Spatial Influence of Fault-Related Stress Perturbations in Northern Switzerland

Abstract. The spatial influence of faults on the crustal stress field remains a topic of active debate. While it is well documented that faults often cause perturbations in the stress field at a meter scale, their lateral influence over greater distances, from a few hundred meters to several kilometers, remains poorly understood. This knowledge gap largely results from the lateral resolution limit of stress data. To address this, we use a 3D geomechanical numerical model based on 3D seismic data from northern Switzerland. The model is calibrated with 45 high-quality horizontal stress magnitude data obtained from micro-hydraulic fracturing (MHF) and sleeve re-opening (SR) tests conducted in two boreholes in the Zürich Nordost (ZNO) siting region. The 3D seismic and stress data were collected as a part of site characterization for a potential Deep Geological Repository (DGR) for radioactive waste. This 3D geomechanical numerical model serves as the reference model in our study and includes seven faults, implemented as contact surfaces with Coulomb friction. It is then systematically compared to three fault agnostic models i.e., models without any implemented faults. These fault agnostic models use identical rock properties and model input parameters, are calibrated with the same 45 horizontal stress magnitude dataset and have the same model extent, but differ in their discretization and mechanical properties’ assignment procedure. The results show that at distances of < 1 km from faults, differences in maximum horizontal stress orientation between models range from 3°–6°, and horizontal stress magnitude differences are about 1–2 MPa. Beyond 1 km distance, the differences reduce to < 1.5° and < 0.5 MPa, respectively. These stress differences are far smaller than the uncertainties associated with the horizontal stress magnitude measurements at the ZNO siting region, which average to ±0.7 MPa for the minimum horizontal stress magnitude and ±3.5 MPa for the maximum horizontal stress magnitude. An important implication of this lateral quantification of fault influence on stress state is that explicit representation of faults may not be necessary in geomechanical models predicting the stress state of rock volumes located a kilometer or more from major active faults, an important prerequisite for any DGR campaign. This structural simplification allows for faster model set-up and discretization, leading to a significant reduction in the set-up phase and computational time by more than one order, without compromising the reliability of stress field predictions.