Seismic anisotropy within the uppermost mantle of southern Germany

Abstract

This paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the Pn phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the Pn phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P-wave anisotropy of 3-4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 km s-1 at a depth of 30 km a direction of N31°E was obtained. The azimuthal dependence of the observed Pn amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06s-1 in the fast direction to 0 s-1 in the slow direction of horizontal P-wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived.