Bootstrap inversion for vertical and lateral variations of the S wave structure and the υp/υs-ratio from shallow earthquakes in the Rhinegraben seismic zone, Germany

Using S wave arrival times of ~ 400 earthquakes in the southern Rhinegraben, 1-D and 3-D inversions for the shear (S)-wave crustal structure of that region are performed. These models are then combined with previously obtained 1-D and 3-D P wave models to compute vertical and lateral variations of the υp/υs (Poisson's)-ratio. Such an independent P and S wave inversion approach circumvents many of the limitations of the classical Wadati-diagram method usually used for the determination of crustal υp/υs-ratios. For the statistical evaluation of the 1-D models we have used a new Monte-Carlo bootstrap resampling technique which has been found useful in many statistical estimation problems where the underlying statistics of the data is poorly known. The 1-D bootstrap υp/υs crustal models exhibit significant vertical variations. Whereas the upper sedimentary and crystalline layer of the crust is characterized by a significant increase of υp/υs over the reference value, υp/υs in the upper and middle crust is lowered. This correlates well with several findings from Wadati-diagrams for υp/υs in other seismically active zones and might be explained through the gradual closing of saturated microcracks in that depth range. However, the υp/υs-reduction can also be interpreted in terms of a particular petrological composition such as orthoclase feldspar without the indispensable need for fluids. A lowering of υp/gus is also observed for the lower crust, which might reflect the strong lamination found here from previous deep seismic sounding experiments. For the upper mantle a strong increase of υp/υs is observed which is primarily triggered by a decrease of υs, and is best explained by an eclogite petrological assemblage. For the 3-D S wave models significant reductions of more than 100% were obtained for the unexplained travel-time residuals. Variations of υp/υs in the upper and middle crust are mostly induced by changes in υp rather than υs. For the upper crust slight decreases of υp/υs are observed for the Rhinegraben, but increases are detected for the adjacent Vosges and Black Forest areas. In the middle crust the refraction-seismic deduced reduction of υp/υs under parts of the Black Forest can neither be corroborated nor rejected. However, because of some experimental ambiguity on the effects of petrological composition and interstitial pore fluids, this does not rule out the presence of the latter. In the lower crust a local reduction of υp/υs is observed under the Kaiserstuhl and graben-Black Forest shoulder and has been explained in terms of a metamorphic, dehydrating phase change of the hydrous amphibolite facies to the anhydrous granulite facies. Depending on the pore-pressure of the dehydrated water and the fractional amount of interstitial CO2, granulite facies metamorphism occurs in the 650–800°C temperature range. Temperatures at the upper end of this interval can be generated by the mechanism of accretionary magmatic underplating as it has been proposed for some extensional rift zones where asthenospheric upwelling, as in the Rhinegraben, occurs.