Site amplification model for use in ground motion prediction equations
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The characteristics of earthquake shaking are affected by the local site conditions. The effects of the local soil conditions are often quantified via an amplification factor (AF), which is defined as the ratio of the ground motion at the soil surface to the ground motion at a rock site at the same location. Amplification factors can be defined for any ground motion parameter, but most commonly are assessed for acceleration response spectral values at different oscillator periods. Site amplification can be evaluated for a site by conducting seismic site response analysis, which models the wave propagation from the base rock through the site-specific soil layers to the ground surface. An alternative to site-specific seismic response analysis is site amplification models. Site amplification models are empirical equations that predict the site amplification based on general characteristics of the site. Most of the site amplification models that already used in ground motion prediction equations characterize a site with two parameters: the average shear wave velocity in the top 30 m (VS30) and the depth to bedrock. However, additional site parameters influence site amplification and should be included in site amplification models. To identify the site parameters that help explain the variation in site amplification, ninety nine manually generated velocity profiles are analyzed using seismic site response analysis. The generated profiles have the same VS30 and depth to bedrock but a different velocity structure in the top 30 m. Different site parameters are investigated to explain the variability in the computed amplification. The parameter Vratio, which is the ratio of the average shear wave velocity between 20 m and 30 m to the average shear wave velocity in the top 10 m, is identified as the site parameter that most affects the computed amplification for sites with the same VS30 and depth to bedrock. To generalize the findings from the analyses in which only the top 30 m of the velocity profile are varied, a suite of fully randomized velocity profiles are generated and site response analysis is used to compute the amplification for each site for a range of input motion intensities. The results of the site response analyses conducted on these four hundred fully randomized velocity profiles confirm the influence of Vratio on site amplification. The computed amplification factors are used to develop an empirical site amplification model that incorporates the effect of Vratio, as well as VS30 and the depth to bedrock. The empirical site amplification model includes the effects of soil nonlinearity, such that the predicted amplification is a function of the intensity of shaking. The developed model can be incorporated into the development of future ground motion prediction equations.