The SpannEnD project: 3-D stress prediction in the upper crust of Germany
Authors: Reiter Karsten , Ahlers Steffen , Morawietz Sophia , Röckel Luisa , Hergert Tobias , Henk Andreas , Müller Birgit , Heidbach Oliver 
Published: November 2021

Abstract. Assessment of the stability of deep geological repositories is a key task in
the site selection process for high-level radioactive waste. Geomechanical
stability is affected by endogenous and exogenous processes as well as by
geotechnical operations. Stability prediction requires both an estimate of
future stress changes as well as the initial, i.e. current stress
state. However, data records on the current stress state in the upper crust
are incomplete, sparse and spatially unevenly distributed. Therefore,
geomechanical-numerical models are the only way to estimate the full stress
tensor at locations where stress observations are not available. The main components of such a 3-D geomechanical model are the distribution of
the elastic rock properties and rock density as well as stress data for the
model calibration. The aim is to find the optimal initial and boundary
conditions that result in a best-fit with respect to available stress data
within the model volume. For this reason, the first open access database for
stress magnitude data has been developed as an extension of the existing
database on stress orientation data (world-stress-map.org). The new database
contains 568 data records from Germany and surrounding areas, but only 15 % of
these data records are of acceptable quality. Thus, only one reliable set of stress magnitude data is available for an area of 100×100 km2. Based on existing compilations of the crustal structure in and around Germany, data were merged into one model with the aim of estimating the 3-D stress state in Germany. Geomechanical models for stress estimation in Germany were created with varying geometrical and mechanical resolution. While the first model consists of four mechanical units and 1.3 million finite elements, the subsequent model consists of 12 units and 3.6 million finite elements. The results of the best-fit model with respect to the stress data reveal that there are regional differences when calculating the fracture potential, i.e. the distance to failure of intact rock as well as different values of slip tendency, which provides a measure of the reactivation potential of pre-existing faults. The observed variability of the modelled stress field can be used as a first-order assessment. Since model uncertainties are still high, the absolute values are not yet reliable. However, the model can be used to derive consistent initial and boundary conditions for models on a regional scale. Furthermore, it makes it possible to investigate the influence of the large-scale crustal structure on the overall stress pattern. The modelling workflow is set-up in a way that new information and higher resolution if needed can be implemented when more data are provided. This will improve the reliability of both, the model prediction on the large scale as well as the initial and boundary conditions for high-resolution regional models for selected areas during the site selection process.