CGRG Bibliography of Canadian Geomorphology
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Author : Adams, J.
Date : 2006.
Title : Where are all the postglacial faults scarps in eastern Canada?
Publication : Joint Annual Meeting of the Geological Association of Canada and the Mineralogical Association of Canada. University of Quebec in Montreal (UQAM) May 14-17, 2006.
Issue :
Page(s) :
Abstract
Current seismicity rates in eastern Canada (region west of the Rockies) suggest a rate of 7 M=4 (approximate completeness threshold) per year and a beta value of 2.09. Thus we predict about 0.1 M=6 events per year, each of a size that had it occurred in the top of the crust should have produced a surface rupture. This rate needs to be reduced by about a (conservative) factor of ~10, viz a factor of ~5 to allow for distribution of events in depth through the seismogenic crust and a further factor of ~2 to allow for the chance that the earthquake is underwater. For the past 100 years (when knowledge of M6 events is fairly complete) we should have had 0.1*100/10 = ~1 ruptures; we know of just one: the 1989 Ungava surface rupture, though a few other 20th Century earthquakes might have undiscovered surface ruptures. Furthermore, a rate of 0.01 p.a. suggests that in the circa 10,000 years since eastern Canada was deglaciated, approximately 100 surface ruptures should have been formed. Arguably we know none, though some potential fault candidates are beginning to emerge, and evidence of earthquake shaking events is accumulating. It is believed that the weight of the Greenland and Antarctic ice sheets inhibits current earthquake activity (Johnston), and that when the Scandinavian ice sheet melted the stored strain energy was released in a burst of large earthquakes in immediate deglacial times. A new assessment of the Swedish deglacial earthquakes suggests that they indeed represent the release of ~50,000 years accumulation at the level of seismic activity typical of stable cratons. If the combined Canadian ice sheets had the same effect, we should have had ~500 M=6 events occurring near the ice margin of the day, i.e. a rate 5 times higher than the postglacial rate. None have been confirmed as yet. Excuses might be made for the Canadian craton, as behaving differently from the Scandinavian craton due to the scale of its glaciation, or due to all the deglacial scarps forming under the ice and thus rapidly destroyed (as might have been the case in southern Sweden). But the gap between the predicted and the known remains profound.
Bibliography of Canadian Geomorphology