Anisotropic measurements in the Rhinegraben area and the French Massif central: Geodynamic implications

As part of an integrated seismic study, polarization of shear waves has been analyzed for teleseismic events recorded at a set of permanent broadband, semi-permanent long-and short-period and temporary short-period seismological stations located in two geodynamically important areas in western Europe, namely the Rhinegraben-Urach area and the French Massif Central volcanic field. While for the semi-permanent and the permanent stations there is a good azimuthal coverage of teleseismic earthquakes which allowed us to investigate the azimuthal dependence and the spatial variation over short distances of an anisotropy direction, no even azimuthal distribution of teleseismic recordings with a clear elliptical (or linear) polarization of the S phases could be obtained in the case of the temporary stations. While the mean values of the splitting parameters phi and delta t are geographically coherent for adjacent stations, our results show a large scatter of the individual splitting parameters for the set of events used. The magnitude of the splitting time suggests that the deformation extends below the lithosphere and that the thickness of the anisotropic structure is at least 100-200 km. For some stations located in the Rhinegraben-Urach area (ECH, RG-N, RG-S, RBG), the variations of phi are consistent with a two-layer anisotropic model as suggested by VINNIK et al. (1994) for the South German Triangle. For the stations ECH (Vosges mountains), RG-N and RG-S (Rhinegraben proper), the resulting estimates of fast direction are around N10 degrees E-N30 degrees E and N80 degrees E-N100 degrees E for the upper and lower layers, respectively. For the station RBG (Urach), the results are N60 degrees E-N70 degrees E and N125 degrees E-N135 degrees E, respectively. In the Rhinegraben-Urach area, the estimates of the effective fast direction for a one-layer model show a rotation from a graben-related (30 degrees) pattern to an Alpine belt-related pattern in the eastern part (approximate to E-W). In the French Massif Central region, the results reveal two distinct fast polarization patterns. While to the west of the Sillon Houiller, phi is parallel to this late-variscan transformlike fault zone and perpendicular to the variscan belt, it is to the east rather perpendicular to the Alpine belt. The results suggest a mixture of both a lithospheric and an asthenospheric component of the seismic anisotropy for the Rhinegraben-Urach as well as for the French Massif Central areas.