I have a couple of possible storage solutions, depending on your degree of handiness and available cash. All of them are variations of under bed storage.

The first is a storage bed. It's like a captain's bed, but lower to the ground with a single row of drawers or cabinets. We put them in our kids' rooms because of the same storage constraints you have. The amount of large item storage can be considerable, but the weight of the water may be too much for the drawer glides during an earthquake. This is the most expensive option.

This one is similar to my son's.
http://www.ikea.com/us/en/catalog/products/40228708/

The second is a trundle bed, minus the second mattress. The trundle will roll easily, even when loaded with considerable amount of weight. Put a piece of plywood (if necessary) over the trundle's slats, and you have instant storage. Use a dust ruffle or bedskirt under the regular mattress to hide the contents. For emergency supplies, this would be my first choice.

In college, I put my bed on 8 inch risers similar to these. Added to the 6 inches of clearance normally found under a bed, and I had space for several large plastic tote boxes. It's cheap, but a solid shake could knock the bedframe off the lifters and onto your supplies.
http://www.ikea.com/us/en/catalog/products/50314319/

And, lastly, one of my college roommates used milk crates. Our rooms were a little over 110 square feet each, with a common bathroom and kitchen. He put all of his stuff in milk crates, put a plywood board on top of the crates, and his mattress on top of the plywood. This will require a bit of strength because you will have to move the mattress to get to your supplies.

EDIT: The USGS E-quake Hazard Maps might help quantify your risks.
http://earthquake.usgs.gov/hazards/hazmaps/