Neotectonism of the region surrounding Chalk River Nuclear Laboratories, Ontario

The area of the Grenville Structural Province surrounding the site of Chalk River Nuclear Laboratories is one that is relatively seismically active compared to other more stable parts of the Canadian Shield. Some, but not all, of this activity is probably related to faults associated with the Ottawa graben of the St. Lawrence Rift System, which was active in late Precambrian-early Palaeozoic and mid-Mesozoic times. Earthquakes as large as magnitude 7 (Richter scale) have occurred along the system; for instance, the Lake Timiskaming earthquake of 1935 had a magnitude of 6.2. Studies of glacial varves indicate the seismicity in the Lake Timiskaming region has been continual since at least 8,500 years ago. New probabilistic ground motion maps of Canada show Chalk River located between the 16 and 23% g (g = gravitational attraction) contours of peak horizontal acceleration, which translate roughly into earthquakes of magnitudes 5.2 and 5.8, respectively. the maps are based on the premise that indicated values have a 10% chance of being exceeded in 50 years. A variety of evidence suggests that regional crustal stresses are consistent with a thrust-fault or strike-slip seismotectonic regime. The observed siesmicity may reflect adjustment to the resultant of stress fields arising from regional density variations, deglaciation and intraplate forces induced by interaction of lithospheric places