SITE RESPONSE STUDIES IN VICTORIA, B.C., ANALYSIS OF MW 6.8 NISQUALLY EARTHQUAKE RECORDINGS AND SHAKE MODELLING

摘要

The largest earthquake generated by the Cascadia subduction zone in over 50 years occurred on 28 February 2001. The Mw 6.8 Nisqually earthquake was located 150 km SE of Victoria, the capital city of British Columbia. The shaking of the earthquake caused peak horizontal ground accelerations (PGA) varying from 1.0 to 3.5 %g, and variation in Modified Mercalli Intensity (MMI) from not felt (I) to slight damage (VI) in greater Victoria. Amplification of seismic waves due to the local geology is investigated as the cause of varying PGA and MMI across greater Victoria. Site-specific comparisons of earthquake intensity and geology indicate significant differences in observed felt effects between high and low shearwave velocity substrates (bedrock and Pleistocene till versus soft clay and peat). Overall, a trend of increasing intensity with decreasing shear-wave velocity was found for geologic units in greater Victoria. Standard spectral ratios (bedrock reference) and horizontal-to-vertical spectral ratios computed for each site are similar in period and amplitude. Thin soil sites (< 3 m) exhibit a relatively flat site response at periods > 0.1 s (< 10 Hz) like bedrock, whereas thicker soil sites (5-11 m) show peak amplification up to six times that of bedrock at 0.2-0.5 s (2-5 Hz). SHAKE modelling was conducted to compare with the observed Fourier amplitude spectra. The comparison showed that the peak amplification at each site could be attributed to the local geology amplifying the ground motion. A flat site response at periods > 0.1 s occurs for < 2.5 m of soil, and peak amplification between 0.2-0.5 s was modelled by 10 m of soft clay, thicker amounts of Pleistocene till, or a combination of both.