The Role of Faults in Shaping Present-Day Stress Fields: Implications for 3D Subsurface Models.

Characterizing the crustal stress field is essential for understanding global processes such as earthquakes and plate tectonics, while also being critical for local applications, such as interim subsurface storage, and deep geological repositories for nuclear waste. A key challenge lies in understanding the interactions between the crustal stress field and pre-existing geological structures, especially with faults. Previous studies have aimed to understand the impact of faults on the stress field by making interpretations based on variation of stress magnitudes or rotation of the maximum horizontal stress (S_Hmax) orientation in larger regions. This approach cannot attribute the local perturbations in the stresses exclusively to the faults. Another common approach is the use of generic geomechanical-numerical models. Although instructive, generic models usually have limitations from a lack of site-specific calibration with in situ stress data.

The S_Hmax orientation is the only component of the reduced stress tensor that is systematically documented and accessible through databases such as the World Stress Map. The S_Hmax orientation reflects consistency on large scales, primarily driven by first-order tectonic forces and second-order buoyancy forces. However, significant S_Hmax rotations over shorter distances are often linked to third-order sources such as faults, and are challenging to model accurately due to computational complexity and the risk of numerical artifacts.

The hypothesis in this study is that the impact of local faults with a few tens of meters displacement on the in-situ stress state might be overstated. Here, we use 3-D geomechanical-numerical models that are calibrated against a unique and robust dataset of 50 stress magnitude data records. This dataset was acquired for evaluating the suitability of Zürich Nordost which is one of the three potential Swiss siting regions to build a deep geological repository for high-level nuclear waste. We vary the numerical resolutions and investigate the spatial scale at which faults influence the individual components of the far-field stress tensor and in particular the S_Hmax orientation. Finally, we compare models with and without faults.

Our results reveal that faults of this scale do not have a significant influence on the stress tensor orientation or principal stress magnitudes beyond a few 100s meters distance from the fault. Comparisons between the models reveal that the stress differences are not necessarily controlled by the mechanics of faults. The impact is rather due to lateral stiffness variations and density contrasts due to the offset between units that occurs at faults. Small lateral variations could be attributed to the mechanical behaviour of faults but these variations are generally less than the stress variations due to uncertainties in the rock property variability.

Our findings suggest that faults could be safely excluded from the modeling workflow for models focusing on large-scale stress predictions and not on stress changes close to the faults, such as those that characterize the geomechanics of potential deep geological repository regions. Removing faults from the modeling workflow reduces computational complexity and accelerates modeling process, without causing any significant differences in the model results at a distance of few 100s meters from the faults.