ABSTRACT: Seismicity and seismic hazards in Eastern Canada: Needs from crustal deformation studies.

CGRG Bibliography of Canadian Geomorphology
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The historical earthquake catalog is the basis for many hazard assessments that explicitly use the pattern of past earthquakes to assess hazard - the current USGS maps for the eastern U.S. rely heavily on smoothed seismicity rates and even classical seismic source zones give a similar smoothing, albeit with non-objective human insight. However, past southeastern Canadian activity has been a poor indicator of future large earthquakes. Though Charlevoix represents the site of repeated M$>$6 earthquakes, other M circa 6+ earthquakes in eastern Canada (Grand Banks, Timiskaming, Cornwall, Saguenay) appear to be one-off events (albeit with long aftershock sequences). Saguenay, the last large earthquake south of 60N, occurred in an essentially aseismic region (no M$>$3 event for over 40 years). To address the problem, Canada's 4th generation seismic hazard model, intended for the 2005 National Building Code, uses two models for Canadian earthquakes "H" and "R". "H" describes the earthquakes in their historical clusters while "R" associates seismicity clusters with continent-scale seismotectonic features like the passive Atlantic margin and the ancient margin of Iapetus. It expresses the alternative hypothesis that future large earthquakes (comparable to those named above) could occur anywhere along these features. The modeling of postglacial rebound data (to understand the relative roles of rebound and plate tectonic stresses) together with paleoseismological studies to establish the locations and rates of pre-historic earthquakes could help decide between the models. Direct horizontal strain measurements in eastern Canada will also help, but come from an extremely short period and need to be reconciled with the circa 350 year historical earthquake record. That record is, however, both (i) too short relative to likely earthquake occurrence rates and (ii) flawed by inaccurate and incomplete information especially regarding the magnitudes for the oldest, largest events. Those events are the most important for deciding the hazard and also contribute almost all of the seismic deformation. The pattern of contemporary crustal deformation could be definitive, and furthermore its rates could constrain the frequency of large earthquakes and their maximum size, both key factors in assessing the seismic hazard.

Bibliography of Canadian Geomorphology