Laterally and Vertically Varying Seismic Anisotropy in the Upper Mantle below Central Europe

The observations of (no) shear-wave splitting of core-refracted S-phases, mainly SK(K)S-phases, at (semi-) permanent broadband seismological recording stations in the Upper Rhine Graben (URG) area, Central Europe, show backazimuthal and small-scale lateral variations (inter-station distance 10 km to 80 km). Our analysis of long-term recordings (partly > 20 yrs.) allows us to study a wide backazimuthal coverage and to appropriately resolve and account for complex anisotropy. The observed variations of the splitting parameters, the fast polarization direction relative to north, , and the delay time accumulated between the two quasi shear waves, t, indicate small-scale lateral and partly vertical variations of the seismic anisotropy at depth within a regional extension of ca. 200 km by 200 km. We tested structural anisotropy models with one layer with horizontal axis as well as with tilted symmetry axis and two layers with horizontal symmetry axes (assuming transverse isotropy with the fast axis being parallel to the symmetry axis). Differences are found between the east side and the west side of the URG and between the Moldanubian Zone in the south and the Saxothuringian Zone in the north. Inconsistent splitting parameters and the observation of numerous null splits, especially below the URG may be at least partly related to scattering of the seismic wavefield or a modification of the mantle material. It seems that so far proposed large-scale deformation pattern, e. g. asthenospheric flow, may be a too simple explanation for SKS-splitting observations in our region.