SEOUL By the time North Korean state radio teased listeners on Wednesday morning that a "special announcement" was due at midday, those outside the isolated country were already fairly sure what was coming.
A 5.1 magnitude earthquake detected near a known nuclear test site signalled that North Korea had probably conducted a fourth nuclear test, and that is what state TV told its audience, in the reverential tones saved for historical events.
While North Korea uses its nuclear tests to project power abroad, they are also a domestic propaganda tool for its third-generation dictatorship.
The years since Kim Jong Un took power following his father Kim Jong Il's death of a heart attack in late 2011 have been turbulent, marked by high-level purges and executions.
"The North Korean government generally looks weak from the outside but nuclear tests like this strengthen the image of a powerful state domestically," said Christopher Green, a North Korea expert at Leiden University in the Netherlands.
After a brief musical interlude, a male newsreader dressed in a dark suit and tie said Kim had personally ordered the country's first hydrogen bomb test.
That was followed by a full announcement from Ri Chun Hui, a well-known figure in North Korea best remembered outside the country as the emotional middle-aged woman who delivered the news in 2011 that then-leader Kim Jong Il had died.
The elder Kim's death sparked an unsettled period marked by two nuclear tests, the purge of Kim Jong Un's powerful uncle, and the violent execution of the militarised state's defence chief, Hyon Yong Chol.
The latest dramatic death or disappearance in the close circle of deputies to the country's leader was Kim Yang Gon, Pyongyang's top negotiator with South Korea, who was killed in what state media said was a traffic accident a week before the test.
"These personnel shuffles and so forth are always part of North Korean political culture," said Michael Madden, an expert on North Korea's secretive and opaque leadership circle.
"Under Kim Jong Un, what we have seen from 2012 to 2015 was very similar to what happened from 1994 to 1998 as Kim Jong Il took the formal reigns of power".
The nuclear test comes a few months ahead of what is scheduled to be just the seventh congress of the country's ruling party, which was founded 70 years ago, and two days before what is believed to be the young leader's birthday.
He is understood to be in his early 30s.
"The test is being used to build a legacy for Kim Jong Un, ahead of the seventh congress of the Workers' Party rather than as a bargaining chip with the outside world," said Chang Gwang-il, who headed the South Korean defence ministry's policy department from 2009 to 2011.
The state news agency, KCNA, quoted the head of a metals plant and chairwoman of the board at a collective farm lauding the test and saying it would help inspire them and their co-workers to increase productivity.
Alongside Wednesday's announcement, state television showed images of Kim signing what it said was the order to begin the test.
A close up of that order showed a note, attributed to Kim Jong Un, bearing a hand-written promise to "open 2016 with the thrilling boom of an H-bomb".
(Additional reporting by Ju-min Park and Eunsol Yoo; Editing by Tony Munroe and Mike Collett-White)
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