Azimuthal anisotropy in the upper mantle from observations of P-to-S converted phases: application to southeast Australia

Converted-waves analysis is used to map the anisotropy as a function of depth in the upper mantle. Stacking of SV- and T-component records of several seismic events well distributed in azimuth helps to detect these converted waves. Weighting coefficients are used to extract the anisotropic signal in the records. Synthetic examples show that a harmonic analysis helps to separate azimuthal anisotropy effects from the effects of lateral heterogeneity, such as those resulting from dipping structures. A good azimuthal coverage of the data set is important in order to separate the different effects in the stack sections obtained for different azimuthal orders. The procedure is applied to data observed at GEOSCOPE station CAN and gives a model with two anisotropic layers in the upper mantle. The upper layer, located between the crust and a depth of 140 km, has its fast axis in an east-west direction. The lower layer, 40 km thick, has its fast axis in a north-south direction. The fast axis directions are in agreement with results obtained from surface-wave analysis and tectonic information.