Local seismic network at the Olkiluoto site. Annual report for 2011

This report gives the results of microseismic monitoring during 2011. Excavation of the underground characterisation facility called ONKALO started in 2004. Before that, in February 2002, Posiva Oy established a local seismic network of six stations on the island of Olkiluoto. After that the number of seismic stations has increased gradually. In 2011 Posiva's permanent seismic network consists of 15 seismic stations and 20 triaxial sensors. The purpose of the microearthquake measurements at Olkiluoto is to improve understanding of the structure, behaviour and long term stability of the bedrock. The investigation area includes two target areas. The larger target area, called seismic semiregional area, covers the Olkiluoto Island and its surroundings. The purpose is to monitor explosions and tectonic earthquakes in regional scale inside that area. The smaller target area is called the seismic ONKALO block, which is a 2 km x 2 km x 2 km cube surrounding ONKALO. It is assumed that all the expected excavation induced events occur within this volume. At the moment the seismic ONKALO block includes ten seismic stations. An additional task of monitoring is related to safeguarding of the construction of ONKALO. The configuration of the seismic network as well as the software packages applied in data processing and analyses have remained during the previous year. The design model of ONKALO and the brittle fault zone model of the Olkiluoto of the seismic visualization package Jdi were upgraded in 2011. The network has operated nearly continuously. There was a 14 minutes and 30 second long operation failure in December 2011. That was the first network operation failure in five years. Altogether 1223 events have been located in the Olkiluoto area, in the reported time period. Most of them (1098) are explosions that occurred inside the seismic semiregional area and especially inside the seismic ONKALO block (1064 events). The magnitudes of the observed explosions inside the semi-regional area range from ML = - 2.2 to ML = 1.5 (ML = magnitude in local Richter's scale). Most of them are explosions. Three of the events are classified as induced microearthquakes. Three excavation induced earthquakes were observed in 2011. The magnitude of the first event (9 February 2011) was significantly larger (ML = -0.5) than the magnitudes of the two events (ML = -2.0 and -1.9) that occurred on 14 May 2011. Estimated peak slip values of the earthquakes are 1 - 20 μm and the source radiuses about 10 meters. Any known geologic structure could not be associated with the location of the first induced microearthquake, but it is possible to see an interconnection between the location of the events in May 2011 and the pervasive foliation of the bedrock surrounding the ONKALO. According to the seismic monitoring the rock mass surrounding ONKALO has been stable in 2011. Indications of undeclared or inappropriate works, which would have influence on the safety of ONKALO, have not been found. (orig.)