Site Response in Victoria, British Columbia, from Spectral Ratios and 1D Modeling

This article examines site response in Victoria, British Columbia, using weak ground-motion recordings (<3.5%g) of the 2001 Mw 6.8 Nisqually earthquake in Washington State, 150 km distant. Significant variations are observed in acceleration spectra across the city that can be largely attributed to local site conditions. Thin soil sites (<3 m) generally have flat-amplitude spectra at frequencies less than 10 Hz, whereas thicker soil sites (5-11 m) show peak amplitudes at 2-5 Hz. Standard spectral ratios (bedrock reference) and horizontal-to-vertical spectral ratios are computed for sites with varying geology. Thin soil sites show flat-site response (like bedrock) at frequencies <10 Hz, whereas the thicker soil sites show peak amplification of up to six times that of bedrock at frequencies of 2-5 Hz. There is good agreement between these standard spectral ratio site-response estimates with the horizontal-to-vertical spectral ratios at each site and spectral ratios from four other earthquakes between 1996 and 2002. Ground-motion spectra obtained from numerical 1D modeling indicate that <3 m of soft soil (National Earthquake Hazard Reduction Program [NEHRP] class E) generates a flat response at <10 Hz, whereas thick layers of either "grey Victoria clay" (NEHRP class E) or Pleistocene till (NEHRP class C) produce peak amplitudes at 2-5 Hz. The correlation of near-surface soft soil conditions on crystalline bedrock with observed weak ground motions provides an important baseline toward future seismic hazard studies in the Victoria area.