title: Geomechanical-numerical modeling of the crustal stress state of Germany
Creators: name: Ahlers, Steffen
Publisher: name: UNSPECIFIED
Year: 2023

description: The stress state in the earth crust is an important quantity for many scientific and technical questions, e.g., seismic hazard assessment, borehole stability and underground storage. However, the level of knowledge about the recent stress field in Germany is still limited. There are basically two larger data sets available: (1) the World Stress Map (WSM) providing mainly information about the orientation of the maximum horizontal stress (SHmax) as well as the stress regime and (2) a stress magnitude database of Germany providing magnitude information about the individual components of the absolute stress tensor. However, the data are sparse, pointwise and unequally distributed. Therefore, a continuous prediction of the recent crustal stress state of Germany by linear interpolation between these data points is not suitable, in particular due to vertical and lateral inhomogeneities, e.g., mechanical properties, or faults leading to stress variations within in the crust. The presented cumulative dissertation is part of the SpannEnD project that aims to enhance the state of knowledge of the stress field in Germany. This dissertation contains three manuscripts: two dealing with the continuous prediction of the recent crustal stress state of Germany by large scale (1000 x 1250 x 100 km3) geomechanical-numerical models and one with the analysis of slip tendency (TS) of faults in Germany using results of one of these models. The two geomechanical-numerical models contain units describing the present geological conditions, which are parameterized with individual rock properties. Linear elasticity is assumed and the Finite Element Method (FEM) is used to solve the equilibrium of forces. The models enable a continuous prediction of the absolute stress state based on continuum mechanics within the upper lithosphere for the entire area of Germany for the first time. The first model presented in this cumulative dissertation contains seven units: a sedimentary unit, four laterally overlapping units of the upper crust, the lower crust and parts of the lithospheric mantle. It is calibrated against magnitudes of the minimum horizontal stress (Shmin) and compared with orientations of SHmax of the WSM and some additional data. The results show an overall good fit to the orientation of SHmax with a mean of the absolute deviations of 15.6° and a median of 5.6° and to the Shmin magnitudes with a mean of the absolute differences of 3.3 MPa used for calibration. However, the SHmax magnitudes show some larger differences especially too low values within the upper part of the model. The second model is an improved version of the first model with focus on a higher stratigraphic resolution of the sedimentary unit, containing 22 units. In combination with an 18-time higher mesh resolution and an additional calibration with SHmax magnitudes the results show an overall good fit to the magnitudes of all principal stresses (Shmin, SHmax and the vertical stress (SV)) and the WSM orientation data. This is indicated by absolute differences of 0.0 MPa for SV, 4.6 MPa for Shmin and 6.4 MPa for SHmax and by a median of 0.3° and absolute differences of 11.9° for the SHmax orientations within the central part. The third manuscript shows a possible application for the results of a large-scale geomechanical-numerical model. It is a TS analysis of faults for Germany using the results of the first model of this study and three different sets of faults with increasing complexities. The analysis show a good spatial agreement between the calculated TS and earthquakes within the study area. However, the fit between the depth of earthquake occurrence and the highest predicted TS show some discrepancies. In general, the study shows the influence of the fault geometry, the fault orientation in relation to the stress field and the crucial influence of the pore pressure. The results of this cumulative dissertation are a step towards a better understanding of the recent stress field of Germany. By two geomechanical models providing - for the first time the 3D stress tensor - and results, which are in good agreement with different calibration and comparison data sets. However, there are still some local and few general deviations that need to be further investigated, for example, with smaller more complex models, for which the stress field of this study can be used as an initial stress state.