The Upper Rhine Graben (URG) is the central part of the Central European Cenozoic Rift System (ECRIS) and one of the tectonically active regions in Europe. The URG extends from the Jura thrust and fold belt in the South near Basle city (Switzerland) to the Rhenish Massif in the North near Frankfurt and is limited by sinistral strike-slip faults on either sides. Several studies suggested particularly the fault system at the eastern margin to be active, as the Basle earthquake 1356 AD was situated there. Ongoing earthquake activity testifies to active faulting, however, there are many faults and fault strands, many of them are regarded as normal faults, others as reactivated normal faults with a sinistral sense of movement. This is due to a major change in stress direction from SW-NE towards a SE-NW direction in Miocene times. The URG is a low-strain setting with long recurrence intervals of large earthquakes. Moreover, the fault morphological signal is perturbed by anthropogenic land-use, the climate of the area, which is located in the temperate zone, and the erosion and sedimentation of the Rhine River. The eastern margin faults lack any neotectonic and paleoseismological investigations, in contrast to the western border faults that have been examined during the last decades, while the area encompasses critical facilities in a vulnerable region such as, dense population, agriculture, mining, geothermal facilities. Our projects in the frame of an IRSN study and the DFG-SPP AlpArray aim to fill this gap of knowledge in large and infrequent earthquakes, through a paleoseismological investigation of eastern side faults.
We use a multidisciplinary approach to improve the input data of seismotectonic models, which include faults and their activity potential in the calculation of seismic hazard assessment and are based on a weak data set at the moment. First results of different locations around Freiburg in the southern URG and around Karlsruhe are presented: the Rhine River fault system about 20 km SW of Freiburg, NE of Freiburg at the eastern border fault system about 10 km. At both sites we used a digital elevation model (DEM) derived from LiDAR-data (5x5m) and together with geophysical measurements performed with ground-penetrating radar (GPR) and electrical resistivity tomography in order to track and identify possible fault scarps. A detailed geomorphological study of the DEM was performed to identify and map superficial expressions of the neotectonic activity of the faults and deformed and offset alluvial terraces and fans. We apply ERT and GPR for imaging the geophysical contrasts at depth, such as faults and stratigraphy in detail. Nevertheless, for the unambiguous verification of these structures and the determination of key fault parameters, such as magnitude, age of last events, slip rate and return periods, additional paleoseismological trenches are needed.