Abstract： The time-frequency method of wavelet transform was used to analyze the ground motion at offshore site. The acceleration records of the offshore site ground motions were collected from California, U.S and Sagami trough zone, Japan, and decomposed into six layers of wavelet packets by Matlab software. The energy spectrum on the different sub-bands and time-frequency distribution characteristic of acceleration records were analyzed, simultaneously contrasted with the outcome of records collected from the closed sites on the coast and recorded from the earthquakes in land，like Wenchuan earthquake and Lushan earthquake. The energy of the ground motion at offshore site was concentrated in the primary 8 sub-bands. Meanwhile, the low frequency-band was dominant in the frequency range, which was different from that of the closed sites in the land. However, the energy of ground motions in land has significant distribution in high frequency region. The horizontal and vertical ground motion at offshore site demonstrated the wide band characteristic of energy distribution, while the energy of the vertical ground motion at inland site is far less than that of the horizontal ground motion. In the time domain, the energy showed obvious secondary released phenomenon.
