The recent stress state of Germany – results of a geomechanical–numerical 3D model

Abstract

Abstract. A decisive criterion for the selection and the long-term safety of a deep geological repository for high radioactive waste is the crustal stress state and its future changes. The basis of any prognosis is the recent crustal stress state, but the state of knowledge in Germany is quite low in this respect. There are stress orientation data provided by the World Stress Map (WSM, Heidbach et al., 2018) and stress magnitude data from a database (Morawietz et al., 2020) for Germany, both providing selective information on the recent stress field. However, these data are often incomplete, of low quality and spatially unevenly distributed. Therefore, a 3D continuous description is not possible with these data so far, at most for the orientation of the maximum horizontal stress (SHmax), but not for the most important magnitudes of the minimum (Shmin) and SHmax. In the course of the SpannEnD project, a geomechanical–numerical 3D model of Germany is created, with which a continuous description of the complete tensor of the recent stress field in Germany is possible. The model covers an area of 1250×1000 km2 from Poland in the east, to France in the west, from Italy in the south to Scandinavia in the north. The depth extent is 100 km. Even though the focus is primarily on Germany, the model area was chosen to be so wide to minimize boundary effects and for a simplified definition of the displacement boundary conditions, which are ideally oriented perpendicular or parallel to the orientation of SHmax. The model contains a total of 21 units: The upper part of the lithospheric mantle, the lower crust, four laterally overlapping units of the upper crust, and 14 stratigraphic units of the sedimentary cover. The stratigraphic subdivision of the sedimentary cover is only done in the core area of the model; because this area is the focus of our study, our calibration data are mainly from this region and well-resolved geometry data are available. Outside of the core area, the sediments are grouped into an undifferentiated unit. The units are parameterized with density and elastic material parameters (Poisson's ratio and Young's modulus). The model has a lateral resolution of 2.5×2.5 km2 and a vertical resolution of a maximum of 240 m; in total it includes 11.1 million hexahedral elements. The equilibrium of forces between body and surface forces is solved by finite element method. The model is calibrated with Shmin and SHmax magnitudes from the WSM and data from the stress magnitude database. First, an initial stress state is generated and in a second step displacement boundary conditions are defined at the model edges, which are adjusted until a best-fit to the calibration data is found. The results show good agreement with both the SHmax orientation data from the WSM and the magnitudes of the two principal horizontal stresses (Shmin and SHmax) from the magnitude database.