I am not the expert in Fluent, but I know there are two approaches to cause fluid to slosh.
The simple approach is to apply transient acceleration to the water. There is a gravity force pulling the water down. If you take the lateral acceleration time history I show in my September 2021 reply to Baloch, and add the Y coordinate acceleration to represent gravity, that is all that is needed.
The more complicated approach is to create a moving mesh to the surfaces of the tank, and move those with the lateral accelerations in the time history and have a simple constant gravity load.
After you have solved the transient model for the fluid motion, you can transfer the pressure on the sides of the tank to a structural model. This is called one-way FSI. The deformation of the tank wall caused by the pressure has an insignificant effect on the fluid motion. The reason for transferring the pressure to the structural model of the tank is to see the stress in the tank walls and support structure and check the strength of the design.
I am also not the expert in acoustics, but I know that the unknown quantity the solver is computing is nodal pressure. The nodes don't move. Contrast this with the Structural solver where the unknown quantity the solver is computing is nodal displacements. The nodes do move.
Since the nodes in the water don't move in an acoustics model, I don't think a Modal Acoustics solution has any relevance to the motion of the fluid in a tank during an earthquake.