Hi:

I want to consider the forces due to earthquake for my system as a uniform ground acceleration in the 4 horizontal directions. Is following the correct procedure for specifying it?

1) Specify U1(1,0,0) and U2(0,0,1) as the +X and +Z vectors.
2) Create the following Operating load cases:

W+T2+P2+U1
W+T2+P2+U2
W+T2+P2-U1
W+T2+P2-U2

Thanks

Yes, this is correct.