Seismic Site Characterization and Response in Metropolitan Vancouver

Jamal Assaf, The University of Western Ontario

Abstract

Metropolitan (Metro) Vancouver is the largest city in British Columbia and has the highest earthquake risk in Canada due to vulnerability of persons and infrastructure located near the Cascadia subduction zone. The effects of local geology on seismic waves (site effects) play a major role in determining surface shaking levels and seismic hazard. Site effects are typically accounted for in ground motion prediction by inclusion of important seismic site characteristics. This thesis first evaluates the seismic site characteristics across Metro Vancouver then facilitates the development of a region-specific site amplification model.

To evaluate site characteristics in Metro Vancouver, a comprehensive database of seismic site condition measures is compiled to develop important predictive site characteristic models for site-specific and regional application. Non-invasive surface wave measurements are conducted to obtain the depth to glacial till (z_gl) and deep shear wave velocity (Vs) profiles with their uncertainties in the Fraser River Delta (FRD). A model to predict z_gl based on the 2^nd peak frequency of the microtremor horizontal-to-vertical spectral ratio (MHVSR) is proposed. Seismic Cone Penetration Testing (SCPT) soundings are compiled to derive a CPT-to-Vs model using traditional linear and nonlinear regressions along with two machine learning approaches and a combination of CPT parameters. Machine learning approaches show slightly improved prediction accuracy compared to traditional regression due to the availability of a large database. A comprehensive Vs database is collected from available invasive and acquired non-invasive in situ measurements in the region. Generic Vs-depth relationships of post-glacial, glacial and rock geologies are developed. A comprehensive Vs₃₀, the time-averaged shear wave velocity to depth 30 m, database is established from direct Vs measurements and via conversion of other in situ measurements using the developed predictive models. Region-specific Vs₃₀ prediction models are developed based on mapped geology and topographic slope or z_gl.

To facilitate the development of a region-specific site amplification model, one-dimensional (1D) site response analyses (SRA) of the Metro Vancouver region are conducted. Input time histories for the three earthquake source types are scaled at 2 and 10 % probability of exceedance in 50 years consistent with the national probabilistic seismic hazard model. The predicted linear and nonlinear SRA de/amplification spectra at 8 selected sites are compared to a site amplification model for a western North America (WNA) ground motion model (GMM). The WNA amplification model overestimates or underestimates linear site amplification for deep and shallow sites in Metro Vancouver respectively. For thick post-glacial sediments in the FRD and North Shore, stronger nonlinearity is predicted than exists in the WNA amplification model. Potential sources of 1D SRA uncertainties in Metro Vancouver are quantified via sensitivity analysis. The 1D SRA accomplished in this thesis are an important step towards developing a region-specific site amplification model that can be combined with developed site characteristics mapping to produce the first 1D site amplification map for Metro Vancouver.