IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS004 Poster presentation 1902
Where are all the young faults scarps in eastern Canada
Dr. John Adams
Geological Survey of Canada Geological Survey of Canada IASPEI
Seismicity rates in eastern Canada (east of the Rockies) suggest a rate of 7 M>=4 (approximate
completeness threshold) per year and beta = 2.09. Thus we predict about 0.1 M>=6 events/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 factor of ~10, viz ~5 to allow for distribution of events in depth
through the seismogenic crust and also ~2 to allow for underwater surface ruptures. 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 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, ~100+ surface ruptures should have been formed. Arguably we
know none, though some candidates are beginning to emerge, and evidence of earthquake shaking
events is accumulating. If the Canadian ice sheets had the same effect of suppressing the seismicity for
~50,000 years as in Fennoscandia, we should have had ~500+ deglacial M>=6 events occurring near
the ice margin of the day, i.e. 5 times more than the postglacial number. 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 (perhaps as in southern Sweden). But the gap between the predicted and the known
remains profound.
Keywords: paleoseismic, fault scarp, canada