Effects of comprehensive site conditions on the nonlinear seismic response of the Hangzhou Bay seabed site

Introduction The Hangzhou Bay seabed is characterized by complex site conditions, including seafloor topography, laterally variable marine deposits, and bedrock undulation. This study investigates how these conditions affect nonlinear seismic response by comparing one-dimensional (1D) and two-dimensional (2D) numerical simulations. Methods A large-scale 2D ABAQUS model was developed from borehole investigation data, and corresponding 1D soil-column models were extracted at three representative boreholes, ZK1, ZK3, and ZK6. Far-field bedrock motions from the KiK-net were applied as input motions, and nonlinear seismic response analyses were conducted in ABAQUS with the non-Masing DCZ hysteretic constitutive model. Results The results show that the seismic response of the Hangzhou Bay seabed is strongly influenced by the combined site conditions. At ZK1, relatively large horizontal surface accelerations were observed, which may be associated with nearshore topographic variation and shallow wave propagation. At ZK3, where bedrock relief and lateral sedimentary variability are more evident, the 2D model showed pronounced acceleration amplification at depths of approximately 60–150 m. Under the investigated conditions, the vertical peak ground acceleration (PGA) ratio between the 2D and 1D models at ZK3 reached 2.15–2.92. Frequency-domain and response-spectrum analyses also indicated amplification of motions below 1 Hz, localized amplification around 1–2 Hz, shifts in predominant periods, and multiple peaks in the normalized acceleration response spectra. Discussion and conclusion The observed 1D/2D differences are interpreted in relation to the combined site conditions; the individual contributions of topography, bedrock relief, and sedimentary variability are not independently isolated in this study. At ZK6, relatively similar normalized spectral shapes can coexist with differences in absolute response amplitude, because normalized spectra and PGA characterize different aspects of ground motion. Thus, 1D analysis may provide preliminary information on the frequency characteristics of horizontal seismic response at sites with relatively flat bedrock and approximately horizontal sediment layering, whereas absolute response amplitudes should be evaluated with possible 2D site effects in mind.