Upper crustal stresses and vertical stress migration in eastern Canada

Abstract

Diversified phenomena are contributing to our knowledge of various aspects of the stress field and the origin of the crustal stresses in eastern Canada. Both in situ stress measurements (to 2100 m) and earthquake fault plane solutions in the upper crust (∼5–20 km) indicate that the maximum horizontal stress is greater than the vertical. Absence of earthquakes in the lower crust (>20 km) implies a comparatively lower deviatoric horizontal stress in this layer. Stress directions as determined from both in situ measurements and earthquake fault plane solutions indicate an overprint confined to the ENE octant, which correlates with the direction predicted by modeling of plate tectonic stresses. Thus spreading (Mid‐Atlantic) ridge stress is considered to be one of the important contributors to the stress field in eastern Canada. Viscoelastic relaxation of this stress is considered to enhance the deviatoric stress level in the upper crust, while simultaneously reducing that in the lower crust. The crust acts as a stress guide within which stresses in the upper crust that are lost through brittle failure (i.e., earthquake stress drop) can be replenished from the lower crust; moreover, the directionality predicted by modeling of spreading (Mid‐Atlantic) ridge stress is maintained. Incomplete postglacial rebound also contributes to deviatoric compression. Other possible contributors to the upper crustal stress field are basal drag and membrane stress, but the relative contributions from these sources are not properly understood.