western quebec and southern ontario - Department of Geology

and Adams (1989), in contrast to studies that favor a regionally homogeneous easternNorth American stress field (Sbar and Sykes, 1977; Zoback and Zoback, 1991). Forexample, Sbar and Sykes (1997) suggest that the Adirondack and western Quebec form acontinuous seismic zone with a uniform stress pattern of ENE compression. In contrastAdams (1989) proposes that, within the mid-plate stress province, there are stressanomalies. Mazzotti and Townend (2010) argue that the intraplate state of stress may beaffected by stress perturbation sources: postglacial rebound and local stress (induced byspatial and temporal interplay of regional stress sources) concentrators, such as lowfrictionfaults. This implies that local conditions can significantly modify the regionalstress field. Locally, some earthquake focal mechanisms (including those in a 50 kmradius around Ottawa) indicate P-axes normal to the regional trend. These variations maybe caused by local stress in a nearly isotropic field. The present study confirms the lack ofuniformity of the stress field inferred from focal mechanism studies (Figure 5.3).Comparisons of the axis of maximum horizontal compressive stress determinedfrom earthquake focal mechanisms in this study and those proposed by Mazzotti andTownend (2010) show a similar stress orientation for western Quebec (Figure 5.3). Theaverage orientation of the P-axes in southern Ontario is slightly rotated compared to theaverage orientation of the compression obtained by Zoback (1992a). Details for the stresspatterns proposed by Mazzotti and Townend (2010) for eastern Canada and thosedetermined in this study for the defined clusters of events in western Quebec andsouthern Ontario are shown in Figure 5.3.65