Earthquake recurrence data for eastern Canada are used to derive estimates of probabilistic seismic risk at the low levels desirable for critical structures. The historical earthquake data (1661 to 1975) are employed to produce a seismicity model consisting of six zones of earthquake occurrence in the continental region and an isolated zone on the Grand Banks. Magnitude recurrence relations of the form
are derived for each of the zones. N5(N ≧ M5) ranges from 0.0003 earthquakes per annum per 104 km2 in the general background zone to 0.08 in the confined Charlevoix Zone in the St. Lawrence Valley; β values range from 1.5 to 2.4.
Seismic risk estimates (probability of exceedence of peak ground acceleration) are derived using the Cornell method of integrating risk contributions from the zones of earthquake occurrence. For sites within the larger zones of relatively low seismicity, e.g., Northern Appalachian Zone, with little influence from adjacent zones, the risk estimates near 10−3 per annum are relatively insensitive to the uncertainties in the magnitude recurrence parameters; the maximum risk contribution comes from the lower magnitude earthquakes at the near distances. For sites within the range of influence of highly active areas such as the Charlevoix Zone, peak ground motions with risk of exceedence near 10−3 per annum can vary by a factor of two depending on assumptions of the magnitude recurrence extrapolation and the maximum magnitudes. Constraints on the seismicity based on tentative geological correlations increase ground motion for sites near some active zones by factors of up to 1.5 at the same risk level compared to that derived from the model based only on historical earthquake distributions.
With the present knowledge of tectonic processes in eastern Canada, the uncertainties in magnitude recurrence extrapolations and maximum magnitudes, the assumptions about the temporal and spatial stationarity of earthquake zones, and the uncertainties on near-field effects and attenuation, seismic risk estimation at probabilities below 10−3 per annum becomes increasingly deterministic.