Effect of seaquake on a floating body in an open sea and in an enclosed basin was studied theoretically and experimentally. First, sea bottom and breakwaters were replaced by the distributions of highly pulsating bubbles, and then formulas for surge, sway and heave accelerations of the floating body were derived in term of the acceleration of sea bottom or breakwaters. The theoretically predicted responses of the floating body were well supported by laboratory experiments using a small water tank on a vibrator. From the present study it was found that the vertical oscillation of large area of sea bottom or of the enclosed basin of any water depth induces the vertical acceleration on the floating body as if it were on the ground. Horizontal shake of breakwaters induces both vertical and horizontal accelerations on the floating body, and their magnitudes decrease substantially with increase of distance from the breakwaters.