The key to understanding how Earthquake Spells work is this: they deal damage based on a percentage of the target’s maximum HP. For instance, a level 4 Earthquake Spell deals 25% damage, so using it on a building with 400HP (like an Army Camp) deals only 100 damage. However, that same Spell on a building with 4000HP (like an Eagle Artillery) will deal a whopping 1000 damage! The tougher the target, the more damage an Earthquake Spell deals, and you can increase that damage percentage by upgrading the Spell.

Would this mean that you can destroy any building with just a handful of Earthquake Spells? 

Well, no - there is a catch. Repeated Earthquake Spells dropped on the same building become less and less effective. After a few they stop doing much damage at all, so Earthquake Spells are better used to soften up or finish off tough buildings.

The best use of Earthquake Spells