Characteristics of Earthquake Ground Motion on the Seafloor

The differences between onshore and offshore ground motions were studied based on the records from the K-net and SEMS. The horizontal response spectrum and vertical-to-horizontal response spectral ratio for offshore and adjacent onshore ground motions were analyzed. The results indicate that the characteristic period of the response spectra for offshore ground motions is obviously larger. The epicentral distance can influence the horizontal response spectra for offshore ground motions in a moderate earthquake, while the influence of water depth is not noticeable. Moreover, the vertical-to-horizontal response spectral ratio for offshore ground motions is significantly smaller for periods less than 1 s.