Effects of source, path, and site conditions on damping modification factor for the horizontal response spectrum using offshore ground motions in the Japan Trench area

In the seismic design, analysis, and risk assessment for engineering structures, the intensity of ground shaking was typically predicted using ground motion prediction models with a 5% damping ratio. However, given the broader range of damping ratios observed in marine structures, it was necessary to scale spectral ordinates using damping modification factor (DMF). This statistical study was conducted based on 6676 records of offshore ground motion, obtained from S‐net located in the Japan Trench. Conducting an examination into source parameters (magnitude, focal depth, earthquake types), source distance, and site parameters (sediment thickness, water depth, burial conditions), the study explored the effects of these parameters on DMFs, assessed their influence rankings through correlation analysis, and scrutinized a model form incorporating these parameters via residual analysis of the default model proposed in the study. It was shown that significant differences in DMFs among buried conditions of ground motion recordings, with magnitude being the most influential factor, followed by source distance and water depth, while focal depth and sediment thickness have the least impact on DMF. Furthermore, this study proposed a default model that takes into account buried conditions, along with a proposed median model that incorporates the default model, source and path effect function, as well as their standard deviation models. It was observed that the reduction in uncertainty of the proposed median model is primarily attributed to the decrease in between‐event uncertainty, resulting in an average decrease of 20% in total standard deviation.