In order to prevent damages by tsunami a large composite breakwater is projected in deep sea of -60 m at the bay mouth of Kamaishi in Tohoku district. For the
purpose of examining the aseismisity of the breakwater a analysis of coupled hydrodynamic response characteristics and water pressures of the breakwater was carried out by means of the shaking table model test and the earthquake response calculation by Finite Element Method.
　In case the caisson of the composite type breakwater does not resonate, the model tests show that the hydrodynamic pressures acting on a caisson is calculated by the formura of Westergaard using the water depth and the seismic coefficient of the top of the mound.
　In case the breakwater resonates, the hydrodynamic pressures acting on a caisson are calculated by means of structures-water coupled systems of Finite Element Method. In order to use earthquake response calculation for practical application, it is essential to investigate whether the modeling and material property values simulate adequately or not. It is concluded that the method for analyzing structure-water systems using Finite Element Method is appropriate for simulating the dynamic response and hydrodynamic pressures of the fill type breakwater.