Prediction of the present-day stress field of Germany by a new 3D geomechanical-numerical model

The present-day crustal stress state is a key parameter for the assessment of a potential siting region for a deep geological repository (DGR) of radioactive waste. It is also crucial for the DGR design and the evaluation of its long-term safety. Therefore, a three-dimensional description of the stress state, including both orientations and magnitudes, is required not only for the repository host rock but also for the underlying and overlying formations that act as additional geological barriers.

For the site selection process in Germany, we present results from an updated large-scale 3D geomechanical-numerical model developed within the SpannEnD project. For the model calibration, we employ a new compilation of data records of the orientation of the maximum horizontal stress S_Hmax as well as stress magnitude data of S_Hmax and the minimum horizontal stress S_hmin. The model geometry is based on a new geological model of Germany and comprises 50 individually parameterized units. We assume linear elasticity and assume that the stress state is a superposition of gravitational volume forces and surface forces related to plate tectonics. The resulting partial differential equations describing the force equilibrium are solved numerically using the finite element method. The model consists of approximately 10⁷ hexahedral finite elements allowing a vertical resolution of ~50 m within the uppermost 5 km of the model.

To avoid overrepresentation of data clusters, we compare our modeled stress orientations with estimates of the mean S_Hmax orientation on a regular grid using the data records from the new WSM release 2025. The model results show good agreement with the mean S_Hmax orientation with a mean of the absolute differences of ~10°. Furthermore, our model results indicate an improved prediction of S_hmin in comparison to previous models with a mean of the absolute stress differences with regard to the calibration data records of 2.5 MPa. The modeled S_Hmax magnitudes exhibit larger deviations from the calibration data with a mean of the absolute differences of 7.5 MPa. These discrepancies are probably attributed to uncertainties associated with the common derivation of S_Hmax magnitudes from the S_hmin data. To further improve the reliability of the model results, additional reliable data records of the S_Hmax magnitude are required as they currently represent the largest source of uncertainty in the model results.