The Spatial Reach of Faults: How They Shape Regional Stress Fields.

Characterizing the crustal stress field is crucial for understanding global processes like earthquakes and plate tectonics, as well as for local applications such as subsurface storage, geothermal exploration, and nuclear waste repositories. A key challenge lies in understanding how pre-existing geological structures, particularly faults, influence crustal stress distribution. While some studies infer fault impact from variations in stress magnitudes or maximum horizontal stress (S_Hmax) orientation over large regions, this approach cannot isolate fault-induced perturbations. Generic geomechanical models, though informative, often lack site-specific calibration. The S_Hmax orientation, systematically documented in databases like the World Stress Map, reflects consistency on large scales due to large-scale tectonic and buoyancy forces but can exhibit significant local rotations due to faults. Accurately modeling these third-order perturbations remains difficult due to computational challenges 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 far field in-situ stress state beyond a certain spatial scale might be overstated and overinterpreted by many studies. Here, we use 3-D geomechanical-numerical models that are calibrated against a unique and robust dataset of 45 stress magnitude data records. This dataset was acquired for evaluating the suitability of potential siting regions to build a deep geological repository for high-level nuclear waste in Switzerland. 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 along faults and lateral juxtaposition of units with contrasting mechanical properties. Small lateral variations could be attributed to the mechanical behavior 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 have been excluded from the modeling workflow for models that focus on large-scale stress predictions and not on stress changes close to the faults Removing faults from the modeling workflow reduces computational complexity and accelerates the modeling process, without causing any significant differences in the model results at a distance of few 100s meters from the faults.