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 Earth Sciences Sector > Priorities > Canadian Hazards Information Service > Earthquakes Canada |
Lower Probability Hazard, Better Performance?
Understanding the Shape of the Hazard Curves from
Canada's Fourth Generation Seismic Hazard Results
John ADAMS, Stephen HALCHUK and Dieter WEICHERT
presented at the 12th World Conference on Earthquake Engineering,
Auckland, New Zealand, January 30 - February 4, 2000
Canada's fourth generation seismic hazard model was released for public comment in 1996 and will be revised to form the basis for seismic design codes in the year-2003 edition of the National Building Code of Canada (NBCC). The Cornell-McGuire method is used, with two complete earthquake source models together with a deterministic Cascadia subduction earthquake and a floor hazard for the low seismicity regions. A "robust" method is used to combine the model results: the mapped value is the largest of the four values. Spectral-value and peak-acceleration maps and Uniform Hazard Spectra for main cities have been produced for median (50th percentile) ground motions at a 10% and a 2% probability of exceedence in 50 years. Despite different methods and strong ground motion relations, our results generally agree at the border with the 1997 US NEHRP maps. Engineers consider that the current NBCC, formulated with California experience in mind and based on 10%/50 year hazard values, provides adequate seismic safety in Vancouver, and thence proportionately across the country. However, the complexity of seismological inputs is such that no simple engineering factor ("overstrength" or "experience factor" or other) can convert those moderate-probability hazard maps into design values intended to provide uniform protection against low-probability building collapse (even across California). Thus the 2%/50 year hazard results are considered a better basis for achieving a uniform level of building safety across Canada, as they are closer to the acceptable frequency of collapse. Use of the 2%/50 year values increases design levels in the low-moderate seismicity eastern Canada by about 15-30% relative to the moderate-high seismicity western Canada.
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