Read about a bloke in Christchurch who had a wide and thick lip all the way around the base of the tank. The quake caused the water to slosh back and forth - forced the wood lip right off, pulling the nails clean out.
The force of the water sloshing back and forth can topple even the biggest aquariums - in the right circumstances.
If both the bottom of the tank and the hood is secured water will wash quite high - what does not fall back in will no doubt risk wetting electrical fittings and any water flowing down the outside of the tank can easily travel down pipework and electrical wiring. Fit drip loops to all wires and a splash guard to stop water flowing down and into your cabinet etc.
Above all - aquariums are very top heavy - not a good thing in an earthquake!
Oh and found these pictures from an email that was going around: