Yes, USGS has detected it. The question is whether it was a nuke or conventional explosive or something else.

http://earthquake.usgs.gov/earthquakes/eventpage/us10004bnm#...

It has been detected as a 5.1

http://www.cnn.com/2016/01/05/asia/north-korea-seismic-event...

It might even cause a news organization to write the first two sentences of its article about that.