2014:58 Technical Note, Rock Mechanics - Assessing probability and extent of blind faults and fault-end growth around the KBS-3 repository at Forsmark – Main Review Phase

To get a general understanding of the stability conditions of the fault and deformation zone inventory at Forsmark a reactivation potential analysis was carried out. At present-day stress conditions the deformation zones are stable and show no large deformations or detected seismicity. Any changes in stress, and in particular increase of differential stress on deformation zones, might cause slip. Therefore, several stress field models have been considered in this study. In general, steeply dipping deformation zones show the lowest reactivation potential at all depths, while the gently dipping deformation zones show highest reactivation potential today at shallow depth.

During glaciation, the alterations of stress at repository depth produce a large potential for activation of shallow dipping deformation zones during ice retreat. A large potential for reactivation of steeply dipping deformation zones during forebulge periods is predicted while a stabilizing effect for all deformation zones during maximum ice cover periods is inferred. From this analysis the critical deformation zones could be identified for further analysis to help to better interpret numerical simulation results. In addition, this analysis clearly shows that a good understanding of the stress field is essential for any mechanical analysis of geological system behaviour.

Simulation of the influence of the thermal phase showed that i) sub-vertical deformation zones parallel to the repository contour are stabilised, ii) sub-vertical deformation zones at an angle to the repository contour may become less stable, iii) shallow dipping deformation zones show highest reactivation potential as for the case where thermally induced stresses might lead to growth of ZFMA2. Accordingly blind-faults with similar orientations within the stress field might become reactivated.