Check out the photo that accompanies
this one LA Times article on the Shakeout. The guy takes cover
under a shopping cart. I had never even considered the possibility of doing that!
Just to focus on this one little thing, what do folks think of that? Could you ever see yourself doing that? I think most of us have seen video from major quakes like Fukushima and numerous other locales, so we have a sense of how violent or how long a major quake can be and what can happen in a supermarket-like setting.
Rather than mimicking what that guy did, if I did want to take shelter in that situation with a shopping cart, I think it would make more sense to tip over the shopping cart onto its side and then taking cover inside it. It should provide more protection for more of your body than just hiding your head under the bottom rack of the shopping cart. With young children, this method would also seem very practical and pretty natural for them, since it seems like a natural play behavior.
One nightmare scenario for me is shopping at a big warehouse store when the shaking starts. You'd have tons of merchandise stacked 35 feet high over your head. Sometimes I think ducking underneath the stack should be safest if a more open area wasn't close by since the merchandise is most likely to tip over into the aisle.
However, I do recall seeing one video of a stack collapsing straight down (I forget if it was a quake video). Although they always say not to run outside when you're inside a building, or run inside when you're outside because of the risk of falling debris along the outside of the building, I think the warehouse store is one situation where if an exit was close by, I would consider bolting outside to get away from all that stacked merchandise.
If I were in certain sections of a home improvement store, like Home Depot, that would be another exception to the "don't rush outside," I think. A rack of toilets or lumber or roofing material falling on me would be "not a good thing," too.