The Koreans report that there is no radioactive fallout resulting from this event.
They also fail to mention the size of this device.
Honestly, I'm not skilled enough in nuclear physics to even guess, but my gut instinct tells me that no atom bomb could be discharged within 40 miles of a city, and not show any signs of radioactive fallout unless it was done so underground.
Yet, the USGS reports the seismic event at the surface. Just how percise that is? I cant say.