User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10137020-20130721103612/@comment-5927468-20130721124619

In mythology when the big three drew lots for who would rule what, the Earth's surface was considered neutral ground that they would share. Hades/Pluto has dominion over the underworld so Hazel only has control of underground earth. I think Percy's earthshaker powers may also be centered below the surface as well, like at the tectonic plates. Plus even though she's asleep, I doubt Gaea would allow them to control her domain/body