Seismic force distribution on silo

This answer is probably too late but my approach for what iys worth is:
If this silo rests on ground say, then conservative case is to assume seismic load as an inverted triangular distributed force (static equivalent). Thus CoG effetively at 2/3 height. This is the classical first mode response of an elastic vertical beam of uniform mass. It may be too conservative, depending on whether you are dealing with a 'deep' silo or 'shallow' silo.
If the silo is on legs then the response will be different. In this case I would probably assume seismic force acts at the real CoG.