Yeah, but Japan, South Korea, and China have all said that they haven't detected any radioactivity, so at the moment i am more inclined to believe those who actually live there.
The simple fact that everybody is wondering if that was a nuclear explosion or not is proof enough that something is not quite right. When a nuclear explosion happens, even underground, there's no debate, everybody immediately knows that it's a nuclear explosion from its sheer power. This explosion was only 500t while a typical nuclear explosion will be considerably larger around 10kt to 20kt, so if that was indeed a nuclear test, it was a failure, that's the one thing everybody agrees on.