A Methodology for the Development of 1D Reference VS Profiles Compatible with Ground-Motion Prediction Equations: Application to NGA-West2 GMPEs

Abstract

ABSTRACT Site response in ground-motion prediction equations (GMPEs) is primarily characterized as a function of the time-averaged shear-wave velocity over the top 30 m of the site profile (VS30). Although the use of VS30 as a main site-response predictor parameter is practical, GMPE site adjustments to different target regions or target site conditions require characterization of the GMPE’s rock-site response in terms of host VS profile and host kappa. Regional VS profiles and kappa values have been traditionally used to characterize GMPEs host site conditions. These regional site properties may not reflect the average site response in GMPEs. We present a methodology, based on the quarter-wavelength principles, that allows the derivation of GMPE-compatible host 1D VS profiles and kappa values. This methodology is applied to the Next Generation Attenuation-West2 (NGA-West2) GMPEs to derive GMPE-specific host VS profiles and kappa for western United States (WUS) site conditions with VS30 of 360, 490, 620, 760, and 1100 m/s. This application uses, for input, the GMPEs’ site response in Fourier amplitude spectra domain relative to a reference VS30 of 1000 m/s and requires an assigned VS profile for the reference site condition. The impact of the choice of reference VS profile on the results is not large. Comparisons of the derived GMPE-specific VS profiles for VS30 of 760 m/s show differences in the host VS profiles among the NGA-West2 GMPEs for the same site condition in WUS. Differences are also observed when comparing the derived GMPE-compatible VS profiles with the commonly used profiles for WUS for VS30 of 760 m/s. These differences highlight the importance of using GMPE-compatible VS profiles and kappa in GMPE adjustments and in site-response analyses. Limitations of this approach for soft site conditions are discussed.