Hey UchiaSasuke, I'm afraid your question is a bit unclear, first you ask how you can make Earthquake's spell to stop shaking, and then to start shaking, which one will it be? If you wish to modify the spell itself to stop shaking, I'm afraid that's not possible. If you want to add terrain "shaking" or deformation to a spell, there's a function for that. Will need to know a bit more before I can help you out though.
~ Angelusz