The prevailing orientation of S_Hmax in the siting regions was constrained by new borehole data using the orientation of borehole breakouts, drilling-induced tensile fractures and fractures generated as a result of MHF testing (Section 4.4.3). The results (Fig. 4‑76) are very much consistent with those predating the new boreholes (Heidbach & Reinecker 2013):

• S_Hmax trends in a NNW-SSE orientation, with a small counter-clockwise rotation of S_Hmax from east to west. The mean S_Hmax orientation based on the borehole data is 172 ± 9.3° (ZNO), 166 ± 10.8° (NL) and 163 ± 7.6° (JO) (Fig. 4‑76), indicating that S_Hmax is approxi­mately perpendicular to the arc-shaped Jura Fold-and-Thrust Belt.

• The dominant S_Hmax orientation is also stable in the vertical direction, i.e. orientations derived from boreholes (to a depth of 1.3 km) are in general agreement with S_Hmax orientations derived from earthquake focal mechanisms at much greater depths (up to 30 km; Nagra 2024o).

• In individual boreholes, the S_Hmax orientation is relatively stable across the Mesozoic sedi­ments, and across the Opalinus Clay in particular. The few exceptions presenting local variations of S_Hmax orientations are discussed in Nagra (2024o).

• A comparison with palaeo-strain data (Madritsch & Hammer 2012) indicates broad consistency of the S_Hmax orientation with deformation features since Miocene times.

Fig. 4‑76:Orientation of maximum horizontal stress S_Hmax in the siting regions

Black lines indicate S_Hma orientations derived from borehole data. Red lines (normal faulting stress regime) and green lines (strike-slip stress regime) indicate S_Hmax orientations derived from earthquake focal mechanisms. Black squares indicate the extent of site-specific 3D geo­mechanical-numerical models. Note that only a selection of the structures from the 3D seismic interpretation on Top Villigen Formation were projected to Base Cenozoic and are shown to highlight the structural trends of regional importance.