New probabilistic strong seismic ground motion maps of Canada

Abstract

Abstract New probabilistic strong seismic ground motion maps of Canada are derived by applying the “Cornell-McGuire” method to 32 earthquake source zones in Canada and adjacent regions. Peak horizontal acceleration and peak horizontal velocity are mapped to depict strong ground motion in the frequency ranges near 5 Hz and near 1 Hz, respectively. Some earthquake source zones are well defined on the basis of both tectonics and average seismicity rates, but a lack of understanding of the near-field effects of the large earthquakes prohibits accurate estimates of ground motion in their vicinity. Some source zones have no known large-scale crustal features or zones of weakness that can explain the seismicity and must, therefore, be defined simply as concentrations of seismic activity with no geological or geophysical controls on the boundaries. Other source zones lack information on low-magnitude seismicity that would be representative of longer periods of time. A significant source of uncertainty in the ground motion estimates is introduced by the uncertainty in strong ground motion attenuation relations, particularly in the near-field of large earthquakes and particularly in eastern Canada where no strong motion data are available for earthquakes larger than magnitude 5. Valid estimates can be made of relative levels of probabilistic strong ground motion on a regional basis at moderate probabilities; however, the simple peak ground motion parameters are not sufficient for engineering design purposes in the immediate vicinity of large earthquakes. A comparison of the new probabilistic ground motion estimates at the Canada-United States border with equivalent estimates made in the United States reveals that peak acceleration contours match well in the British Columbia-Washington State, Yukon-Alaska, and Lake Ontario border regions. Canadian peak velocities are a factor of two higher near the British Columbia-Washington State border. Canadian peak accelerations are higher in the border regions of eastern Canada influenced by the Charlevoix zone, and in regions of the Alaska Panhandle-British Columbia border influenced by the active Queen Charlotte and Fairweather fault zones. The new ground motion estimates are used to produce new peak acceleration and peak velocity zoning maps to replace the 1970 (peak acceleration) zoning map in the National Building Code of Canada. The acceleration and velocity maps provide independent ground motion reference levels for small rigid structures and taller flexible structures, respectively. Compared to the 1970 map with four zones, the new maps with seven zones have a finer subdivision of zoning in moderate risk areas and additional zones in the high risk areas. A change in probability of exceedence from 0.01 per annum on the 1970 map to 10 per cent in 50 yr on the new maps, and the addition of velocity as a zoning parameter, have necessitated changes in the seismic design provisions. These revisions will be incorporated into the 1985 edition of the Code.