Seismic activity in southern Ontario: annual progress report for the period January 1 – December 31, 2020

As part of geoscientific investigations for Ontario Power Generation (OPG) and Nuclear Waste Management Organization's (NWMO) Deep Geologic Repository (DGR) projects, the Canadian Hazards Information Service (CHIS) of Natural Resources Canada has been monitoring and reporting on microseismic activity surrounding the Bruce nuclear site near Tiverton and regional seismic activity in southern Ontario since 2007. This report summarizes seismic activity observed to the end of 2020. Following the decision by OPG to not proceed with the proposed low and intermediate level waste DGR at the Bruce nuclear site in June 2020, the emphasis of this reporting year (2020) is maintained on the Bruce nuclear site as an interim measure. Future regional reporting in southern Ontario will be recentered around the proposed site for used nuclear fuel near Teeswater. The proximity of Teeswater site to the Bruce nuclear site means that the data and results presented in this report need not be fundamentally altered. Seismic activity is observed using the existing Canadian National Seismic Network (CNSN) and Southern Ontario Seismic Network (SOSN) stations in the region, as well as three additional borehole seismographs located within 50 km of the Bruce nuclear site commissioned for the DGR project in August 2007. This network was designed to be capable of recording seismic events of magnitude 1 and above within 50 km of the Bruce nuclear site. All stations transmit continuous digital data in real-time to a central acquisition hub in Ottawa. CHIS staff in Ottawa integrate the data from these stations with those of the rest of the CNSN and provide monthly reports of the seismic activity within 50 km and 150 km of the Bruce site. In 2020, three earthquakes occurred within 150 km of the Bruce nuclear site, all less than m_N 2. A group of earthquakes 275 km southeast of the site, near Niagara Falls, included the largest in the region used for earthquake-recurrence estimation. This was a 2.9 m_N for which the depth was estimated to be 6.0 km based on regional depth phase modeling (RDPM). In all, 41 earthquakes have been catalogued within 150 km of the site since 1952. Since 2004, 13 earthquakes have been catalogued within 50 km of the site. These events have all been very small, ranging in magnitude from 0.1 M_L to 2.5 m_N, and grouped in a single cluster under Lake Huron, approximately 34 km to the north of the site. More work is needed to relocate these events accurately with respect to one another, to find out, for example, if they resolve into a linear structure. A new area has been proposed for estimating earthquake recurrence parameters in Southern Ontario, one which is more seismotectonically uniform. The rate of M>5 events using the new recurrence region is estimated to be once every 145 years. (author)