I agree with Stella, you can do all you like to try and secure your tank but at the end of the day bad stuff happens.. You will have to worry about your TV, Chimney, windows, fridge or various other things in an earthquake of that size, is anyone aware of any deaths/injuries caused by fish tanks in the quake? Everyone even escaped from the commercial aquarium down down without injury.
If your tank is well designed (well braced correct thickness glass) and has a good solid stand (which I am pretty sure yours does because I think I know who made it and he over engineers everything) then you should be fine.
It should always be a consideration but it would be hard to live your life not having things you enjoy just in case.. I think if we thought of all the things that have injured/hurt/killed people in the past we would be wrapped in cotton wool and have empty houses.
But like I said to you despite all the above if it stops you from sleeping and really has you upset then it has to go because it is not worth the stress.
But I think you should keep it and enjoy and be proud of it, it looks awesome and I know how many years you have been working on sourcing descent fish for it.