Seaquake is caused by propagation of seismic motion of sea-bed through seawater. Although aseismatic design has been providing applicable marine structures for earthquake zone, the effects of seaquake on a floating structure may be one of an item to be taken into further consideration from a view point of the structure safety. Although the effects of vertical motion of sea-bed due to earthquake may not be small and seaquake forces on a floating structure should not be ignored, the study on effects of seaquake has not been performed much. This paper is presenting a method of numerical calculation of seismic wave propagation in seawater. The hydrodynamic pressure caused by seaquake is analyzed numerically and its dynamic effects on the three-dimensional floating structure is investigated. First of all, we are introducing Green function which is a solution of differential equation governing the seismic wave propagation and fulfills free surface condition and the sea bottom surface condition. Secondary, we are introducing the boundary integral equation by making use of Green function and Green's theorem. Finally, we are obtaining the pressure and the seaquake forces on a three-dimensional floating structure by solving the integral equation. And then we'll discuss the effects of seaquake on the floating structure.