Searching for Empirical Nonlinear Site Response Applicable to Greater Vancouver, British Columbia

Greater Vancouver, British Columbia, Canada, is an area prone to strong earthquakes, located at the northern end of the Cascadia subduction zone. Nonlinear soil response is often quantified by comparing strong main-shock recordings with weak after-shock recordings. Such empirical studies have described nonlinear behavior occurring when peak ground accelerations (PGA) exceed 0.1 g which manifest as a decrease in shaking amplitude in combination with a shift in the predominant frequency towards lower frequencies.

In Vancouver, all the recorded earthquake motions are weak; maximum PGA of 0.053 g during the 1976 magnitude 5.3 Pender Island earthquake. To constrain the expected nonlinear soil response in Greater Vancouver, we are searching for seismic stations elsewhere in the world with available weak-to-strong motion recordings that have similar linear site response to Greater Vancouver as well as similarities in peak frequency, shear wave velocity (Vs) depth profile and geological setting, to then constrain the degree of empirical nonlinear soil response for future large earthquakes affecting the ground in Vancouver.

We have identified seismic stations in Alaska, Japan, Mexico City, and Spain that meet our equivalency-to-Vancouver criteria. The Spain station Vs depth profile is most similar to the Fraser River delta, but it also lacks strong motion recordings and is therefore eliminated. We have proceeded to downloading weak-to-strong acceleration records of three selected stations in Mexico City and calculating earthquake Horizontal to Vertical Spectral Ratios (eHVSR) to quantify variation in amplification spectra with shaking intensity. Two Mexico City lakebed stations (CE23, TH35) have equivalent 0.1-0.3 peak frequency with the deep Fraser River delta in Greater Vancouver, in contrast to the Mexico City transition zone station DR16 that has an equivalent 1-2 Hz peak frequency with the Fraser River delta edge in Greater Vancouver. Even though the equivalency in site condition criteria is met, the number of strong motion records for which nonlinear soil response could be triggered is few (< 3 events). Overall, this study highlights continued need for performing and documenting in situ site characterization at seismic stations worldwide to achieve international standards in station metadata.