I wouldn't worry about earthquakes. I live in northen California myself. My parents had a 55gal for years and it never fell in a quake and we lived in a mobile home, talk about a ride when the ground shakes.
Earthquakes large enough to knock over 500 to 1,000 are so few and far between isn't worth the sweat. You probably have a much a better chance of getting into a nasty car accident going to the local stop and rob for milk than your tank taking a spill in an earthquake.
There is always insurance. Earthquake insurance will not only cover structures, but also belongins lost or damaged as a result. I know you can also insure specific items in your home for a specified value. That may cost you $5 to $20 a year. It will also cover any damaged caused by the water or the tank breaking something else on it's way down. If you have a lot of money into a tank it may be a good idea to cover it. My wife and I have individual coverage on her jewelry and some of the pieces cost as much as I have invested in my tank.
HA HA I even have specific coverage on my TV, I'm such a loser.
I'm glad you brought this topic up, I had not even thought of it before. I'm going to give my agent a call in the morining.