Structure of the uppermost mantle from long-range seismic observations in southern Germany and the Rhinegraben area

The crustal structure has been determined in the area between the Lorraine, the Bohemian massif and the northern Alps with considerable detail in recent years. But up to now little has been known about the velocity—depth structure of the uppermost mantle in this area. The situation changed recently when two recent seismic events near the northern and southern end of the Rhinegraben rift system were recorded to distances of 400 km. The explosions at the westernmost shotpoint of the international alpine refraction profile in 1975 were also observed in the Rhinegraben area up to the same distance. Earlier refraction seismic experiments between Steinbrunn near Basle and Boehmischbruck at the western border of the Bohemian massif also reach distances of 400 km. All these data lead to the rather high P-wave velocities of 8.5–8.6 km/s at depths between 40 and 50 km. These velocities are considerably higher than the average velocities of 8.2 km/s under other areas of western and central Europe, as for example the Bretagne in northwestern France and the North German Plain. There are indications of a minor velocity inversion in the uppermost mantle between Steinbrunn and Boehmischbruck. From the dispersion of surface waves there is good evidence that the regional high P-wave velocities are limited to a certain depth range only. This indicates rather pronounced lateral variations of the velocity—depth structure in the uppermost mantle of central Europe.