These issues are critical in evaluating the threat posed by subduction earthquakes. A magnitude 9 earthquake releases approximately 30 times the energy of a magnitude 8 event. Should the entire Cascadia subduction zone release during a magnitude 9 event, all cities in western Oregon, western Washington, and southwestern British Columbia might be affected. Vancouver, Seattle, and Portland are fortunate that the earthquake-generating portion of the subduction zone is located offshore and extends little if at all beneath the coast. However, shaking would decrease slowly with distance inland, and the hazard to these cities is considerable, even though they are 100 to 200 km from the source. The intensity of shaking in the frequencies that most structures are sensitive to would be no greater than that of a large crustal earthquake, although strong shaking would last much longer, perhaps several minutes. In addition, the energy spectrum of a subduction earthquake would have seismic waves with relatively long periods that might be especially damaging to some taller buildings.
A magnitude 8 subduction earthquake would affect a smaller total area than a magnitude 8 event. The shaking would not last as long, but would still be severe along the coast adjacent to the rupture. Although a magnitude 8 earthquake centred on the southernmost part of the Cascadia subduction zone might have no effect on Vancouver, a succession of such earthquakes, rupturing different parts of the subduction zone over a short period of time (days to perhaps years), might produce more damage to the entire coastal Pacific Northwest than a single magnitude 9 event. A disturbing prospect is that a magnitude 8 earthquake would not fully release the accumulated strain along part of the plate boundary and might lead to another earthquake in the same area some time later. The fear of a second or third large earthquake would impede reconstruction and the resumption of normal economic activity and life in cities affected by the first earthquake.
On a probability basis, estimates of accelerations and velocities in the Vancouver and Victoria areas might not be increased by including great earthquakes in seismic hazard assessments; however, the greater duration of shaking is something new that must be considered in the design of some buildings. In general, the hazard from great earthquakes is largest for sensitive structures (for which failure, even at low probabilities, is not acceptable) as well as structures likely to be damaged by lengthy shaking. It is also important to note that the provisions of the NBCC and most seismological studies estimate ground motions at bedrock sites. Additional site-specific studies are required to determine the effects of local soil conditions, as considerable amplification of ground motion can occur at resonant frequencies at some sites on thick soils.