Region-specific ground-motion model for fourier amplitude spectra and its interfrequency correlation of epsilon for the Sichuan–Yunnan region, China

Abstract

Fourier amplitude spectrum ground-motion models (FAS-GMMs) provide an alternative framework for characterizing ground motions in the frequency domain; however, regional FAS-GMMs for China remain limited despite the high seismicity of the region. This study develops a regional FAS-GMM for the horizontal effective amplitude spectrum (EAS) applicable to earthquakes with moment magnitudes 3.0 ≤ M_w ≤ 6.7 in the Sichuan–Yunnan region of Southwestern China using available strong-motion records. A functional form incorporating source, path, site, and focal-depth effects is calibrated over the frequency range of 0.23–30 Hz using a mixed-effects regression approach. Residuals are decomposed into between-event, between-site, and within-site components to assess model stability and uncertainty. Residuals show no systematic dependence on magnitude, distance, or site conditions. The model exhibits magnitude-dependent spectral scaling consistent with classical source-spectrum theory and captures key effects including the near-source saturation effect, distance attenuation, and linear site amplification. Compared with the FAS-GMM developed using the NGA-West2 strong-motion database, the proposed model predicts higher EAS amplitudes at intermediate to high frequencies for small-to-moderate earthquakes, consistent with previously reported relatively higher stress drops for small-to-moderate earthquakes in this region. Based on the proposed ground-motion model, a region-specific interfrequency correlation model is developed, resulting in improved agreement with empirical correlations, particularly at frequencies above 1 Hz. The proposed models provide a foundation for regional FAS-GMM development and FAS-based probabilistic seismic hazard analysis in Southwestern China.