Storage Tank Prescribed Movement

I am currently working on a project in seismic zone 3 which includes 3 large diameter storage tanks. The client and I are concerned that the differential movement between the the tank and the relatively stiff piping during a seismic event may cause rupture in the tank. Per API 650, the tank is not anchored to the foundation. The tank supplier basically says "tell us how much you want the tank to move and we'll design it for you". Is there a process to determine what the movement would be in a large tank, or is there an accepted movement to subscribe in this case?