Simultaneous Inversion For 3-D Crustal Structure and Hypocentres Including Direct, Refracted and Reflected Phases-Iii. Application to the Southern Rhine Graben Seismic Region, Germany

Abstract

SUMMARY The new technique of simultaneous inversion for hypocentres and crustal structure (SSH) is applied to the southern Rhine Graben. P and S wave first and later arrival times of direct, refracted and reflected phases of 350 seismic events are used in the study. Substantial improvements of over 100 per cent in the non-linear residual fit are obtained for the 3-D and less for the 1-D models. Many of the structural results of the 3-D SSH models are consistent with major findings from other seismic refraction experiments and fit into the general geological, petrological and geodynamica1 frame of this area. Thus the thick sedimentary layer in the Rhine Graben proper manifests itself by large negative seismic velocity anomalies. For the middle crust the SSH models show a strong seismic differentiation between sections of the graben proper and the areas under its western and eastern shoulders of the Vosges and the Black Forest, respectively. Using a new a priori Bayesian inversion technique it has been attempted to substantiate the refraction-seismic low velocity zone of Gajewski & Prodehl (1987) in the middle crust under the Black Forest. The results demonstrate that such a velocity low is compatible with the observed traveltimes only if one assumes it as a more local crustal phenomenon in this area. In the lower crust the SSH models are able to clarify some controversies from earlier refraction experiments in the region. The models reveal many seismic features which are typical for continental rift zones such as high transitional velocity anomalies (-7.4 km s-l) under parts of the graben proper (‘rift-cushion’). In the upper mantle, the models show a ring-like high velocity structure of 8.30-8.40 km s-l, centred around a low velocity area (-7.9 km s-l) beneath the Kaiserstuhl. I conjecture this to be a ‘thermal fingerprint’ of a partially molten upwelling asthenospheric diapir bulging under the Moho. The simultaneously relocated hypocentres for the 1-D models show some systematic downward movements for the events in the lower crust of the southern Black Forest area, but otherwise do not differ too much from the initial ones to alter the present seismotectonic picture of the southern Rhine Graben. Thus the hypothesized phenomenon of the seismic gap along the southern section of the Graben Black Forest wedge can both structurally and seismically be substantiated. The relocated hypocentres suggest optically even a widening of this seismic gap. The later has thus to be accepted as a, possibly more general, characteristic of a graben formation.