UPDATE: North Korea confirms a nuclear test was conducted today. From KCNA, the state news agency:
It was confirmed that the nuclear test that was carried out at a high level in a safe and perfect manner using a miniaturized and lighter nuclear device with greater explosive force than previously did not pose any negative impact on the surrounding ecological environment.
"Amid talk of a possible nuclear bomb test, North Korea vowed Tuesday to carry out a 'high-intensity, all-out action," CNN reported earlier today.
They just made good on that threat.
Voice of America Northeast Asia bureau chief Steve Herman tweeted just now that "Seismic activity [has just been] detected in #DPRK... believed to be "man-made" per various #ROK sources... Speculation now that #DPRK may have conducted a nuclear test."
Here's a first briefing from the Associated Press, and another from CNN. The government of South Korea estimates the intensity of what would be North Korea's third known nuclear test at 6 to 7 kilotons, though it's too early to independently confirm.
Here is the USGS event data.
A quick Google Maps zoom-in on the coordinates shown in that USGS link reveals that the "earthquake" took place near "Nuclear Test Road."
Here's the official seismic data from the PRC, indicating a "suspected explosive."
The area in which the event occurred has little or no history of natural seismic activity.
We don't know yet whether the bomb was plutonium-based (as past tests were), or uranium-based. Here's why uranium would be even more worrisome.
(URGENT) S. Korean official:'high possibility' of NK nuclear test after detecting artificial quake bit.ly/12HH5Nb— Yonhap News Agency (@YonhapNews) February 12, 2013
The "earthquake" is near N. Korea's nuclear test site. Presumed nuclear test, but no confirmation yet.— Chico Harlan (@chicoharlan) February 12, 2013
Mb 4.9.10 kilotons, maybe more.— Jeffrey Lewis (@ArmsControlWonk) February 12, 2013
If depth estimate is correct--and it might not be--yield could be higher than 10 kT.— James Acton (@james_acton32) February 12, 2013
They broke the Monday/Holiday pattern so, with a second tunnel I'd prepared for another in the next few days.— Jeffrey Lewis (@ArmsControlWonk) February 12, 2013
Yonhap reports that North Korea notified the US and China of its nuclear test plan a day early.— Martyn Williams (@martyn_williams) February 12, 2013
Kim Jung Il's birthday on the 16th - #nuclear test marks Day of the Shining Sun as this celebration is called in North Korea— Angus Walker (@anguswalkeritv) February 12, 2013
So, here's the big question. North Korea prepared two tunnels. Will there be two tests?— James Acton (@james_acton32) February 12, 2013