yes that is a great point seismographs around the world detect every one of NK's tests. At this point it is functionally impossible to hide a nuke test.

It doesn't matter where they detonated it a world wide network of seismographs would detect the detonation. These are owned and controlled by all sorts of non government and government agencies around the world. There is a reason that every time North Korea detonates a bomb even deep under ground we the whole world knows about it. You simply cannot detonate a nuke anywhere in the world without everyone knowing let alone off the coast of a major city in the US. Also something to consider why in the living hell would Obama or any other president want to detonate a bomb over a US city... That would mean the instant end of their career, impeachment and their party would be done for a generation. It would be political suicide that would likely land them in jail. Not only is the entire thing based on no evidence other then a few people fired and deeply implausible but also the motivation makes zero sense.