Poseidon is also called the "Earthshaker" and is the God of Earthquakes in both the PJ series and classic Greek mythology.<br/><br/>http://www.theoi.com/Olympios/Poseidon.html (http://www.theoi.com/Olympios/Poseidon.html)<br/><br/>You will find that a lot of "godly powers' overlap but causing an earthquake is within the realm of both deities and their demi-gods, I'd think.  I'd just think that the demigods of Poseidon would be able to do the ability more readily and with more power than the children of Hades but the kids of Poseidon could not use geokinesis to throw boulders and the like as they can only technically "shake the earth" or cause an earthquake.<br/><br/>Does that help?