Estimation ofκ0Implied by the High‐Frequency Shape of the NGA‐West2 Ground‐Motion Prediction Equations

Abstract

The spectral‐decay parameter κ is often used to account for the reduction of the high‐frequency amplitude of ground motion caused by attenuation within the site profile. In this study, we used the inverse random vibration theory approach described by Al Atik et al. (2014) to calculate Fourier amplitude spectra from predicted values of response‐spectral acceleration for all five Next Generation Attenuation (NGA)‐West2 ground‐motion prediction equations (GMPEs). We used these spectra to estimate κ using the spectral‐decay method. Each GMPE was evaluated for a National Earthquake Hazard Reduction Program B/C site condition and for default estimates of depth to the top of rupture, hypocentral depth, and sediment (basin) depth. We derived estimates of κ for magnitudes ranging from 3.5 to 8.0 and distances ranging from 5 to 20 km and used a mixed‐effects model to derive equations for these estimates as a function of magnitude. We also calculated κ from the geometric mean of the response‐spectral accelerations of the GMPEs to check the sensitivity of the results to the two approaches and found that the values of κ derived in this study using a mixed‐effects model are in good agreement with these estimates. The values of κ obtained in this study do not necessarily represent the physical high‐frequency damping within the site profiles used to develop the NGA‐West2 GMPEs. Instead, they are intended to represent the high‐frequency shape of the median predicted spectral accelerations from the GMPEs. The κ model developed in this study can be used in inversions to develop stochastic models that are intended to mimic the predictions from the NGA‐West2 GMPEs. Online Material: Tables of random effects result on coefficients κ , and figures of Fourier amplitude spectrum.