western quebec and southern ontario - Department of Geology

5.4.6 Southern Ontario cluster (ON1) (The Burlington-Niagara Falls Cluster (BNFC))The Burlington-Niagara Falls (ON1) cluster also known as the Western Lake Ontariocluster is located in the west end of Lake Ontario from latitude 43.3 to 44° and longitude-77° to -79.5°. This area is structurally underlained by the northeast striking relativelyshallow southeast-dipping thrust faults of the Grenville orogen (Easton, 1992). Theseismicity in Lake Ontario has been characterized by Stevens (1994) as “intermittentscattered” based on the available data at that time but other researchers have attributedapparent linear trends of seismicity (e.g., Mohajer et al., 1992; Mohajer, 1993, 1995a,b;Wallach et al., 1998; Dineva et al, 2004).Eight earthquakes, with magnitude of m ≥ 3.5 were recorded in this region after1992. Focal mechanisms for the Western Lake Ontario cluster are typically oblique withpressure orientation almost NNE and NE. The focal depth of the events in this clustervaries from 5 km to about 10 km depth. Figure 5.20 shows the distribution of the majorfaults and geological structures in southern Ontario. The nodal planes in this cluster dip atangles from 66 to 77° in E-S direction and 45° in N direction (Figure 5.19c). Theorientation of the major horizontal principal stress, estimated from the focal mechanismsin this region differs from the direction of regional tectonic field. The orientation of theP-axes varies from EES to WWN direction. The compressive stress field instead appearsto be oriented sub-parallel to the major terrain boundaries such as the Grenville Front, theCentral Metasedimentary Belt boundary zone and the Elzevir Frontenac boundary zone.This suggests that the stress field has been modified by these deep crustal scaledeformation zones (Baird & McKinnon, 2007).93