Probabilistic seismic liquefaction hazard mapping of western Metro Vancouver, British Columbia, Canada

Alireza Javanbakht Samani, Western University

Abstract

The Metro Vancouver region lies in a seismically active region with numerous areas exhibiting thick liquefiable layers and shallow groundwater levels. The liquefaction susceptibility classification is verified by considering the thickness of liquefiable layers from 808 cone penetration tests (CPT). Probabilistic seismic-induced liquefaction assessment based on the national seismic hazard model of the 2020 National Building Code of Canada is accomplished to account for the contribution of all seismic sources to the liquefaction evaluation. The liquefaction hazard curves from 900 CPT and shear wave velocity profiles are generated by incorporating the liquefaction potential index (LPI) and Ishihara-inspired liquefaction potential index (LPI_ISH) into the performance-based liquefaction assessment. The first probabilistic liquefaction hazard maps of Metro Vancouver are presented using the targeted return periods of 475 and 2,475 years. These liquefaction hazard maps reveal that liquefaction manifestations are expected in Richmond and Delta regions. In addition to considering seismic loading uncertainty, variability of soil resistance and the liquefaction triggering model are also accounted for through Monte Carlo simulation to embed the factor of safety against liquefaction (FS_L) and the required soil improvement to prevent liquefaction (Δq_L) into the performance-based assessment. It yields the FS_L and Δq_L hazard curves to determine the targeted design level. The probabilistic FS_L, Δq_L, liquefaction return period and H_cr maps built on a large number of in situ data and semi-sophisticated analyses should be used together to mitigate against liquefaction hazard in the region. Useful regression models to predict Δq_L for return periods of 475 and 2,475 years from the soil resistance are developed. Deterministic analyses are performed for Cascadia subduction zone earthquakes using ground motion prediction equations of the 6^th national seismic hazard model to determine the range of M_w-a_max combinations that result in the observed paleo-liquefaction features in the region. A probabilistic framework is developed to account for the uncertainties of soil resistance, groundwater table depth, maximum ground acceleration, and the liquefaction model. Our probabilistic results also reveal that Cascadia interface earthquakes with M_w > 8.9 lead to a 31-57% probability of liquefaction triggering in the region. Magnitude-bound curves are also developed for Cascadia interface earthquakes.