Felipe Kuncar

V _{S 30} ‐based site amplification factors are commonly used to account for local site effects in hybrid broadband ground‐motion simulation when site‐specific data are limited. Validation against observations allows for testing alternative semi‐empirical V _{S 30} ‐based models and establishing proper protocols for their application in forward analyses. This article validates alternative implementations of V _{S 30} ‐based site factors using 5218 ground motions from 479 small‐magnitude events recorded at 212 sites in New Zealand, which represent a wide range of site conditions. The investigation of (Fourier) effective amplitude spectra (EAS) residuals allows for a direct assessment of the low‐frequency (LF; f < 1 Hz) and high‐frequency (HF; f > 1 Hz) components of the hybrid broadband simulation approach, and the corresponding effect of the site “adjustment.” Before applying the site factor, the LF simulation exhibits systematic underprediction and the HF simulation displays systematic overprediction. The use of site factors based on semi‐empirical models for EAS, developed in the last decade, results in comparable, or slightly better, prediction performance compared with response spectra‐based models commonly used in previous validation studies. A greater LF underprediction was identified at sites located within sedimentary basins that are less constrained by site data or not properly modeled in the LF simulation due to limitations in spatial discretization. A simple protocol is proposed for the application of the LF site factor based on a basin‐type and geomorphic site categorization, which reduces the variability of EAS site‐to‐site prediction residuals by up to 0.1 natural log units. The explicit incorporation of the parameter Z _1.0 , or alternatively, the use of a host‐to‐target correction factor demonstrates that a significant portion of the HF systematic overamplification in the frequency range 1.0–3.0 Hz can be explained by V _S profile features not accounted for by V _{S 30} alone. These findings can inform the application of site adjustments in future studies and guide advancements in the LF and HF simulation components.

Proper incorporation of shallow site effects in hybrid broadband ground-motion simulation is essential to improving predictions at soil sites. This paper validates and compares four methods to account for these effects in the predominantly linear regime, using 1446 ground motions from 213 small-magnitude earthquakes ( 3 . 5 ≤ M W ≤ 5 . 0 ) recorded at 38 sites in New Zealand representative of a wide range of soil conditions. These methods require V S 30 or a shear-wave velocity profile, with the second approach relying on either the square-root-impedance method or the theoretical one-dimensional (SH1D) transfer function. Incorporating shallow site effects with any of the considered methods significantly improves accuracy and precision for several intensity measures, including pseudo-spectral acceleration at short vibration periods. Although the reduction in prediction variability was comparable between methods, the SH1D-based approach showed particular benefits in the case of (1) stiff sites for which the simulation overestimates attenuation effects; (2) relatively stiff sites with high-frequency resonances; (3) relatively soft sites with strong resonances in the intermediate frequency range; and (4) sites with a near-surface predominant frequency lower than the frequency range considered for the application of the V S 30 -based adjustment. This study shows that the potential prediction improvements obtained by using more advanced site-adjustment methods that incorporate additional site-characterization data are highly dependent on the site characteristics and the quality of the ground-motion simulation in a given region. These findings can help to inform site response method selection in forward applications.

Shallow site effects are indirectly considered in conventional hybrid broadband ground‐motion simulations, and their proper incorporation may be key to improving ground‐motion predictions at soil sites. This article presents and examines five methods to adjust hybrid simulations to account for these effects. These methods use different approaches to modeling site response and require different amounts of site‐characterization data: Methods 1 and 2 use only proxy parameters (e.g. , ) to describe the site conditions, with Method 1 relying solely on proposed site response scaling factors in existing semi‐empirical ground‐motion models, and Method 2 incorporating a host‐to‐target velocity‐profile 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 1D time‐domain nonlinear site‐response analysis and generally requires additional data to constrain nonlinear constitutive‐model input parameters. The five methods are applied to four sites subjected to two levels of ground‐motion intensity to illustrate the challenges involved in their implementation and to compare and contrast the resulting adjustments in terms of site amplification. The suitability and performance of each method will depend on multiple factors, including the availability of site‐characterization data and complexity of the site, and hence, their relative advantages and disadvantages are discussed considering a broad range of scenarios.

This study develops a method for estimating site amplification that combines instrumentally observed site‐specific amplification factors with adjustment factors from nonlinear site‐response analyses. This approach provides estimates of site response for large‐strain motions based on observations and sophisticated nonlinear modeling. 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 scaled strong ground motions to assess the influence of nonlinearity on site amplification factors and demonstrate the approach. Different shear‐wave velocity ( V _S ) profiles, constitutive models, and modeling approaches (e.g. one‐dimensional (1D) site‐response analyses vs empirical ‐based approaches) are used to quantify the sensitivity and modeling 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 amplification from the effects of soil nonlinearity. However, at stiffer sites, which generally experience less basin amplification in observations, there may be moderate amplification at longer periods when nonlinearity is considered due to softening of the soil profile. Empirical ground‐motion models were found to under‐represent the observed amplification between basin sites and the nearby reference site, especially at intermediate to long periods, corresponding to resonant frequencies of these basin sites. In addition, the empirical nonlinear site amplification models (‐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.