AZIMUTHAL ANISOTROPY IN THE EARTH FROM OBSERVATIONS OF SKS AT GEOSCOPE AND NARS BROADBAND STATIONS

Abstract

Abstract We have analyzed observations of low-frequency SKS waves recorded on horizontal components at several broadband stations of the GEOSCOPE and NARS arrays. Splitting of SKS is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis, in the upper mantle beneath the receivers. We have designed an inversion method, which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi-shear waves. In general, the directions of fast velocity obtained, both for stations around the Atlantic and around the Pacific, appear to be related to directions of past and present plate motions.