How does the Earthquake spell work, and why does it require mana?

Thank you for your answers; as a result, I realized that the developers of Heroes 5 simply really wanted to rip us off by adding (or rather, not adding) a spell for Earthquake, which, as it turns out, doesn't work at all.

Actually, there is only one tiny nuance. =)
It doesn't affect the standard spell because it doesn't deal damage to creatures. But in the Machine branch, there is an "alternative" version that damages those behind the wall... and it is likely that this spell can affect that formula. The only question is whether there will be even one player in the community who will study "Earthquake" in the Machine branch instead of Ballista?