SpannEnD 2.0 – New insights into the present-day stress of Germany by a new 3D geomechanical-numerical model

For prediction of the short and long-term geomechanical behaviour of a deep geological repository for nuclear waste, the present-day 3D in-situ stress is a key parameter. However, in Germany the knowledge concerning the crustal stress state is still quite low. It is mainly based on data from the World Stress Map (WSM) project providing data of the orientation of the maximum horizontal stress (S_Hmax) and a new compilation of stress magnitude data providing magnitudes of S_Hmax and the minimum horizontal stress (S_hmin). However, these two databases still provide only unequally distributed data records and in particular horizontal stress magnitude data records are only reliable at a dozen locations all over Germany. Thus, for an in-situ stress field prediction geomechanical-numerical models - calibrated on available horizontal stress magnitudes – are used. They enable a continuum-mechanics based description of the 3D present-day stress state and can resolve lateral and especially vertical variations. Two 3D geomechanical-numerical models of Germany have been published during the initial phase of the SpannEnD project (2018-2022). In the follow-up project SpannEnD 2.0 a new model has been set-up based on a new geological model enabling new insights into present-day crustal stress field of Germany, in particular due to higher vertical resolution. We also use a significantly enlarged stress magnitude database for model calibration.

The new 3D geomechanical-numerical model combines information of 27 regional geological models and comprehensive additional data. It comprises 49 geological units parametrized with elastic rock properties (Young’s modulus and Poisson’s ratio) and rock densities. Linear elasticity is assumed and the finite element method (FEM) is used to solve the partial differential equations that describe the equilibrium of gravitational and surface forces. Overall, the model contains ~10 million hexahedral elements providing a lateral resolution of 4 km and a vertical resolution of up to 45 m in the uppermost 5 km. The model results show an overall good fit with stress magnitudes used for calibration indicated by a mean of the absolute stress differences of ~3 MPa for S_hmin and of ~5 MPa for S_Hmax.  Furthermore, the results agree well with additional data sets - not used for calibration - e.g., an absolute mean deviation of the orientation of S_Hmax with regard to WSM data of ~10°.