[loramin]

First off, the real answer here is that Clerics only (realistically) use AoE spells during faction pulls, when the damage doesn't matter, or in trash clearing groups (eg. in Plane of Fear) ... where you'll have other real AoE casters like Druids and Wizards (so the Cleric's damage contributions aren't that important).

(Historical Note: AoE experience groups used to be a (huge) thing here on P99. Countless Blue toons "leveled" to 60 by paying to sit in a Chardok AoE group ... back when AoE groups monopolized that zone ... but the staff changed the rules to make such groups impractical.)

However, to answer your question, yes, Earthquake is less mana efficient ... but mana efficiency isn't always your priority. Let's say you're a 44 Cleric, and you just cast Word of Souls. Great, now you're sitting on your hands doing nothing for nine seconds while you wait for the gem to refresh ... or you can cast Earthquake.

If your goal is to be as mana efficient as possible, you'll skip the Earthquake, but if you just want to burn down a mess of mobs as quickly as possible, you're going to cast Earthquake.