Update, North Korea’s announcement: Full text of the Korean Central News Agency (KCNA) announcement of the nuclear test, presumably a TV broadcast (via the Independent):

“The Democratic People’s Republic of Korea successfully conducted one more underground nuclear test on May 25 as part of the measures to bolster up its nuclear deterrent for self-defence in every way as requested by its scientists and technicians.

“The current nuclear test was safely conducted on a new higher level in terms of its explosive power and technology of its control and the results of the test helped satisfactorily settle the scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology.

“The successful nuclear test is greatly inspiring the army and people of the DPRK all out in the 150-day campaign, intensifying the drive for effecting a new revolutionary surge to open the gate to a thriving nation.

“The test will contribute to defending the sovereignty of the country and the nation and socialism and ensuring peace and security on the Korean Peninsula and the region around it with the might of (the military first policy) Songun.”

Also see a slightly different translation at the Daily NK, and the online KCNA announcement from 2006.

A major difference between 2006 and now is the tone. In 2006 “scientific research,” “scientific consideration,” and “a great and prosperous nation” were all stressed before mention of a, “self-reliant defence capability.” In 2009 “nuclear deterrent” was stressed before science and “thriving nation” speak.

Original post: North Korea has announced it conducted a second underground nuclear test. The alleged nuclear test occurred at 09:54 local time, 25 May (00:54Z 25 May, 20:54 EDT, 24 May) at site the first nuclear test was conducted in October 2006.

Currently the only indication of the test is Pyongyang’s claim and a seismic event at the location of the 2006 test. The U.S. Geological Survey reported a magnitude 4.7 seismic disturbance at the site. In 2006 there was a 3.58 magnitude seismic tremor. The larger magnitude seismic measurement does not necessarily equate to a vastly more powerful device being tested since the size of the underground test chamber and other factors can effect seismic readings, though it does tend to suggest North Korea did get things right this time.

South Korean and Russian officials said the explosion was up to 20 kilotons, while the U.S. bis still analyzing the data. The 2006 test was less than one kiloton.

North Korea’s nuclear weapons likely are plutonium based, though it’s possible there is or was a secret uranium enrichment program. For more on North Korea’s nuclear programs, see this.

There will be an emergency meeting of the UN Security Council today. I’m sure they’ll be very, very angry and will write Kim Jong-il a letter telling him how angry they are.