RSC-Programme - Interim Report. Approach and Basis for - Posiva

44earthquake M>6 by Bödvarsson et al. (2006) and Fälth and Hökmark (2006), also thebasic assumption in the RSC-project. This means that all deformation zones orlineaments larger than 8 km could act as hosts for a large earthquake (Fälth andHökmark 2006). These zones and lineaments are therefore being defined as a layoutdetermining features.GPS measurements (Ahola et al. 2008) and precise levelling campaigns (Lehmuskoski2008) on Olkiluoto Island suggest that certain areas are moving in relation to oneanother. Mattila et al. (2007b) hypothesised that in certain places, movement could beconnected to deformation zones or lineaments. These zones are here defined as layoutdetermining features, although movement seems very slow and incomparable to that oflarge earthquakes. According to Mattila et al. (2007b), further studies are required inthis area.An ice load covering the site during the glacial period will cause local changes in thestress field depending on geometry of the faults at the site. To be able to further definewhich zones and lineaments might move in the changing stress field, two types ofanalyses of different ice loads were conducted (Appendices in Wikström et al. 2009). Inboth cases, the main sets of data have been stress data from Olkiluoto Island, calculatedstress changes due to glaciation and the geometry of the main geological brittledeformation zones and lineaments. Analyses were conducted using the 3 DEC -programme and the Poly3D -programme (presented in the Appendices in Wikström etal. 2009). Based on these analyses, a few of the brittle deformation zones wereconsidered layout determining features. However, these zones slipped under veryconservative assumptions and if more realistic values were used, no slippage wouldprobably occur. Nevertheless, results indicated that these zones are favourably orientedfor slippage to occur under modelled stress states. The lineament interpretation is basedon observed geophysical anomalies (Korhonen et al. 2005). The geologicalcharacteristics of these zones are not known at the moment, but they are assumed topresent mainly possible deformation zones and thus, due to their size, layoutdetermining features. These features are located mainly in the straits surroundingOlkiluoto Island, but also in the eastern part of the island. Those features must becharacterised in more detail if located close to the planned deposition tunnels.Layout determining feature due to hydraulic propertiesThe layout determining feature may not always be very significant in terms of geologyor rock mechanics. However, a feature with high transmissivity (T > 10 -5 m 2 /s) and oflarge size or connected to a high-transmissive feature may diminish the radio nuclidetransport time significantly, hence affecting long-term safety of the repository. Theeffect of up-coning of saline groundwater and other potential changes in thegroundwater chemistry enhanced by the high flows have also been taken into account indefining hydrogeologically important zones. Thus it is not solely the T-value, but ratherthe understanding of site hydrogeology and geochemistry in general that determinescertain zones as more important than the others.In Ahokas et al. (2007) and in Vaittinen et al. (2009) zones estimated to be hydraulicallysignificant in the site scale are modeled. These hydrogeological zones have large aerialextension and high transmissivity and thus need to be avoided by deposition tunnels andholes. These hydrogeological zones are included in the layout determining features.Nevertheless, many of the hydrogeological performance targets (Table 4-3) suggest that,