Numerical Modelling of Earthquakes and Induced Seismicity Under Various In Situ Stress Conditions at Forsmark, Sweden, the Site for a Final Repository of Spent Nuclear Fuel

This study concerns dynamic simulations of earthquakes and the seismicity associated with induced slip of preexisting fractures in the rock mass for a repository for spent nuclear fuel planned to be constructed at Forsmark, Sweden. We use Particle Flow Code 2D (PFC^2D) to model the repository volume that contains the faults and the pre-existing fracture system. The heterogeneous structure of the faults and the pre-existing fracture system are explicitly modelled in PFC^2D using the smooth joint model. Earthquake at a fault is simulated by instantaneous release of the strain energy stored along the fault after build-up of the rock stresses. The release produces earthquakes and the seismic waves propagate and attenuate through the model. The earthquakes are simulated under different present day in situ stress conditions and under estimated future glacial cycles of Weichselian type. In particular, the time of ice cover and related forebulge, maximum thickness of the ice cover, and the retreat of the ice cover are considered. Modelling results demonstrate that the magnitudes and the stress drops of the induced seismic events associated with fracture slip tend to be the largest under stress condition of high anisotropy, in other words, where the ratio of the maximum and the minimum horizontal stresses is large. Among the seven tested in situ stress conditions, the occurrence of an earthquake under the stress condition at the time of forebulge in front of the ice cover is found to produce the largest induced moment magnitude (M -3).