Amplification and Attenuation Due to Geologic Conditions in the Sichuan Basin, Central China

Abstract

Abstract The Sichuan basin is an intracontinental basin in central China containing extensive Paleozoic, Mesozoic, and Cenozoic sedimentary sequences of varying thicknesses. We used seismic recordings from 189 regional earthquakes with magnitude Ms 4.0–7.0 occurring from 2010 to 2020 recorded by 22 stations from the China Digital Seismological Observation Network to estimate the kappa values (κ0) in the Sichuan basin using Lg waves and coda waves. The Lg and coda κ0 values agree well with each other and show positive correlation with the thickness of sediments in the basin. The average κ0 for the Sichuan basin is estimated as 0.0475 s, similar to results reported in previous studies. The average estimate of κ0 is 0.06 s for stations in the western part of the basin that are underlain by sediments approximately 8 km thick. The quality factor Q of Lg waves in the crustal waveguide is estimated as Q(f)=335f0.72. We estimated site response in the Sichuan basin from Lg spectral inversion, coda spectral ratios, and Lg horizontal to vertical spectral ratios, respectively, and investigated correlation between the site response values and sediment thickness. Consistent patterns are observed in the three sets of site response, which are clearly dependent on the thickness of sediments. Maximum averaged amplification factors of the sediments occur around 2–4 Hz reaching ∼4.5 for sites overlying 8 km sediments. Stronger attenuation effects are observed at stations lying on thicker sediment columns at frequencies higher than 9.66 Hz. This spatial dependence of low-frequency amplification and high-frequency attenuation should be incorporated in ground-motion prediction models for the Sichuan basin. The results of this study can be applied to sites in the Sichuan basin where the thickness of the sedimentary section exceeds 2 km.