Seismic emissions induced by excavations of the rock repository in Loviisa

The low and intermediate level waste from the Loviisa nuclear power plant will be disposed of in the bedrock of Haestholmen, in Loviisa. Two field experiments were conducted to examine the stability and structure of the rock mass surrounding the repository and its access tunnel. In addition, it was evaluated to what extent it is possible to observe the excavation induced behaviour of the Precambrian bedrock by means of a seismic emission method. Altogether 40 induced microearthquakes were located. These events revealed structures of fractured rock within 50 metres from the excavation. Mostly the events verified the already existing 3D rock model of Haestholmen. Some observations extended and adjusted the modelled fracture geometry. The sequences of seismic emission can determine the plane surface. Three microearthquake sequences were observed. They illustrated quite clearly the fracture geometry as well as the mechanism of the strain release. The events induced by the excavation of the access tunnel were related to horizontal weakness zones. The events induced by the excavation of the repository seem to be associated with right- lateral strike-slip movement in the set of vertical fractures running in the direction SW-NE. (orig.) (9 refs.)