Seismic microzonation of a region with complex surficial geology based on nonlinear site amplification modelling

Seismic microzonation helps quantify the unpredictable impacts of seismic hazards through site amplification models that predict ground shaking intensity. A novel methodology for regional seismic microzonation is applied to Saguenay, Eastern Canada, a region with highly heterogeneous surficial geology. The high impedance contrast between bedrock and surficial sediments, combined with strong site resonance at specific periods, may contribute to relatively high seismic amplification. Site amplification maps were developed using a robust site amplification model based on one-dimensional (1D) nonlinear ground response analyses of fifty soil profiles, with real and synthetic input motions scaled to two National Building Code of Canada (NBCC) 2020 hazard levels: 2%/50yrs and 10%/50yrs probability of exceedance. The nonlinear site period (T NL ), computed from nonlinear ground response analyses, is a more effective parameter under nonlinear conditions than the natural site period (T 0 ) and is used to model nonlinear site amplification. Correlation equations were developed between site amplification and proxies such as average shear wave velocity over the top 30 m (V s30 ), average shear wave velocity for the soil column (V savg ), shear wave velocity ratio of rock and soil (V f ), T NL , total soil thickness (H soil ), total thickness of postglacial soils (H pg ), and total thickness of glacial soils (H till ). The optimal combination was found using V s30 , V savg , T NL , and H till , which enabled the development of seismic microzonation maps for site amplification in the Saguenay region. • Nonlinear response of 50 profiles shows max amplification ∼5 (10% PE) and ∼4 (2% PE). • Log(Vs 30 +T NL + Vs avg + H till ) gives the best amplification prediction. • NBCC 2020 misestimates both short- and long-period site amplification. • Numerical and empirical amplifications correlate strongly (R 2 = 0.8–0.9). • Seismic Microzonation maps show that shallow soils amplify short-periods, while deep soils amplify long-periods.