Yes. Earth Power would do more Damage with your Higher SAtk.
Of course, it does depend on the foe you face and also your Attack EV investment or nature if you have any, but a Dialga with a Hardy Nature (or other neutral nature) and no EV investment will do more damage with Earth Power against a foe with equal Defense and Special Defense than an Earthquake would do.
Earthquake will end up doing more damage to a foe with equal defensive stats if Dialga has at least 120 more Attack EVs than Special Attack EVs. Or only 12 more Attack EVs than Special Attack EVs if you have an attack beneficial nature such as Lonely.
Earthquake will also end up doing more damage (neutral nature, no EV investment) if the foe has a Special Defense stat that is 27 points (not EVs, just points) higher than its Defense.
All these calcs hold true only if IVs are equal.
Also, because Dialga gets an Attack boosting move (Bulk Up) you can also use that to make Earthquake do more damage.