CTW,
The amount of product that may be used to resist seismic overturning is entirely depended upon the details of construction.
If the silo is tower or skirt supported, then all of the product that is present will resist overturning.
If it is a ground supported silo, its more complicated. For an unanchored silo, the amount of uplift resistance from the product will depend on the thickness, material strength and details of construction of the silo bottom at the shell-to-bottom joint. If the silo is anchored, the uplift resistance of the silo wall will not include any product component and will be entirely dependent on the anchorage details.
In addition, the foundation design will also govern. If its a solid slab, you may be able to count all of the product. If its just a ringwall foundation, you will not be able to assume all of the product is effective.
Unfortunately, it's one of those "depends" answers. More info would be need to provide proper guidance.
Hope this gets you started.
Steve Braune
Tank Industry Consultants