Abstract

The surficial geology of the city of Ottawa primarily consists of soft soil sediments with low shear‐wave velocities (forumla) underlain by hard bedrock with very large shear‐wave velocities (forumla). Earthquake recordings show unusually large seismic amplification values for weak motion. These unusually large seismic amplification factors were reconfirmed with the earthquake spectral ratio method using two stations, the horizontal‐to‐vertical earthquake spectral ratio method using a single station, and the horizontal‐to‐vertical spectral ratio technique using background noise. These findings were the motivation for carrying out an extensive site response analysis, using finite element modeling (FEM), as a part of the seismic microzonation studies for the city of Ottawa. The FEM results confirmed the large amplification ratios for weak‐motion recordings. FEM analysis was also carried out using a selection of strong‐motion time series for the study area. The combined effect of the soil–bedrock acoustic impedance contrast and the level of ground shaking on the variation of soil amplification factors for the fundamental frequency were investigated. The maximum value of the soil amplification factor for the fundamental frequency increased with increasing impedance contrast ratios until the soil/bedrock acoustic impedance contrast ratio reached values that were usually greater than 12; however, the change in peak amplification was much less with subsequent increases in the contrast ratio beyond that value. As expected, the value of the soil amplification factors for the fundamental frequency decreased with increasing peak ground acceleration (PGA) of the input motion due to nonlinear soil damping. Finally, for the Ottawa region, a mathematical model is suggested for soil amplification factors at the fundamental frequency, as a function of the soil/bedrock acoustic impedance contrast ratio and the PGA of the input motion.

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