Damping Modification Factors for Horizontal and Vertical Acceleration Spectra from Offshore Ground Motions in the Japan Sagami Bay Region

ABSTRACT Damping modification factors (DMFs) are used to scale the 5%-damped response spectra for the seismic design of structures with different damping ratios. However, there is no DMF model for offshore engineering response spectra at present. Using 892 offshore and 4033 onshore ground-motion records from the Kyoshin net, the difference between offshore and onshore DMFs are revealed by DMF ratios and t tests and whether site conditions need to be incorporated in offshore DMF models are determined. The between-event residual distribution shows that DMFs depend on the focal depth. The results show that the DMFs from the offshore and onshore ground motions differ significantly for most spectral periods, and the impact of the site condition on offshore DMF is very slight. The offshore DMF models for the horizontal and vertical acceleration spectra from the Japan Sagami Bay region are presented from spectra of 34 spectral periods and 10 damping ratios, in which only spectral periods and damping ratios are considered as variables. The uncertainty in the DMF model mainly depends on the path and site effects, and the standard deviations of DMFs for the vertical component are slightly less than those for the horizontal component. All standard deviations are modeled by a set of quadratic functions related to damping ratios. A comparison between the offshore DMF model in this study and those derived from onshore ground motions indicates that the differences are significant. The spectra from the Hu et al. (2020) study for offshore ground motions adjusted by the DMF models are used as our example applications.