apply seismic acceleration to a model with a fixed base

The most simple way to do this will most likely be use DEFINE_CURVE to input the data for the acceleration you'd like to impose. Then use a prescribed motion card to apply that acceleration to a node set or rigid body.

If your goal is to apply the seismic acceleration to the base, make sure it is not actually 'fixed' - it must not be constrained in the direction(s) you wish to apply the prescribed motion.