Resolving upper mantle anisotropy in 3-D: A case study in the Upper Rhine Graben area, Central Europe

Abstract The analysis of long-term data (15-25 yr) regarding shear wave splitting (SWS) of SKS, SKKS, PKS (XKS) phases at six (semi-)permanent broadband seismological recording stations (BFO, WLS, STU, ECH, TMO44, TMO07) indicates vertical and small-lateral variations of the seismic anisotropy underneath the Upper Rhine Graben (URG) area.In this study, the two splitting parameters, the fast polarization direction Φ and the delay time δt, are measured, and show characteristic backazimuthal variations. Additionally, the splitting intensity is calculated. SKS-SKKS phase pairs show no significant discrepancies and therefore do not indicate a significant contribution from the lowermost mantle. Thus, the observed anisotropy is assumed to be located in the upper mantle (lithosphere and asthenosphere). To explain the SWS measurements, structural anisotropy models with one layer with a horizontal (simple anisotropy) as well as a tilted symmetry axis and two layers with horizontal symmetry axes (complex anisotropy) are tested in a modeling procedure based on forward-calculated synthetic splitting parameters. Besides fitting the whole backazimuth (BAZ) range, the modeling is limited to station-specific BAZ ranges. The consideration of differences and similarities in the splitting pattern across the recording stations allows to outline subregions with different anisotropy regimes and by this to propose a structural 3-D model of the anisotropy in the upper mantle underneath the URG: Differences are found between the east and the west sides of the URG (change of the fast polarization directions) and between the Moldanubian Zone in the south and the Saxothuringian Zone in the north (change from two layers to one layer). Inconsistent splitting parameters and the observation of numerous nulls (no SWS), especially underneath the southern URG in the southwest of BFO, may be at least partly related to scattering of the seismic wavefield or a modification of the mantle material. Additional materials Talk mentioned in the section "Modeling: strucutral models": https://doi.org/10.5281/zenodo.18524089