Assessment of Seismic Site Amplification at Newly Deployed Earthquake Early Warning Stations in Canada

Abstract

A well-implemented Earthquake Early Warning (EEW) system can warn populations and critical infrastructure operators about imminent shaking from earthquakes. The Canadian EEW network covers regions of high seismic hazard, dense population, and critical infrastructure. In some regions, site amplification due to soft soils and sediments are expected to cause deviations from predicted ground motion, which impacts the dissemination of alerts from the EEW system. The goal of this study is to provide empirical site amplification factors at EEW stations which reflect deviations from expected ground motion with respect to a hard rock (reference) site. We calculate the horizontal-to-vertical spectral ratios (HVSRs) at each site using ambient seismic noise and estimate the predominant site frequency (fₛᵢₜₑ) using a systematic Gaussian curve-fitting algorithm. From a total of 172 Canadian EEW stations investigated, 34% show site amplification compared to 14% of the stations outside of the EEW network, which are predominantly located on bedrock. Site amplification is assessed at stations with available earthquake data using the ground motion spectral ratio (GMSR), and station magnitude residual methods. We show consistency in fₛᵢₜₑ calculated from both the HVSR ambient noise or GMSR strong motion methods. We evaluate and compare the calculated ground motion amplifications with station magnitude residuals to show that both provide a reasonable estimate of overall site amplification and magnitude overestimation. Finally, by defining a relationship between fₛᵢₜₑ and site amplification, we calculate site amplification at the newly deployed EEW stations using only ambient noise data.