It would do 12X damage.
earthquake has 100 base damage
because aggron has a 4x weakness to ground it becomes 400 damage
and since it is underground it doubles to 800 damage
It varies on what's its attack stat but it doesn't really do much if the Pokemon 's attack stat is low and if it's a ground type the power of earthquake will go up fifty percent(read the new additional pokedex where it says hints on any pokemon)also the power is 450