Near-Surface High-Frequency Spectral Decay Kappa (κ) Model of <i>S</i> Waves in Western-Central Sichuan, China

Abstract The Longmenshan fault zone in the western-central region of Sichuan is one of the active earthquake areas in China and is characterized by frequent earthquakes. We used 4260 seismic components of 165 earthquakes with magnitudes ranging from 3.5 to 6.0 recorded by 168 stations in western-central Sichuan, China, to estimate the high-frequency decay parameter kappa (κ) of the horizontal and vertical components in this region. The estimated horizontal kappa values are almost twice those of the vertical kappa values, and the difference in the kappa values between the two horizontal components is not significant. In addition, we investigated the influence of earthquake size and site classification on the κ0 value. The results revealed that the κ0 value gradually increased as the soil layer changed from rock to soft, and the κ0 value strongly depended on the near-surface site conditions. When the site type was the same, the κ0 value increased with magnitude, indicating that the κ0 value is influenced by both site and source effects. The relationships between the kappa values and the local magnitude were defined as κ=0.00403ML+0.01748 for the horizontal component and κ=0.00496ML+0.00369 for the vertical component. The single-station κ0 value also indicated that there was not much difference in the κ0 values between the two horizontal components, which were significantly greater than the κ0 value of the vertical component. Finally, the relationships for the horizontal and vertical components between κ0 and VS30 were defined as κ0=(0.1032±0.0989)−0.0118ln(VS30) and κ0=(0.1491±0.1043)−0.0222ln(VS30), respectively, which indicates that the relationship is weak without considering the influence of magnitude. The estimated kappa values for western-central Sichuan, China, may provide a fundamental basis for the study of seismic attenuation and fine simulation of ground-motion fields in this region.