Modelling End-Glacial Earthquakes at Olkiluoto Expansion of the 2010 Study

The present report is an extension of Posiva Working Report 2011-13: “Modelling Endglacial Earthquakes at Olkiluoto”. The modelling methodology and most parameter values are identical to those used in that report. The main objective is the same: to obtain conservative estimates of fracture shear displacements induced by end-glacial earthquakes occurring on verified deformation zones at the Olkiluoto site. The remotely activated rock fractures (with their fracture centres positioned at different distances around the potential earthquake fault being considered) are called “target fractures”. As in the previous report, all target fractures were assumed to be perfectly planar and circular with a radius of 75 m. Compared to the previous study, the result catalogue is more complete. One additional deformation zone (i.e. potential earthquake fault) has been included (BFZ039), whereas one deformation zone that appeared to produce only insignificant target fracture disturbances (BFZ214) is omitted. For each of the three zones considered here (BFZ021, BFZ039, and BFZ100), four models, each with a different orientation of the target fractures surrounding the fault, are analysed. Three of these four sets were included in the previous report, however not as systematically as here where each of the four fracture orientations is tried in all fracture positions. As in the previous study, seismic moments and moment magnitudes are as high as reasonably possible, given the sizes and orientations of the zones, i.e., the earthquakes release the largest possible amount of strain energy. The strain energy release is restricted only by a low residual fault shear strength applied to suppress post-rupture fault oscillations. Moment magnitudes are: 5.8 (BFZ021), 3.9 (BFZ039) and 4.3 (BFZ100). For the BFZ100 model, the sensitivity of the results to variations in fracture shear strength is checked. The BFZ021 and BFZ100 models are analyzed for two additional in situ stress orientations, specified to maximize the potential instability of these two deformation zones. For the moment magnitudes, this appeared to have practically no effect. The new analyses confirm that the results of the previous report can be taken as reasonable upper bound estimates of fracture shear displacements induced by endglacial earthquakes at the Olkiluoto site. Inclusion of an additional fracture set and all possible combinations of fracture positions and fracture orientations did not increase the maximum induced fracture displacement by more than a few millimetres. The largest induced displacement, 28 mm, was found for a gently-dipping fracture at 100 m distance from the BFZ100 fault. The other deformations zones did not produce induced displacements larger than 12 mm. No fracture belonging to the steeply-dipping sets displaced by more than 6 mm. Alternative assumptions regarding the present-day in situ stress field and the shear strength of the target fractures did not produce more than marginal, about 3 mm, changes to the results. Target fracture shear velocities did not exceed 160 mm/s for any fracture in any model. The implications of this finding are not discussed. Here it sufficient to observe that this velocity is significantly lower than the default velocity assumed in the stressdeformation analyses of the buffer-canister system performed to establish the canister damage threshold applied in the Swedish safety analysis (SKB 2010).