Shear‐wave velocity and Poisson's ratio structure of the upper lithosphere in southwest Germany

Abstract

Knowledge of shear‐wave velocity structure, when used with compressional‐wave velocity, can help constrain interpretations of composition from seismic refraction results. We present shear‐velocity (vs) models from high‐quality S‐wave data from Southwest Germany, which combine with existing P‐wave velocity (vp) models to give Poisson's ratio in the crust and uppermost mantle. In general, Poisson's ratio is 0.25 in the uppermost crust, 0.22‐0.23 in the mid‐crust, and varies laterally from 0.24‐0.30 in the lower crust; the average Poisson’s ratio for the whole crust is everywhere about 0.25. The laminated signature of the lower crust prominent on reflection and P‐wave refraction data is absent on the S‐wave refraction data, suggesting strong vertical variations of Poisson's ratio within the lower crust. No shear‐wave refractions from the upper mantle are present, even where strong P‐wave refractions are observed, possibly indicating an increase of Poisson's ratio with depth in the upper mantle. We interpret these results to indicate a high quartz content or fluid phases at low pore pressure in the mid‐crust, a high feldspar content in the lower crust, and an increase in depletion of basaltic components with depth in the upper mantle.