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

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 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.

Southern Rhine Graben: small-wavelength tomographic study and implications for the dynamic evolution of the graben

“…In the tentative model of the ECORS-DEKORP working group (Brun et al 1991) a region of decreased lower crustal reflectivity below the eastern half of the graben is interpreted as a detachment zone in the ductile lower crust superimposed on a broad upwarp of the Moho (Edel et al 1975). Compared to the teleseismic model, a graben-related anomaly pattern is less pronounced in the detailed 3-D model of the crust as obtained by the inversion of local events (Koch 1993). This crustal tomography study yielded high velocities of 6.2 to 6.3 km s-l in the middle and lower crust down to 20km below the Black Forest and major parts of the Vosges.…”

Fluid trapping at the brittle–ductile transition re‐examined

“…1987). However, seismicity persists to a depth of at least 20 km (Ebel & Bonjer 1990; Koch 1993). These data from the Black Forest are some of those included in the global compilation of Marquis & Hyndman (1992), in which fluids trapped below the brittle–ductile transition are discussed as a possible mechanism for explaining deep crustal conductors and seismic layering.…”

2-D anisotropicPn-velocity tomography underneath Germany using regional traveltimes

“…In any case, the derived velocity structure in Fig. 11 is seismologically completely unrealistic, since, other than in certain areas of the southern Rhine graben rift zone (Koch 1993b), Pn velocities <7.9 km s −1 , as indicated in Figs 11 (a) and (b) for the upper mantle of southern Germany, have never been reported in related studies. Thus, one may believe that the actual Pn data cannot be explained with such an isotropic model alone, and that anisotropy is needed in addition.…”

“…In a study of the 3‐D inversion of teleseismic traveltimes (Koch 1992) and of local traveltimes for simultaneous structure and hypocentres (Koch 1993a,b), it was demonstrated that a reliable inversion should comprise the concurrent use of several of these ORTs to obtain stable, optimal models. Because of the different optimum criteria used in each of the methods, they usually do not perform equally well in restricting an optimal solution for some of the models.…”