The characteristics of offshore ground motions were studied on the basis of the records from the Kyoshin network（K-NET）in Japan and Seafloor Earthquake Measuring System（SEMS）in the USA.The horizontal response spectra

the vertical-to-horizontal response spectral ratios（V/H RS ratios）

and the inelastic response spectra were analyzed to compare the differences in offshore and onshore ground motions.It is found that the PGA of vertical components for offshore ground motions is lower.The horizontal response spectral values for offshore ground motions are higher for periods longer than 0.2s.The characteristic periods of offshore ground motions is longer than that of onshore ground motions

the larger the epicentral distance

the longer the characteristic period will be.Moreover

it is very different in the V/H RS ratios between offshore and onshore ground motions.The V/H RS ratios for offshore ground motions are obviously higher than that for onshore ground motions at short to middle periods lower than1 s.Further more

the simple design equations of V/H RS ratios for offshore ground motions are derived on the basis of the real ground motion recordings with different earthquake magnitudes and epicentral distances.At last

to compare the inelastic response spectra

the difference in strength reduction factor spectra between offshore and onshore ground motions is slight in general.