OBF-verkon toiminta ja seismiset havainnot vuonna 2021

This report presents the observatory practice and the observations of the Ostrobothnia local seismic
network (OBF) monitoring Fennovoima Oy’s future nuclear power plant site at Hanhikivi, Pyhäjoki,
Northern Ostrobothnia, Finland in 2021. The network operated by the Institute of Seismology of the
University of Helsinki has been designed to automatically detect and locate low magnitude seismic
events, down to M L 0, within 25 kilometres radius from the site. Larger earthquakes are detected and
analysed within 50 km radius from the site.
The OBF network consists of ten seismic stations including a Finnish National Seismic Network
permanent station, OUF. The OBF-stations (OBF0, OBF1, OBF2, OBF3, OBF4, OBF5, OBF6, OBF7 and
OBF8) were built during 2012 – 2015. The OBF network has been operating as a part of the
automatic seismic detection and location routine of the Institute of Seismology since the 1st of June
2013. Detected events have been identified using spectral methods and their automatic source
parameters have been stored in a database. Earthquakes have been revised manually and when
possible, focal mechanism solutions have been determined.
During 2021, within 50 kilometres of the Hanhikivi site, altogether 564 explosions were automatically
located, 1038 false detections were revised, and eleven earthquakes (M L -0.5 – 0.5) were detected
and analysed. Focal mechanisms of the earthquakes could not be determined as the events were
very small. Data coverage of the entire OBF network was 99,1% in 2021 varying between 97,4% and
100% depending on the station.