Is the Vs30-based Site Amplification of the 2020 National Building Code Correct for Use in Canada?

ABSTRACT: The 6th generation Canadian seismic hazard model (6CSHM) introduced a significant change in predicting site amplification, directly affecting the seismic design ground motions adopted in the 2020 National Building Code of Canada (NBCC). This update involves directly using the time-averaged shear-wave velocity (Vs) of the top 30 meters (Vs30) for ground motion prediction. This paper examines whether the inherent gradational-Vs-based linear site amplification of the ground motion models governing the 2020 NBCC seismic design ground motions is appropriate for Canadian site conditions with significant seismic impedance contrasts (leading to seismic resonance) between post-glacial sediments and glacial till or rock. Representative 1-D layered site models are built from a wealth of available Vs depth profiling for 6 Canadian cities to calculate impedance-based site amplification to compare with the 2020 NBCC linear site amplification. Impedance-based site amplification is calculated using the quarter-wavelength (QWL) method, implemented in MATLAB. These results show that NBCC has a strong underestimation of the 6 cities’ site amplification at short spectral periods (< 0.5s). Future work is needed to examine site amplification inclusive of nonlinear soil behavior and considering higher shaking intensities.