Comparing alternative methods to account for shallow site effects in hybrid broadband ground-motion simulations

Abstract

Shallow site effects are usually not explicitly modelled in hybrid broadband ground-motion simulations, and their proper incorporation may be key to improving prediction at soil sites. This paper examines five methods to adjust hybrid simulations to account for these effects. These methods require different amounts of site-characterization data: Methods 1 and 2 only use proxy parameters (e.g., Vs30) to describe the site conditions, with Method 1 relying solely on proposed site response scaling factors in existing ground-motion models and Method 2 incorporating a host-to-target adjustment. Methods 3 and 4 use a shear-wave velocity profile along with two different frequency-domain approaches to predict the linear site response, coupled with the nonlinear component of Method 1; and Method 5 uses time-domain site-response analysis, and generally requires additional data to constrain nonlinear constitutive model input parameters. Partial results of a validation study using 1000+ ground motions recorded at multiple strong-motion stations in the Canterbury Region (New Zealand) are provided. The results show that the incorporation of shallow site effects can significantly improve the accuracy of simulations. Method 1 tends to produce overamplification at long vibration periods. An explanation for this phenomenon is provided in the paper and different strategies to mitigate this issue are discussed.