Quantifying Basin Amplification in Southwest BC from Simulated M9 Cascadia Subduction Zone Earthquakes Kakoty, Preetish; Dyaga, S.; Molina Hutt, C.
Southwest British Columbia has the potential to experience large-magnitude earthquakes generated by the Cascadia Subduction Zone. Buildings in Metro Vancouver are particularly vulnerable to these earthquakes because the region lies above the Georgia sedimentary basin, which can amplify the intensity of earthquake ground motions. Studies of recorded ground motions and simulations have shown that deep sedimentary basins can greatly increase the intensity of earthquake ground motions at medium and long periods. Earthquake design provisions in Canada neglect basin amplification and the consequences of accounting for these effects are uncertain. By leveraging physics-based simulations of a suite of M9 Cascadia Subduction Zone earthquake scenarios, this paper develops site-specific and period-dependent basin amplification factors throughout Metro Vancouver. The M9 simulations, which explicitly account for basin amplification, are benchmarked against ground motion models (GMMs), i.e. BC Hydro, which neglect such effects. The results indicate that for sites outside the basin, the empirical and simulated seismic hazard estimates are consistent. However, for sites within the basin and periods in the range of 1 to 4 s, GMMs significantly underestimate the hazard. The simulated M9 ground motions are also benchmarked against probabilistic estimates of the hazard from the 2015 National Building Code of Canada. Four different hazard levels are considered: 2%, 5%, 10% and 40% probability of exceedance in 50 years. At sites within the Georgia basin, the M9 simulations, which have a return period of approximately 500 years, far exceed the 10% in 50-year probabilistic estimates of the hazard. The proposed basin amplification factors vary across the region as a function of basin depth, reaching values as high as 9.2 at a 2 s period. The proposed factors are intended for use in design until basin amplification is explicitly considered within Canada’s national seismic hazard model.
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