I think trident is at an appropriate power level for such an effect. It requires two elements to work, it gives your opponent one turn of warning to prepare, and it can be destroyed, but if left uncountered it is a very strong effect.
I think earthquake is too strong, but it's hard for me to think how to nerf it without ruining it. Adding too much to the cost makes it too slow and would make it useless. Making it destroy only 2 pillars would weaken it a lot, but I think it would still be playable. Destroying 1 would make it underpowered, unless maybe it also got a cost reduction or something.
Maybe the appropriate thing would be to do something like this...
Earthquake = 2
Spell: destroy up to 2 target pillars/pends in the stack
Quicksand = 3
Spell: destroy up to 3 target pillars/pends in the stack
That way, in unupped play, it's actually cheaper, but destroys less, but in upped play, where it is easier to recover from quicksand due to the faster speed of upped play, it destroys more, but costs more to reflect this. This would also affect trident/poseidon. In upped play, poseidon would simply cost 1 more to use, and in unupped play, it would be cheaper to use but destroy less.