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. In the beginning of 2006, the target area of the seismic monitoring expanded to semiregional scale. Four new seismic stations started in the beginning of February 2006 and the focus of interpretation was expanded to an area, called the seismic semi-regional area. At the end of 2006, two new borehole geophones were installed in order to improve the sensitivity and the depth resolution of the measurements inside the ONKALO block. 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 2007. Also the changes in the structure and the operation procedure of the network are described. The true orientation of the borehole sensor OL-OS13 was calculated. The correct orientation of triaxial seismometer is essential when the fault plane solution of an earthquake is calculated. The other borehole sensor OL-OS14 was permanently disconnected in October 2007. The network has operated continuously in 2007. Altogether 2207 events have been located in the Olkiluoto area, in reported time period. Altogether 2207 events have been located in 2007. Most of them (1912) are explosions occurred inside the seismic semiregional area and especially inside the ONKALO block (1891 events). The magnitudes of the observed events inside the semi-regional area range from ML = -2.1 to ML = 1.5 (ML magnitude in local Richter's scale). All these events are explosions. According to seismic monitoring the rockmass surrounding the ONKALO has been stable in 2007. One of the recorded events was a local microearthquake (ML 1.9) outside the target area of the network. That earthquake occurred 3.1.2007 in Laitila about 40 km from Olkiluoto. Joint interpretation of recordings of three seismic networks (Posiva, Finnish and Swedish national networks) was used when the preliminary fault plane solution of the Laitila earthquake was calculated: The reverse faulting occurred in a nearly vertical N-S oriented fault, which can be associated with mafic dykes in the area.The orientation of the compressional axis related to the microearthquake on (NWSE) is consistent with the estimated maximum in-situ stress field in Olkiluoto and elsewhere in Finland. (orig.)