The seismic design of marine structures so far still relies on civil engineering design codes. The objective of this study was to point out the importance of using offshore ground motion for seismic design and safety assessment of marine engineering. Several earthquakes in history were selected

and the focus was on paired offshore and onshore accelerations with identical epicenter distances. Three key elements of seismic motions were discussed

and Hilbert-Huang spectrum was adopted to distinguish energy distributions in terms of temporal and spectral characteristics. Results show that the peak ground acceleration of offshore horizontal motion was larger than that of onshore horizontal motion. The duration was generally longer

and the response spectrum shifted to moderate and low frequencies. These features may have unfavorable effects on structural dynamic response. As revealed by the Hilbert-Huang spectral analysis

the frequency corresponding to the main energy of offshore ground motion was lower than that of onshore ground motion

and the frequency corresponding to the largest instantaneous energy was almost identical to peak-to-trough frequency rather than predominant frequency in Fourier spectrum.