To simplify it the earthquake may:
a) hit enemy (best output)
b) hit you and enemy (no progress)
c) hit nobody (negative output)
d) hit just you (worst output)
Or maybe rather based on the amount of possible target cells the chance to hit enemy will be based on their number. If only 1 middle cell is available , the propability is 100%, but you also suffer 100% of dmg in same time. If there are 3 cells available, the propability is either 50% (if enemy is exactly 3 cell in line to you) or ~34%. The further the enemy the less chance that he will be hit, so the avarage dmg ratio would be still low.
p.s. btw Earthquake and manifold bramble are circle of 2 not cross of 2 aoe.
Anywho, on Earthquakes damages, if you start getting into "what if" scenario's it can go on for good. As there can be anywhere from 1 to, while not infinite, a vary large number of cells. Since if there is anything, be it a wall or other obstacle directly in the center of where an Earthquake could pop, it won't.
You could then say that is the same reason you don't see all the "what if" scenario's with every AoE spell. Since you could then calculate it's worth for hitting one or multiple enemies. How much it would deal if it was to hit an ally as well.
In the end, the damage calculations are the same. Whether or not you take 75 damage per AP/MP, the enemy if hit, will still take 75 damage per AP/MP. Therefore it really isn't a negative output as the output is still the same. You just sadly are taking some backlash.
(Sadly I cannot look or fix my charts at work as they block most image sites here....)