Observations of laterally inhomogeneous anisotropy in the continental lithosphere

Abstract

Elastic anisotropy may be an important factor when seismic data are used to determine the structure, composition and dynamics of the Earth's interior^1,2. Azimuthal anisotropy has been found in the oceanic and, although less definitely, in the continental lithosphere from field experiments where the P_n-wave velocity was observed to vary with direction^3–5. These studies suggest a 180° periodicity in travel-time variations as the only criterion of the presence of azimuthal anisotropy. This criterion, however, may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium. A new method for measuring azimuthal anisotropy has been described recently⁶, based on the phenomenon of shear-wave splitting in anisotropic media, which suggests three criteria instead of one to identify azimuthal anisotropy. We have now applied this method to the records of permanent seismograph stations in southern Germany. Our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km.