Crustal Stresses and Seismicity in Intraplate Settings: Relationship to Geology and Driving Forces
Baird, William Alan Frank
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This thesis explores the relationship between contemporary stresses, paleotectonic structure, and seismicity within continental interiors. This is achieved by combining a variety of geophysical and geological data coupled with numerical stress analysis to form seismotectonic models for local seismic zones in eastern North America. The work focusses on two main areas: (1) Southern Ontario, a region with low to moderate earthquake activity, which is structurally dominated by the Grenville orogen. Numerical models are used to show that both increased seismicity and an identified perturbation in the local stress orientation could be explained by reactivation of basement faults. (2) The Charlevoix seismic zone, a region of comparatively high background seismicity and large historic events, which is structurally controlled by an Iapetan rift and a meteorite impact structure. Although the rift faults are poorly aligned for reactivation in the regional stress field, modelling indicates that if they are sufficiently weak, they may act as a conduit, channelling higher stresses into the interior of the fractured crater zone, triggering much of the background seismicity. Furthermore, interaction with the crater acts to enhance rift fault slip near their intersection points, matching the observed pattern of large events in the seismic zone. The results also highlight a potential structural bias inherent in intraplate focal mechanisms. The faulting style of large earthquakes is heavily influenced by the regional structural trends, while smaller events, which can take advantage of local complexity in fault orientation, produce mechanisms more indicative of local stress conditions. Concepts developed with these studies as well as a comparison with the eastern Tennessee seismic zone are used to propose a regional model for eastern North American seismicity. Seismicity may be explained by a large deformation zone between the rigid North American craton and the Atlantic oceanic crust. Differential motion across the zone is accommodated by infrequent large earthquakes that localize on the major paleotectonic structures, which may produce a regional counterclockwise stress perturbation within the deformation zone. Stress orientations deduced from smaller magnitude events illustrate this regional perturbation.