found some stuff here,  maybe of some use to you.

http://www.iitk.ac.in/nicee/IITK-GSDMA/EQ04.pdf
http://caltecheerl.library.caltech.edu/165/00/8601.pdf

http://nels.nii.ac.jp/els/contents_disp.php?id=ART0002633551&type=pdf&lang=en&host=cinii&order_no=Z00000007069107&ppv_type=0&lang_sw=&no=1183475298&cp=

http://nels.nii.ac.jp/els/contents_disp.php?id=ART0002632855&type=pdf&lang=en&host=cinii&order_no=Z00000007069188&ppv_type=0&lang_sw=&no=1183475551&cp=

http://www.dec.fct.unl.pt/seccoes/S_Estruturas/Dinamica/Util/SeismicAnalysisTanks.pdf

http://www.google.com/search?q=tank+seismic+analysis&hl=en&start=20&sa=N

http://virtualpipeline.spaces.msn.com

Could you look through the ship rules from the major classification societies (DNV, Lloyds etc) and see what they say about the design of stability tanks, fuel oil tanks, clean water tanks etc?

You can calculate the angular accelerations from the vessel roll, pitch and yaw and direct accelerations from heave, surge and sway.  The angular accelerations can be converted to tangential accelerations based on lever arms relative to the vessel centrelines.

As a rough idea you could try assuming taking the solid mass of water and using the accelerations?  This obviously takes no account of sloshing.

However, your situation is more complicated than that as the movement of your liquid will have a damping effect on the vessel motion as it is unlikey to be moving in phase with the vessel motions.  

In addition the effect of the free surface of the liquid will have an effect on the stability of the vessel so should also be considered.  The liquid will move as the ship does which changes the vessel COG, reduces the lever arm and therefore affects the moment of statical stability