Revisiting Stress and Strain Fields in Switzerland with Focus on the South-Western Alps

Abstract

The state of stress of a region plays an important role in seismic hazard assessment. Especially the evaluation of the potential of reactivating pre-existing faults is thus a crucial component of the hazard assessment of natural and anthropogenic seismicity. In addition, mapping the stress field also helps to understand regional tectonic processes. A common procedure to assess the state of stress and its regional variations is the analysis of earthquake focal mechanisms (FMs). Here, we aim to update the regional stress field of Switzerland and improve its spatial resolution as well as the understanding of the Rawil Fault Zone (RFZ). This goal is achieved by performing stress inversions of focal mechanism solutions calculated by the Swiss Seismological Service (SED). For this study, the existing FM catalog of the SED was updated and the final catalog contains 471 FMs, which represents a significant improvement over previous studies in Switzerland. The stress inversions are performed with the MATLAB package MSATSI by Marinez-Garzon et al. (2014). We grouped the data with different methods in order to get the best possible spatial resolution, while maintaining the stability of the inversions. In many places we achieved a higher spatial resolution. However, the focal mechanisms are quite clustered and this required a zonation approach in order to get meaningful results. The resulting stress field describes a stress regime dominated by strike-slip faulting with a tendency to a transtentional stress field and a normal faulting stress field south of the Penninic Thrust in the internal parts of the Alps. The regional stress field shows a tendency of counterclockwise rotation of the P- and T- axis and a significant change of orientation between the mean stress field in the foreland and the Alpine belt. The ongoing indentation of the Adriatic Plate into the European Plate and the lithosphere delamination and slab rollback mechanism are tectonic processes influencing the stress field in Switzerland. An in-depth analysis of RFZ’s stress field gives an insight into the complexity of the RFZ with strike-slip mechanisms in the crystalline basement and predominantly transtensional tendencies in the shallow parts and the sedimentary cover. Interactions between the stress field in the basement and the pre-existing faults in the sedimentary layer might explain the complexity of the RFZ.