Compressional and Shear-Wave Velocity Models of the Schwarzwald Derived from Seismic Refraction Data

Abstract

To investigate the structure, physical properties and the composition of the crust beneath the Black Forest, southwest Germany, deep seismic sounding experiments were carried out in 1984. They comprised seismic refraction as well as seismic reflection measurements. This paper presents a summary of investigations concerning the Black Forest seismic refraction P- and S-wave three-component data, which were recoreded along a 240-km-long line, following the morphological axis of the Black Forest in a N-S direction. The data underscore the importance of joint P- and S-wave interpretations. Contrary to the common assumption, the reflectivity of the crust for P- and S-waves may differ significantly. The most surprising observations in the Black Forest data come from the middle and lower crust. Whereas the P-velocity model shows a distinct decrease in the middle crust (velocities of about 5.4 km/s are observed), the corresponding S-velocities show no decrease. The result is a low Poisson’s ratio of 0.15 for the middle crust. It is interpreted as an area of fractured rocks with either dry cracks or fluid filled cracks at low pore pressure or both together. The reflected P-wavefield from the lower crust is characterized by long reverberating wavetrains followed by the reflection from the crust-mantle boundary (Moho). The Moho-reflection is also clearly visible in the S-wave seismograms; however, there does not exist a shear-wave response from the lower crust, which is comparable to that for the P-waves. The reflectivity of the lower crust is different for seismic P- and S-waves: the S-wave velocity distribution varies smoothly with depth, whereas the P-wave velocity distribution is laminated (as is, therefore, Poisson’s ratio). The high P-wave velocity layers are of mafic composition, while the low P-wave velocity layers may have a higher quartz content. This supports the interpretation of the lower-crustal reflections as intrusions (underplating) in a previously more homogeneous, less mafic crust. A clear converted wave from the crust-mantle boundary is observed, indicating that the Moho along the eastern shoulder of the Rhinegraben rift is, at least locally, a first order discontinuity.