Site response analysis and seismic surveying of high contrast soil profiles for the city of Ottawa.

摘要

Seismic surveying and site response analysis of several sites in the city of Ottawa were studied. The motivation for this research arose due to two main reasons: First, Ottawa is rated third in terms of highest seismic risk in Canada. Second, some sparse earthquake recordings in the region along with the surficial geology of the city accentuate the hazard. The surficial geology of the city consists of high velocity contrast soil-bedrock formations that can trap seismic waves and lead to large seismic amplification.;Site response analysis using the finite element method confirmed the observed large amplification ratios from the weak motion recordings, but on the other hand, indicated limited peak amplification ratio for the shaking intensities used in geotechnical/structural engineering practice. It was also noted that the variation in the peak amplification for a given shaking intensity is not significant when soil-bedrock velocity contrast ratio exceeds about twelve. Strong correlation between low and high frequency amplification ratios with the average shear wave velocity in top thirty meters was obtained in the examined sites. These amplification ratios are comparable with the values specified in the 2005 edition of the National Building Code of Canada (NBCC) for the Ottawa region.;The peak amplification values obtained from two-dimensional site response analysis of a deep valley in the study area were in good agreement with those obtained for a nearby one-dimensional site. At higher frequencies, minor discrepancies between two-dimensional and one-dimensional cases occurred.;A fast seismic surveying method, the multi-channel analysis of surface waves, was implemented to discern the high shear wave velocity contrast in Ottawa area and the capability of random search inversion was verified for most of the examined profiles. The reflection/transmission coefficients method for site response analysis, which can simulate the internal wave reverberations, was modified using a shaking-intensity damping scheme.