Earthquake targets 1-2 Adjacent Zones, the effects on conjurations and creatures in those zones are incidental and do not imply that they are being targeted. Both Nullify and Reverse Magic state "When this creature is targeted" and since Earthquake does not target any individual creatures those enchantments are not activated.
As to whether the effects of Earthquake count as an Zone Attack, I'm not sure. It certainly sounds like a Zone Attack in every way except for the fact that it is an incantation and does not state that it is a Zone Attack. I assume it does not count as a Zone Attack, but that doesn't get around the fact that no individual creatures or conjurations are being targeted.