User blog comment:SayuriDarling/Full first chapter of The Mark of Athena/@comment-4566556-20120614082311/@comment-3146930-20120614222829

He caused an explosion by calling on water from the sea that erupted into a geyser. Personally, I think he used water from his own body to make the explosion, resulting in his weakened state when he arrived on Calypso's island. However, it could have been him simply calling on too much sea water at once, draining his power. The explosion of water caused the earth to quake, but it wasn't an Earthquake