Strong-Motion Attenuation Characteristics in Sichuan Basin Derived from Densely Distributed Records and Its Implications for Ground-Motion Modeling

Despite being located in a stable tectonic zone, the Sichuan basin is surrounded by several major faults. During the 2008 Wenchuan earthquake, the isoseismal intensities attenuated more slowly eastward into the Sichuan basin than westward. Nevertheless, this effect has not been well assessed due to the long-term lack of strong-motion records. In this study, we investigate the strong ground motion attenuation characteristics in the Sichuan basin using densely distributed records from ten moderate-to-large earthquakes recorded in recent years, including the 2022 Ms 6.1 Lushan earthquake and the 2022 Ms 6.8 Luding earthquake. Our data show that attenuation in the basin exhibits a plateau at source distances of 80–160 km at periods up to approximately 0.5 s. Analysis of the time histories indicates that the observed plateau is attributable to constructive interference between the direct wave and the postcritical reflection from the Moho discontinuity. We also observed lower anelastic attenuation rates and significantly smaller within-event variability for the Sichuan basin than western and southern Sichuan, based on mixed-effects ground-motion models developed for the regions. These results are consistent with the more homogeneous crustal structure and gentler topography of the basin. We recommend a four-segment geometric attenuation function with distance breakpoints at 80, 150, and 240 km for the Sichuan basin, as it provides a statistically superior fit and more effectively captures the complex characteristics of far-field attenuation compared with the conventional three-segment model.