I'm not concerned about saving my tanks from a 7.0. It's the smaller ones where the world won't be ending, but I still won't want 200 gallons of water on my floor, along with my little girl's favorite pets and a lot of glass.
i think we all worry about these kinds of things, but short of not having an aquarium, what can we really do?
if we brace the stand and not the tank; it just means the tank shakes more, and could shake right off the stand.
if you brace the actual tank to a wall, you better brace it all around and very evenly or else you will cause strong and weak points to develop which would lend themselves to uneven pushing etc= broken tank.
i truly believe if you are worried, a few wheels underneath the stand should take most of the side to side vibration out of the picture, and translate that motion into side to side rolling, but that will only protect against P waves, and you would still need to restrict the amount of rolling you would allow or else the tank could roll across the whole room!
now as for S waves... i think the more padding you put between the tank and the stand the better.
but, i live in EQ territory, have been through the worst one in a while and am not sure its really worth worrying about. the amount of damage my tank caused during the last shake was nothing compared to all the other stuff that went wrong. i dont even think i lost any fish and our power was out for at least 24hrs....not even any jumpers.