For a Vessel supported on support lug PV Elite gives below warning-

This is a vertical vessel on support lugs and the earthquake type selected is a building code. Building codes assume all of the mass is located above the point of support. This makes their use on lug supported vessels questionable. It is strongly suggested that based on the seismic zone and applicable parameters, an appropriate lateral (Gx) and (+-) vertical(Gy) acceleration be applied using the G loading option.

My question is, how to compute Gx & Gy accelaration based on seismic zone and applicable parameter.