Offshore earthquake ground motions have not been fully characterized due to a lack of measured data. This study has collected more than 1,000 offshore and concurrent onshore site records from the K-NET and SEMS projects. These data are compared statistically in terms of the peak ground accelerations (PGA), the vertical-to-horizontal (V/H) PGA ratios, the normalized earthquake response spectra of the horizontal and vertical components and the corresponding vertical-to horizontal (V/H) ratios. The analysis indicates that in the same earthquake event and with identical epicenter distances, the offshore horizontal PGA has greater amplification than the onshore. Meanwhile, the V/H PGA ratio of the offshore ground motions is much smaller than that of the onshore counterparts due to the effects of seafloor soft sediments and water layer. For offshore earthquake response spectra (both H & V), the peak spectral plateau occurs at longer periods than for the onshore records. Also, the offshore response spectra at moderate and long periods gain greater amplifications, which is critical to bottom fixed offshore structures. The V/H ratios of earthquake response spectra, offshore versus onshore, exhibit different trends varying with periods. Finally, to fill up the gap of current seismic design of offshore structures, two reference design response spectra (both H & V) by spectral fitting have been proposed for the engineering use. The offshore design spectrum is shown to have distinct features compared to land based design spectra in three codes.
