Near-Source Attenuation and Spatial Variability of the Spectral Decay Parameter Kappa in Central Italy

Abstract

We study the spectral decay parameter κ using S-wave recordings from the central Italy dense regional array. The data set used consists of 266 earthquakes, 353 stations, and 13,952 observations of κ with a mean value of 0.0412 ± 0.0177 within the distance range of 7.1–168.8 km. We model the variation of κ with hypocenter distance r as κ r κ0 κs ̃κ r , in which κ0 and κs represent the near-site and the near-source decay parameters, respectively, and ̃κ r the average κ along the S-wave source-station paths. We first determine ̃κ r with a nonparametric inversion approach and then we solved for κ0 and κs with a second inversion. We found that ̃κ r increases with distance within the whole distance range analyzed (9.2–80.6 km). The near-source decay parameter takes values in the range 0:0 < κs ≤ 0:026 with a mean value of 0.003 ± 0.006, which represents 7.52% of the mean value of the observed κ. The values of the near-site decay parameter vary in the range 0:0035 ≤ κ0 ≤ 0:0823 with a mean value of 0.0298 ± 0.0133, that is, 72.28% of the mean value of the κ observed. We conclude that most of the high- frequency attenuation takes place near the site, because ̃κ r contributes with only 20.2% of the spectral decay. We also investigate the spatial variability of κ by determining ̃κ r within four quadrants that divide the studied region taking as a reference axis the Apennines chain orientation. We found higher values of ̃κ r in the southern quad- rants, where seismicity and faulting are more active, and less attenuation in the more stable northeast quadrant.