User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7153535-20130812003603/@comment-3146930-20130814012635

But Nico's shadow travel is somewhat hinted at before. There are times in the series where Percy will mention shadows bending around Nico. Also, the hellhound way back in TLT that attacked Percy didn't turn to dust, it melted into shadow. Hades also has a reputation of melting into shadows, so it isn't completely out of left field, we just didn't know it had a name.

Percy's powers on the other hand have always been water related (for the most part, I mean he can talk to horses and stuff too). Poseidon is the god of the sea and storms, and the ultimate type of sea storm is a hurricane, which is wind and water. Percy has been shown to cause fissures, but every time water is involved in some way (explosion of water at Mount Saint Helens, salt water erupted on the bridge, and the same thing happened on the glacier). I just don't see earthquake powers in his future.