Local Seismic Network at the Olkiluoto Site Annual Report for 2007

In February 2002, Posiva Oy established a local seismic network of six stations on the island of Olkiluoto. Later, in June 2004, the seismic network was expanded with two new seismic stations. At that time started the excavation of the underground characterisation facility (the ONKALO) and the basic operation procedure was changed more suitable for the demands of the new situation. The purpose of the microearthquake measurements at Olkiluoto is to improve understanding of the structure, behaviour and long term stability of the bedrock. The studies include both tectonic and excavation-induced microearthquakes. An additional task of monitoring is related to safeguarding of the ONKALO. This report gives the results of microseismic monitoring during the year 2006. Also the changes in the structure and the operation procedure of the network are described. The network has operated continuously in 2006. In the beginning of 2006, the target area of the seismic monitoring expanded to semi-regional scale. Four new seismic stations started in the beginning of February 2006. At the end of the year, two new borehole geophones were installed in order to improve the sensitivity and the depth resolution of the measurements inside the ONKALO block. This report presents also new interpretations of the excavation induced earthquakes that occurred in the ONKALO in 2005. Altogether 2041 events have been located in the Olkiluoto area, in reported time period. The magnitudes of the observed events range from ML = -1.1 to ML = 3.1 (ML = magnitude in local Richter's scale). Most of them are explosions. Two of the observed events are be classified as microearthquakes. Evidence of activity that would have influence on the safety of the ONKALO, have not been found. The observed earthquakes occurred in 2006 were small, ML = -0.6 and ML= -0.9. The earthquakes relate to small movements in brittle deformation zones OL-BFZ043 and OL-BFZ034 presented in the geological model of the Olkiluoto site (Paulamäki et al. 2006). Estimated peak slip values of the earthquakes are 14 μm and 4 μm and the source radiuses 11 and 8 meters. The orientation of the compressional axis related to the microearthquake on 14.9.2006 (NWW-SEE) is consistent with the estimated maximum in-situ stress field in Olkiluoto and elsewhere in Finland. The event seems to be related to the vertical structure OLBFZ043. The fault type is normal left-lateral oblique i.e. normal fault with a component of left-lateral strike. The location of the earthquake was about 70 m from the preceding blasts and about 40 meters below the level of excavation. All the other induced earthquakes in the ONKALO area have occurred in the very vicinity of the just excavated tunnel. The estimated orientation of compression related to the 27.11.2006 (SW-NE) event is perpendicular to the commonly prevailing stress field. In the place the latter earthquake occurred, the ONKALO tunnel is perpendicular to the estimated orientations of compression. The fault type is left-lateral strike slip. It seems that the foot wall of the nearly horizontal brittle deformation zone OL-BFZ034 has moved towards the tunnel.