de la Torre CA, Bradley BA, Kuncar F, Lee RL, Wotherspoon LM, Kaiser AE. 2022. Combining observed linear basin amplification factors with 1D nonlinear site-response analyses to predict site response for strong ground motions: application to Wellington, New Zealand. Lower Hutt (NZ): GNS Science. 27 p. (GNS Science report; 2022/37). doi:10.21420/0XB3-0266.
Abstract
This study develops a method for estimating site amplification that combines instrumentally observed site-specific amplification factors with modification factors from nonlinear site-response analyses. This approach provides estimates of site response for large-strain motions based on observations and sophisticated nonlinear modelling. A database of weak- to moderate-intensity ground motions recorded in three basins of Wellington, New Zealand, is used to study the observed site amplification. A subset of nine strong-motion stations was selected to perform nonlinear site-response analyses with strong ground motions to assess the influence of nonlinearity on site amplification factors and demonstrate the approach. Different shear-wave velocity (VS) profiles, constitutive models and modelling approaches (e.g. 1D site-response analyses versus empirical VS30-based approaches) are used to quantify the sensitivity and modelling uncertainty in the nonlinear site-response analyses. It was found that, for soft sites subjected to strong ground motions, there may be a decrease in spectral acceleration amplification factors for periods up to approximately 2 s, relative to the expected linear site response. For longer periods, there is little to no additional amplification from the effects of soil nonlinearity. However, at stiffer sites, which generally experience less basin amplification in observations, there may be moderate amplification, relative to the linear case at longer periods when nonlinearity is considered due to softening of the soil profile. Empirical ground-motion models were found to significantly under-represent the observed amplification between the reference site and basin sites, especially at long periods. Additionally, the empirical nonlinear site amplification models (VS30-based) were found to deviate from nonlinear analyses at large strains (where such models are poorly constrained due to such a limited number of observations). (The authors)
