It is extremely probable, though at this time not positively verified, that North Korea has detonated a nuclear device, as they have been threatening to do. Attached is an overlay showing the location of a tremor of around magnitude 4.2 which probably was produced by a test.
There is already a thread started about the test site
HERE. All of the placemarks in the thread fall within the area of the tremor.
HERE is the article from the New York Times which is the source of the overlay.
Here it is as located by the USGS. (41.294N, 129.134E)