Spatial influence of fault-related stress perturbations in northern Switzerland

Abstract. The spatial influence of faults on the crustal stress field is a topic of ongoing debate. While faults are often known to perturb the stress field at a meter scale, their lateral influence over a few hundred meters to several kilometers remains poorly understood. To address this knowledge gap, we use a 3D geomechanical numerical model based on 3D seismic data from northern Switzerland. The model is calibrated with 45 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, northern Switzerland. This model with seven faults implemented as contact surfaces serves as the reference model in our study. The reference model is systematically compared to three fault-agnostic models, which share identical rock properties, model dimensions, and calibration data with the reference model, but differ in their element resolution and mechanical properties' assignment procedure. Results show that at distances <1 km from faults, differences in maximum horizontal stress orientation between models range from 3–6°, and horizontal stress magnitude differences are approximately 1–2 MPa. Beyond 1 km, these differences reduce to <1.5° and <0.5 MPa, respectively. These differences are significantly smaller than the calibration data uncertainties at ZNO, which average to ±0.7 MPa and ±3.5 MPa for the minimum horizontal and maximum horizontal stress magnitude, respectively, and ±11° for the maximum horizontal stress orientation. An important implication of our results is that, under the specific geological, mechanical, and stress conditions observed at the ZNO siting region, explicit representation of faults may not be necessary in geomechanical models predicting the stress state of rock volumes located 1 km or more from active faults. This simplification substantially reduced our model setup time from 2 months to 2 days, without compromising the reliability of stress field predictions.