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

摘要

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