Earthquake Site Response Studies in Victoria, B.C. Using Weak Motion Recordings, Microtremor Data, and SHAKE Modelling

We have examined the earthquake site response across greater Victoria, British Columbia. The surface geology across this region varies significantly and is dominated by thin soils situated on crystalline rock with a number of deeper pockets of soft clay soils. Detailed geological and geophysical data are available for most of the city. We have utilized a variety of techniques and data sets to estimate seismic site response and compare our results with the predicted amplification based on surface geology. Recordings of six recent earthquakes, including the Mw=6.8 Nisqually, Washington event (150 km from Victoria), provide weak motion data with peak horizontal acceleration varying from 1%g to 3.5%g. These events were recorded on a variety of geological conditions including bedrock, thin (<3 m) soft soil (NEHRP class E), and thick (5-12 m clay and till) soil (NEHRP class C and E). Microtremor measurements were also made at each of these sites. Earthquake site response was estimated using standard spectral ratios (bedrock reference sites) and H/V ratios. The results from the two techniques at each site are remarkably similar both in frequency and, notably, in amplitude. H/V ratios of the microtremor data were in good agreement with those of the earthquake data. Overall, sites with < 3 m of soil exhibit a relatively flat site response at frequencies < 10 Hz. Sites with 5-12 m of clay show peak amplification up to six times that of bedrock at frequencies of 2-5 Hz. SHAKE modelling of the observed Fourier amplitude spectra demonstrated that the peak amplification at each site could be attributed to the local geology amplifying the ground motion. A flat site response <10 Hz occured for < 2.5 m of soil, and peak amplification between 2-5 Hz was modelled by 10 m of clay, thicker amounts of Pleistocene till, or a combination of both.