Investigation of Site Amplification and Attenuation Effects in the Changjiang Delta

Abstract

Abstract Site response in the Changjiang Delta in eastern China was studied using Lg Fourier amplitude spectra. We used broadband seismograms recorded at 70 stations from 62 earthquakes with magnitude (Ms) varying from 3.5 to 5.0 during 2009–2021. The crustal quality factor Q and site response in the Changjiang Delta were obtained simultaneously from regression of Lg Fourier acceleration amplitude versus frequency. The κ0 of each individual station was subsequently calculated from a regression of the high-frequency site terms versus frequency. The site terms exhibit obvious dependence on sediment thickness in the Changjiang Delta. The site amplification factor reaches ∼7–10 for stations overlying sediments ∼8–9 km thick in the northern Jiangsu basin. The site terms were found to behave consistently as a function of sediment thickness over the frequencies of 0.56–24.86 Hz. Site amplification shows a positive correlation with sediment thickness at lower frequencies (<7.26 Hz) but transitions to a negative correlation as frequency increases to 12.86 Hz and higher. Linear functions versus sediment thickness were used to model the site response terms at individual frequencies. We also showed that site terms calculated using the κ0 model as a function of sediment thickness fit the site response terms well at frequencies higher than 9.66 Hz. Results of this study can be incorporated in ground-motion prediction models for the Changjiang Delta. In addition, the site response estimates determined here can be used to reduce bias due to site effects in studies of earthquake source parameters.