The in situ stress state within fault zones is technicallychallenging to characterize. At the Bedretto Underground Laboratory in theSwiss Alps, the breakouts observed in an array of eight inclined boreholespenetrating a major fault zone offer a unique opportunity to characterize thefault-associated spatial stress variations. Synthesizing multiple geophysicallogs, natural geological structures intersecting these boreholes areidentified, revealing a hierarchy of a major fault zone along with secondarystructures. Within the boreholes, breakout rotations occur over multiplescales, spanning individual fractures and the entire major fault zone. Based onthe analytical solution of the stress field around a circular borehole andMohr-Coulomb failure criterion, the observed breakout azimuths are used toinvert the orientations and relative magnitudes of the stresses across thefault zone, revealing a decreasingly critical stress state from afar to thefault zone center. Our work provides a workflow to semi-quantitatively estimatethe in situ stress variations around fault zones in the absence of directstress measurements, which is beneficial to a number of scientific andengineering applications.