Caractérisation de l'activité paléosismique des failles du Fossé rhénan supérieur
Laboratoire d'accueil : Bureau des risques sismiques pour la sûreté des installations (BERSSIN)
Date de début de thèse : octobre 2016
The Upper Rhine Graben (URG) is the central part of the Central European Rift System and one of the most seismically active regions in Western Europe, north of the Alps. This graben is delimited by faults, which are supposed to be active. The objective of the thesis is the analysis of the fault system activity to figure out its seismotectonic parameters useful to estimate the associated seismic hazard. This is a challenging task because of the numerous critical facilities and highly populated cities. A first scientific challenge is the recognition and characterization of the fault activity in such a highly human-modified landscape and subject to erosional/depositional processes, both competing the growth of fault-controlled relief. To eventually determine the long-term deformation rate and the earthquake history, a key topic is the analysis of the sedimentary record along the faults which might have registered those deformation episodes.
The objectives during the first stages of the thesis, before investigating the sedimentary record, are 1) to analyze in detail the morphology with high-resolution Digital Elevation Models (DEM) and 2) to perform a detailed geophysical survey. The geomorphological analysis provides a first order fault location, its lateral extension and significance: interesting spots have already been found out and guided our first geophysical investigations. These consisted in Ground-Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) soundings which aim at imaging the sedimentary layers geometry and anomalies that could correspond to faults. Proven indicators for faults in GPR profiles are displacement of reflectors or changes in dip angle, reflection angle and pattern or signal intensity. The ERT proofed very useful for the revision and verification of features found in GPR profiles. The GPR has limitations in case of high soil water content and there is a tradeoff between achievable spatial resolution and investigation depth. Several faults described in former studies (e.g. geological map of Baden-Württemberg, GeORG, 2012 and Nivière et a1., 2008) could be verified, but partially with deviations in the exact location, possibly related to accuracy and density of the data sets, as well as the geophysical and interpolation methods that were employed. For defining an exact location for further trenches (planned for 2018), investigations with percussion drilling will be conducted to verify if the GPR and ERT anomalies are fault-related features rather than being caused by erosion.