3-D modelling of the crustalS-wave velocity structure from active source data: application to the Eastern Alps and the Bohemian Massif

Abstract

SUMMARY Here, I present results from 3-D modelling of crustal S-wave velocities in the eastern part of the Eastern and Southern Alps and the southern part of the Bohemian Massif. Data were acquired between 2000 and 2003 during three large 3-D wide-angle reflection and refraction experiments. The surveys aimed at the P-wave velocity structure of the crust and uppermost mantle, and therefore only vertical-component geophones were used. Nevertheless, many shot gathers show strongS waves, in particular refractions from the crust (Sg waves) and reflections from the Moho discontinuity (SmS waves). Compared to P-wave arrivals, the traveltimes of S waves can only be picked with a relatively large uncertainty. Furthermore, approximately 80 per cent of the data represent crossline recordings on which it is more difficult to assign certain phases to the arrivals. For these reasons, I apply signal detection and stacking routines to put the 3-D data set to best possible use. Concerning the crust, a smooth 3-D velocity model is based on refractions from within the crust (Sg), whereas the crust–mantle boundary is modelled by reflections (SmS) from the Moho discontinuity. Considering that the projects were designed for P-wave studies, the proposed methodology provides surprisingly detailed images. S-wave velocities and Poisson’s ratio correlate with tectonic units and show significant features in the upper crust. A high Poisson’s ratio is observed in the Moravian part of the Bohemian Massif, in the Southern Alps and in the Northern Calcareous Alps and their basement within the Eastern Alps. The middle and lower crust are only covered in the Moldanubian part of the Bohemian Massif, where Poisson’s ratio is low. This would favour a felsic composition. The evaluation of SmS waves is in accordance with previously proposed P-wave velocity models, which indicate a fragmentation into three crustal blocks in the centre of the Eastern Alps.