As an approach to the analysis of seismic forces this paper deals with the problem of transient fluid waves which are going to develop inside a tank. In the paper, theoretical formulas for internal waves and dynamics fluid pressures in the transient state have been derived under the assumption that a tank is completely filled with two different fluids and forced to move vertically and horizontally at the same time. For the case without vertical movement, the complete solution can be obtained, but in the case of concurrence of horizontal and vertical movements, resonant conditions are investigated by an approximate solution. In the consequence, it is made clear that the vertical movement plays as important a role in the resonant phenomenon as the horizontal movement.
The theory is applicable to the same problem of a ground undersea tank. The validity of the theory is confirmed by the experiments for ground tanks of a rectangular and a circular shape which were vibrated in the horizontal direction only. It is also confirmed by the experiments that the solution for a rectangular tank is applicable to the computation of the propagation of waves generated by a piston type wave paddle.