In summary, North Korea claimed to have successfully tested a nuclear weapon, drawing global criticism. Russia, another country with a history of antagonism with North Korea, estimated the blast's force at 10 to 20 kilotons, far higher than other estimates. The claim and subsequent analysis is likely a tactic to increase pressure on the United States and Japan, two countries North Korea has threatened war with in the past.
Martin B. Kalinowski, a nuclear expert at the University of Hamburg, said a seismic magnitude of 4.7 corresponded to a nuclear explosive yield of about three to eight kilotons of high explosives, with the most likely yield being four kilotons.
...But Alexander Drobyshevsky, a Russian Defense Ministry spokesman, offered a different estimate, saying that the force of the blast, which the ministry confirmed occurred at 9:54 a.m. local time in northeastern North Korea, was 10 to 20 kilotons, the RIA-Novosti news agency reported. It was unclear what information the defense ministry based its estimate on. Russia estimated the force of the 2006 blast at 5 to 15 kilotons, far higher than other estimates at the time.
Now, Kim is poised to succeed again by following precisely the same script. In April, Pyongyang launched a Taepodong-2 missile, and National Security Council official Gary Samore recently confirmed that a second nuclear test is likely on the way. The North is set to try two U.S. reporters for "hostile acts." The state-controlled newspaper calls America "a rogue and a gangster." Kim recently expelled international monitors from the Yongbyon nuclear complex. And Pyongyang threatens to "start" enriching uranium -- a capacity it procured long ago...
They've also been firing (short-range) missiles towards Japan.
It might be worth pointing out to them that they are a nuclear power who is technically still at war with an ally of the United States. There is a non-trivial chance that their attempted nuclear extortion will instead result in Pyongyang becoming a sea of molten glass and steel.