SV-wave azimuthal anisotropy in the Australian upper mantle:: preliminary results from automated Rayleigh waveform inversion

The pattern of azimuthal anisotropy for SV waves in the upper mantle beneath Australia has been determined using a two-stage tomographic procedure for Rayleigh waves. The inversion exploits an automated procedure based on the waveform inversion method of Cara & Leveque (1987) to allow the analysis of 668 vertical-component seismograms within a few weeks. The automated procedure provides an effective choice of the secondary observables used in the waveform inversion for both fundamental and higher modes, allowing the construction of a set of radially stratified upper-mantle models which represent the average structures on paths criss-crossing the Australian continent. The 668 path-average models are then combined in a 3-D velocity model which describes the lateral variation of the SV velocity and its azimuthal anisotropy. The lateral variations in wave speed show a good overall agreement with previous results for shear wave-speed variations in Australia but are accompanied by significant azimuthal anisotropy. At 150 km depth the anisotropy displays a simple pattern with most of the directions of fast SV velocities dominated by a north-south component close to the present-day absolute plate motion. Nevertheless, in the regions with the highest shear wave speed, anisotropy is not aligned with the current plate motion, suggesting a local resistance to mantle flow.