Zealand
AU: Gledhill, K
EM: k.gledhill@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, Box 30368, Wellington, 6000 New Zealand
AB: We observe significant variations in shear wave splitting parameters using teleseismic S and regional ScS waves recorded on an L-shaped array of nine stations (with 10 km spacing) situated above the fore-arc region of the Hikurangi subduction zone, New Zealand. This is in marked contrast to a previous study which found no variation in average SKS splitting values at a network of nine stations with a spacing of 100 km in the same area. Two of the stations were co-located with stations in the array we are using. In a previous study using local earthquakes recorded at the array, station to station variations in splitting were seen, suggesting variations in near surface anisotropy. SKS splitting was also previously studied at the array, but the seismograms were stacked to try to improve the signal to noise ratio, which meant that it was not possible to study any variation in splitting. In this study we have used S and ScS phases, whose frequencies are intermediate between local and SKS events and we are able to obtain high quality splitting measurements at the individual stations without stacking. In general, significantly higher delay times are observed at the three stations on the down-dip arm compared to the other six stations aligned sub-parallel to the strike of subduction. For one of the ScS events (with a wavelength of around 10 km) the delay times change from 1 $\pm$ 0.1 s to 1.5 $\pm$ 0.1 s over a distance of 26 km. The fast polarisation directions also vary between stations but there is no clear trend, and there is evidence for variations in the splitting parameters obtained from different events. The variation in splitting seen across the array suggests that the anisotropy can not be on the source side. The S and ScS waves have similar raypaths beneath the stations as SKS but have higher frequency content. This suggest there is a frequency dependence in the splitting parameters, which is inconsistent with standard models of olivine LPO causing the anisotropy.
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8150 Plate boundary--general (3040)
SC: Seismology [S]
MN: 2003 Fall Meeting