Four seismogenic environments in eastern Canada

Abstract

Although the seismogenic environments in eastern Canada have some common features, the respective seismotectonic environments are influenced by local features. The seismotectonic environment in four seismogenic regions are described in order to illustrate how local features are reacting with the ambient plate-tectonic stress (a common feature) to modify the nature of the seismic activity. The Charlevoix earthquakes are occurring along steeply dipping planes formed during the opening of lapetus Ocean in the Proterozoic, perturbed by a meteor impact during the Paleozoic and reactivated by the opening of the present-day Atlantic Ocean in the Mesozoic. Theoretical curves of maximum shear stress versus depth based on a wet upper crust and a wet quartz rheology in the lower crust, for which the heat flow is 1.0 HFU (42 mW/m 2), can account for the depth distribution of earthquakes in this region. Potential trigger mechanisms are gravitationally induced stress, water infiltration and vertical shear strain changes (approximately 3 × 10 −8/ yr) due to post-glacial rebound. The unusually deep focus (29 km) of the M 5.9 Saguenay earthquake of 1988 could be due, in part, to the lower heat flow in this region as compared with that in the Charlevoix region which is about 70 km to the east. The Miramichi earthquake sequence is occurring on conjugate faults within an intrusion at depths shallower than 10 km. Theoretical curves of maximum shear stress versus depth based on a wet quartz rheology in the lower crust for which the heat flow is 1.5 HFU can account for this depth distribution. The steep gradient in the vertical ground movement rate (5 × 10 −8/ yr) in the epicentral area, which is probably due to post-glacial rebound, is a potential trigger mechanism for favorably oriented faults. The normal-fault seismic regime along northeastern Baffin Island can be explained by a combination of topography and post-glacial rebound, which can generate horizontal extensional stresses capable of overcoming the ambient compressional plate tectonic stress. Post-glacial rebound stress can act as a trigger mechanism for the NE-dipping faults in this region. In the Grand Banks region there is seismic evidence indicating that the slump that caused the destructive tsunami also generated the strong seismic ( M 7.2) signals, but double-couple solutions that satisfy certain criteria are not ruled out. The absence of surface faulting, especially in the epicentral regions of large earthquakes, is not properly understood, but several possibilities are near-surface layering that could mask the underlying fault offsets and a spatiotemporal variation in the ambient plate tectonic stress field. As knowledge of the causative factors of eastern Canada earthquakes and also the data base for empirical earthquake scaling relations increase, an overview of the seismotectonics of these mid-plate earthquakes should be enhanced.