GMPEs for elastic input energy spectra of horizontal and vertical offshore ground motions based on the ETMC database in Japan

In this study, the characteristics of offshore ground motions related to elastic relative and absolute input energy are investigated, and ground-motion prediction equations (GMPEs) for elastic input energy of offshore ground motions are proposed. The GMPEs are developed using mixed-effects models calibrated through regression analysis of elastic responses which are derived from dynamic analyses of single-degree-of-freedom (SDOF) systems subjected to offshore ground motions. To compare the differences between offshore and onshore ground motions with regard to input energy, an onshore GMPE is derived from the onshore database. The ground motion database includes 892 offshore records and 4033 onshore records; the offshore and onshore records are collected from the same set of 273 earthquakes. To quantitatively analyze the elastic input energy of offshore ground motions, the moment magnitude, tectonic source type, focal depth, hypocentral distance and site condition are considered in the proposed equations. A comparison and analysis of the relative and absolute input energy of offshore and onshore ground motions manifest that the horizontal input energy spectra of the entire period band and the vertical input energy spectra of moderate and long period band of offshore ground motions are larger than those of onshore motions of all site classes, except that the vertical input energy of offshore motions is slightly smaller than that of onshore motions for short periods due to the combined effects of water layer and soft sediments under seafloor. The differences between the relative and absolute input energy of offshore ground motions and comparisons of the proposed GMPEs with others from the literature are also discussed in this study. The comparisons show that the proposal of the GMPE for elastic input energy of the offshore ground motion is necessary due to the offshore ground motion being considerably different from the onshore motion and the proposed onshore GMPEs.