Detection and Delineation of a Fracture Zone with Observation of Seismic Shear Wave Anisotropy in the Upper Rhine Graben, SW Germany

TLDR: In this article, local seismic shear wave anisotropy is studied with recordings of microearthquakes near Landau and Insheim in the central Upper Rhine Graben, SW Germany.

Abstract: Local seismic shear wave anisotropy is studied with recordings of microearthquakes near Landau and Insheim in the central Upper Rhine Graben, SW Germany. Although the recordings have a low signal-to-noise level and there is a complex heterogeneous 3-D tectonic structure, a time separation δt between horizontally polarised SH-waves and vertically polarised SV-waves can be observed in seismograms and particle motion diagrams. The observations can be explained by azimuthal anisotropy in the upper crust with a direction φ 0 = 155° east of north for the fast polarisation direction of SV-waves. A gradient of time separation with distance x of δt/x ~ 10 ms/km can explain the data. This model can be interpreted with a classical scenario of fluid-filled (sub-)vertical cracks with a preferred NNW-SSE orientation. Known faults strike NNW-SSE around Landau and Insheim and the seismicity pattern is also oriented in this direction. This direction coincides with the regional orientation of the maximum horizontal stress (σ H ), and fluids apparently exist at depth as known from geothermal water extraction. Furthermore, we find that 3-D seismic velocity heterogeneities have a much larger influence on the precision of the microearthquake location than the anisotropy effect in this complex tectonic region. This is obvious from the up to five times larger travel time residuals (maximum −0.5 s and +0.35 s), which are used as station corrections during the location procedure, compared to the anisotropic δt observations (maximum 0.1 s).