Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

The in situ stress state within fault zones is technically challenging to characterize. At the Bedretto Underground Laboratory in the Swiss Alps, the breakouts observed in an array of eight inclined boreholes penetrating a fault zone offer a unique opportunity to characterize the fault-associated spatial stress variations. Synthesizing multiple geophysical logs, natural geologic structures intersecting these boreholes are identified, revealing a hierarchy of a major fault zone along with secondary structures. Within the boreholes, breakout rotations occur over multiple scales, spanning individual fractures and the entire major fault zone. We first estimate and rule out the effect of the fracture-induced anisotropy on the breakout rotations, which are attributed mainly to the stress variations. Based on the stress field around a circular borehole and Mohr-Coulomb failure criterion, the observed breakout azimuths are used to invert the stress information. Results show that the stress field outside the fault zone features a stress ratio (quantifying the relative stress magnitude) of about 0.9, an inclined overburden stress (inclination: 12 degrees similar to 18 degrees), and a maximum horizontal principal stress (S-Hmax) oriented N100 similar to 120 degrees E. Within the fault zone, a substantial reduction of the stress ratio and complicated stress rotations are constrained, likely induced by the stress drop on local fractures. As a result, less critical stress state inside the major fault zone is expected. Our work provides a semi-quantitative estimation of the in-situ stress variations around fault zones in the absence of direct stress measurements, which is beneficial to a number of scientific and engineering applications. Plain Language Summary Fault zones exert appreciable influences on fluid flow, earthquake processes and subsurface engineering by modifying the local stress state. Due to the structural complexity of the faults and limited stress measurement techniques, we lack a comprehensive understanding of how stress magnitude and orientation change around faults. This study presents an elaborate geomechanical study at the Bedretto Underground Laboratory for Geosciences and Geoenergies, Switzerland. We propose a workflow to describe the changes of stress magnitude and orientation within a fault zone according to the observations in an array of boreholes penetrating it. Our results provide new insights into the spatial relationship between the stress state and fault structure.