S-wave attenuation of the shallow sediments in the North China basin based on borehole seismograms of local earthquakes

Abstract

S-wave velocity and attenuation structures of shallow sediments play important roles in accurate prediction of strong ground motion. However, it is more difficult to investigate the attenuation than velocity structures. In this study, we developed a new approach for estimating frequency-dependent S-wave attenuation (⁠|$Q_S^{ - 1}$|⁠) structures of shallow sediments based on multiple time window analysis of borehole seismograms from local earthquakes. Multiple time windows for separating direct and surface-reflected S-waves in local earthquake waveforms at borehole stations are selected with a global optimization scheme. With respect to different time windows, the transfer functions between direct and surface-reflected S-waves are achieved with a weighted averaging scheme, based on which frequency dependent |$Q_S^{ - 1}$| values are obtained. Synthetic tests suggest that the proposed method can restore robust and reliable |${\rm{\ }}Q_S^{ - 1}$| values, especially when the data set of local earthquakes is not abundant. We utilize this method for local earthquake waveforms at 14 borehole seismic stations in the North China basin, and obtain |$Q_S^{ - 1}$| values in 2–10 Hz frequency band, as well as average |${V_P}$|⁠, |${V_S}$| and |${V_P}/{\rm{\ }}{V_S}$| ratio for shallow sediments deep to a few hundred metres. Results suggest that |$Q_S^{ - 1}$| values are to 0.01–0.06, and generally decrease with frequency. The average attenuation structure of shallow sediments within the depth of a few hundred metres beneath 14 borehole stations in the North China basin can be modelled as |$Q_S^{ - 1} = {\rm{\ }}0.056{f^{ - 0.61}}$|⁠. It is generally consistent with the attenuation structure of sedimentary basins in other areas, such as Mississippi Embayment sediments in the United States and Sendai basin in Japan.