Crustal stresses and seismotectonics in eastern Canada.
Authors: Hasegawa H S , Adams J 
Published: March 2017

Prominent features of the stress field in eastern Canada are discussed with respect to the seismotectonic environment and possible causes of, or major contributors to, the stress field. A ubiquitous feature of the stress field in this region is that the maximum
horizontal stress component is larger than the vertical within the top few kilometres of the crust. Fault-plane solutions of shield earthquakes indicate this trend extends to mid-crustal depths. Although the deviatoric (horizontal) stress component within the top few kilometres of the Canadian
Shield is well documented, uncertainty exists as to whether deviatoric stress at mid-crustal depths is of the order of a few tens of MPa or a few hundreds of MPa. However, a lower limit of the order of 5 MPa, is inferred from the stress drop of two earthquakes. The azimuth of the deviatoric
compression vector, as derived from fault-plane solutions of eastern Canada earthquakes, shows a much greater scatter than the uniform direction (ENE) indicated by northeastern United States earthquakes. However, because of the inherent uncertainty in inferring principal stress directions from
deviatoric stress orientations, the question as to whether or not there is uniformity in direction of the maximum principal stress component at mid-crustal depth is still unresolved. More fault-plane solutions of eastern Canada earthquakes are required to resolve this uncertainty. A similar
situation exists for in situ stress measurements in eastern Canada . On a broad-scale or regional basis, near~surface stress determinations do not indicate a uniform or general trend but show an appreciable scatter, much of which is likely due to varying local stresses predominating over
regional (e.g. plate tectonic) stresses. For~certain localized areas, however, stress determinations may be indicating a more uniform pattern; in situ stress measurements north of Lake Superior and Lake Huron can be interpreted as indicating an east-west trend with a scatter of +45°. High
horizontal stresses may be caused by one or more of the following: erosion; postglacial heating; residual stress from each glaciation - deglaciation cycle; differential postglacial uplift; and plate- tectonic movements. Stress concentration within or surrounding intrusions are induced by density and
elastic parameter contrasts between intrusion and host rock. A search for and documentation of, surface evidence of differential postglacial uplift and monitoring of vertical crustal movements is vital to understanding these inherited and cyclic stresses. As a step towards understanding earthquake
prediction in intraplate environments, the Charlevoix region - the most active in eastern Canada - has been selected for continuing physical measurements of processes related to earthquakes. A wide variety of experiments have been carried out; explosion studies to detect changes in P-wave
velocities; variations in the magnetotelluric and gravity fields; and variations in ground deformation. Compilation of continuing measurements should enhance our understanding of the seismotectonic environment in eastern Canada.