Simultaneous Inversion For 3‐D Crustal Structure and Hypocentres Including Direct, Refracted and Reflected Phases‐Ii. Application to the Northern Rhine Graben/Rhenish Massif Region, Germany

Summary The new technique of simultaneous inversion for hypocentres and crustal structure (SSH) using direct, refracted and reflected crustal phases is applied to the northern Rhine Graben/Rhenish Massif region. P ‐ and S ‐wave arrival times of about 100 seismic events are inverted for locations and crustal structure. the issue of the coupling between crustal structure and hypocentre locations is investigated by comparing SSH models where the hypocentres have been fixed a priori (pure velocity inversion) with ‘full’ SSH inversion models. Although this computationally expedient variant of SSH leads to smaller reductions in the residuals than the corresponding ‘full’ SSH, their features compare surprisingly well. Improvements of over 100 per cent in the non‐linear residual fit are obtained for the P ‐wave 3‐D models and somewhat less when S phases are included which hints of variations in the assumed v p /v s ratio. Some of the structural results corroborate findings from previous deep‐seismic‐sounding experiments while others deliver new information on the petrology and tectonics of this region. For the uppermost crustal layer the SSH models show positive velocity anomalies that reflect the gneiss and granite basements found under most of the stations. the seismic velocities of the 3‐D models for the Odenwald suggest a continuation of the gneiss outcrops down to the middle crust. Both the 1‐D model and sections of the 3‐D models reveal a small velocity decrease in the middle crust, clarifying an ambiguity of earlier refraction models. the lowermost crustal layer of the 3‐D model is characterized by large positive anomalies ( u p = 7.3‐7.4 km s ‐1 ) that are interpreted as a crust‐mantle transition zone. For the upper mantle consistently high P n velocities ∼8.4 km s ‐1 are found for both the 1‐D and 3‐D models. the structural results under the graben proper suggest the absence of many of the thermo‐mechanical characteristics of a typical rift zone in the northern Rhine Graben. the simultaneously relocated hypocentres manifest some dependency upon the parametrization of the crustal model. Although the hypocentral movements are generally larger for the 1‐D than for the 3‐D SSH models, they are not sufficiently large to warrant a reinterpretation of the seismotectonics of the region. Overall the SSH results give evidence for a local correlation between decreased crustal velocity and reduced seismic activity in the middle and lower crust. This appears to reflect some general seismo‐, thermo‐mechanical and rheological concepts which postulate a ductile, aseismic flow regime for various anomalous portions of the middle and lower crust.