Azimuthal anisotropy in the upper mantle from observations ofP-to-Sconverted phases: application to southeast Australia

Abstract

SUMMARY Converted-waves analysis is used to map the anisotropy as a function of depth in the upper mantle. Stacking of SV -a ndT-component records of several seismic events well distributed in azimuth helps to detect these converted waves.Weighting coe⁄cients are used to extract the anisotropic signal in the records. Synthetic examples show that a harmonic analysis helps to separate azimuthal anisotropy eiects from the eiects of lateral heterogeneity, such as those resultingfrom dipping structures. A good azimuthal coverage of the data set is important in order to separate the diierent eiects in the stack sections obtained for diierent 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 of140 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.