Seismic response characteristics of offshore sites in the Sagami Bay, Japan—Part II: Nonlinear behaviors and stochastic simulation of subduction zone earthquakes

Despite their crucial importance for marine engineering, the nonlinear seismic response characteristics of offshore sites remain poorly understood. Consequently, simulating ground-motion at offshore sites poses a significant challenge. To address this, this study begins with a dataset comprising stress drops of 70 earthquakes, region-specific quality factors, and linear site amplification factors (AFs) of six offshore stations in Sagami Bay, Japan, obtained using the generalized inversion technique (GIT). Then, by incorporating additional offshore accelerograms with focal depths up to 333 km and peak ground accelerations (PGAs) ranging from 0.2 to 4.2 m/s², we delve deeper into the effects of nonlinear site behaviors on the high-frequency attenuation parameter (κ₀) and AFs, respectively. A counterintuitive decrease in κ₀ was observed as the peak ground acceleration (PGA) reached 0.5–0.8 m/s², echoing similar observations from previous studies on KiK-net stations. Our results indicate that the high-frequency attenuation characteristics of offshore sites vary under strong motions, potentially attributable to the nonlinear evolution of the frequency-independent quality factor and S-wave velocity within near-surface sediments. Additionally, the degree of nonlinearity (DNL) at these offshore stations exceeded 4 when PGA reached 0.2–0.3 m/s², a threshold significantly lower than the previously reported range of 0.5–1.0 m/s². Furthermore, we observed systematic variations in nonlinear behaviors between flat and steep offshore stations, particularly with peak frequencies shifting towards lower and higher frequencies, respectively. These new findings may be mainly attributed to the intricate interaction of topography and marine sediments. Finally, simulations of two subduction earthquakes (M_W6.2 and 5.9) using the stochastic finite-fault simulation method (SFFSM) showed good agreement with observations at frequencies above 0.1 Hz. Notably, nonlinear AFs outperformed linear ones across a wide PGA range of 0.2–1.2 m/s², highlighting the significance of nonlinear site behaviors in characterizing offshore ground-motions. This finding reinforces the potential of the simulation framework (integrating GIT and SFFSM) for effectively and accurately simulating offshore ground-motion.