Détermination des variations latérales de la lithosphère par l'analyse des ondes de surface enregistrées par des réseaux régionaux
Surface waves of intermediate period (10-100 seconds) are sensitive to the rigidity of the lithosphere. In this work, we studied the lateral variations of the lithosphere by analysis of surface waves. The wavelength of the studied lateral variations is decakilometric. Considering the improved spatial coverage of broad band stations and the recent availability of portable stations, usual methods based on seismological stations hundreds of kilometres apart are no longer satisfactory. This is particularly valid in strongly heterogeneous areas. We therefore developed methods for analysing surface recorded by dense or regional alTays to study propagation anomalies and lateral variations of the lithospheric structure. We applied these array analysis in three different areas. Firstly, we showed the presence of strong, localised lateral variations of the crustal structure across the Tsangpo suture, in the southermost part of the tibetan plateau. For this, we analysed the local phase velocity as well as the amplitude variation across the suture. North of the Tsangpo suture, we identified a low velocity layer in the lower crust while such a layer was not present south of the suture. Secondly, we studied the lithosphere in the French Alps and we showed that it thickens from 85 km beneath the western part to up to 200 km beneath the axis of the range. We also studied the propagation anomalies of the surface waves in this area using a dense array. Strong propagation anomalies were found and interpreted using ray tracing and numerical simulations. Finally, we analysed surface waves in the area around the Sorgenfrei-Tornquist zone which separates Phanerozoic Europe (west) to Paleozoic Europe (east). We showed that the lithosphere is thin (50-100 km) in the westernmost part, thick in the easternmost part (up to 200 km), and intermediate (120 km) within the lateral transition.