Low probability hazard and the National Building Code of Canada
The determination of 1/5,000 or 1/10,000 year (0.0002 or 0.0001 per annum) seismic hazard is normally required only for special facilities such as nuclear power plants or dams which have a large consequence if they were to fail. These low probabilities are beyond the scope of the current National Building Code of Canada (NBCC), which is intended to be used for standard structures at a probability of 1/2475 years (0.000404 p.a.). Extrapolation of the hazard model to lower probability results is mathematically possible, but represents an uncertain extrapolation of the model, and may be unreliable due to (for example) the crudeness of the seismic source zones used in the model.
Having said that, we give some guidance by providing the 10%/50 year (1/475 year or 0.0021 per annum probability) values from the 4th Generation seismic hazard model in addition to the 2%/50 year 2005 or 2010 NBCC probability values. You can determine the seismic hazard at these two probabilities for any point in Canada by using our seismic hazard calculator. You can then plot these two sets of values on a log-log scale and extrapolate them out to the 1/10,000 year return period, with the understanding that we cannot vouch for the validity of these extrapolated values at your particular site. We believe that for most sites in Canada, these extrapolated 1/10,000 year values will be slightly conservative compared to the precise 1/10,000 year values calculated directly from our model (though precise, the values may be inaccurate). These values can be used as a screening tool to determine if a site-specific seismic hazard assessment is warranted.
If your project requires it (because of the consequences of failure), a site specific hazard assessment developed by consulting engineers would be required to determine the 1/10,000 year hazard. They would have to perform detailed investigations of the local earthquakes and nearby earthquake sources and/or faults in order to better determine the very low probability hazard for the site.
Sample extrapolation of 0.0021 p.a. and 0.000404 p.a. hazard values to a 0.0001 p.a. value, to be used for screening purposes only to determine if a site specific seismic hazard assessment is warranted.
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