Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 90

The past few decades have brought to light numerous geological and geophysical char-
acteristics of the SLRS and their links with seismicity. At a continent-wide scale, the
correlation between the SLRS and earthquakes is appealing. When a more local scale is
looked at or when smaller magnitude earthquakes are considered, questions arise and the
picture becomes more complex. We conclude that the sole presence of pre-existing normal
faults related to the SLRS does not explain all the features of the seismicity of Eastern
Canada. Seismicity is concentrated in more active areas, some with conspicuous normal
faults and some with suspected weakening mechanisms such as intense pre-fracturing (e.g.,
due to a meteorite impact), passage over a hotspot, or the presence of intrusive and lateral
crustal density variations. In most cases, the superposition of the tectonic stress field and
the relatively modest post-glacial rebound stresses is likely to play a role. Irrespective of the
presence of rift faults, earthquakes are caused by dynamic instabilities along pre-existing
faults caused by the inherent weakness of the faults, by the locally enhanced stress differ-
ences, or a combination of the two. A general unifying explanation is likely to be useful
only at the large scale, and a detailed understanding of seismicity requires further work
concentrating on local conditions.
Acknowledgments
We thank Allison Bent, Pradeep Talwani, and Thierry Camelbeeck for their reviews of a
draft of this paper. GR's participation in this work has been supported by a grant from
NSERC (Natural Sciences and Engineering Research council of Canada), Earth Sciences
Sector contribution no. 20130049.
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