SETTLING OUR DIFFERENCES: ASSESSING THE VARIATIONS BETWEEN CANADIAN AND AMERICAN SEISMIC HAZARD MAPS IN EASTERN NORTH AMERICA

HALCHUK, S.C., and ADAMS, J.E., Seismic Monitoring and Analysis, Geological Survey of Canada, 7 Observatory Crescent, Ottawa, ON, K1A 0Y3 Canada, halchuk@seismo.nrcan.gc.ca.

The Geological Survey of Canada's new seismic hazard model for Canada forms the basis for the seismic design provisions of the 2005 National Building Code of Canada (NBCC). In eastern Canada the Cornell-McGuire method is used with two complete earthquake source models, based on historical and regional/geological considerations, to represent the uncertainty in where (and why) earthquakes will happen in the future. A third probabilistic seismic source model for the nearly aseismic central part of Canada is assessed based on a global model of stable continental regions. A "robust" method is used to combine the probabilistic hazard estimates from the three source models: the mapped value is the largest of the values. In the eastern United States the USGS employs spatially smoothed representations of historic seismicity to directly calculate probabilistic hazard without the use of subjective source zones. Data from individual faults is also used in the calculation. Despite the different methodologies and the application of different strong ground motion relations, preliminary results generally indicate good agreement at the border. Of course not all the hazard captured by the GSC's "geological" model is represented in the USGS results (eg. where large earthquakes may happen in areas of low historical seismicity). The reasons behind the differences will be explained where they are evident. An inter-comparison of relative hazard levels between representative eastern Canadian and U.S. cities will be made.