Site response effects of thick sediments in the Subei Basin, eastern China

Characterized by extensive sedimentary successions, the Subei Basin stands as the premier hydrocarbon reservoir in eastern China. These thick sediments will have significant influences on earthquake ground motions in the basin which have not been systematically examined. To investigate the site response effects in the Subei Basin, we downloaded seismograms recorded by broadband stations in Jiangsu and Shanghai from 62 earthquakes with magnitude (M_S) 3.5-5.0 occurring between 2009 and 2021. Site responses for the stations were calculated from regression of Lg-wave Fourier acceleration amplitude versus frequency. We compared our site response results with published estimates obtained using Moya’s method. The two sets of site response results show similar characteristics, while the amplitudes of the site terms in this study attenuate less rapidly at high frequencies. A pronounced contrast emerges between stations in Subei Basin and those on thin-sediment in Shanghai: stations situated in the basin display apparent amplification at low frequencies, accompanied by strong attenuation at high frequencies, relative to stations in Shanghai. The peak frequency of amplification for basin stations is approximately 0.6 Hz, substantially lower than that observed in Shanghai (~12 Hz), a difference attributable to the fundamental resonance of the thick sediment column. Meanwhile, the high-frequency attenuation factors ( \kappa _0 ) are systematically larger within the basin compared to those in Shanghai. Amplitudes of site responses in the basin exhibit a clear dependence on sediment thickness and agree well with theoretical site factors calculated from transfer functions of one-dimensional velocity structure in the Subei Basin. For sites overlying thick sediments, the site response can be approximately modeled as a linear function of sediment thickness. Sensitivity tests demonstrate that the relative site response patterns and their dependence on sediment thickness are robust with respect to reasonable variations in the assumed stress drop values, although the absolute amplification levels at high frequencies carry some quantitative uncertainty. These findings demonstrate that site response effects due to the thick sedimentary columns in the Subei Basin are substantial and should be incorporated into regional ground motion predictions and seismic hazard analyses.