Early reports of North Korea detonating a "Hiroshima sized" nuclear weapon over the weekend appear to have been - erm - overblown, with seismic data indicating an explosion less than half as powerful.
The Guardian and other media have widely reported an estimate by the Russian defence ministry that the explosion in North Korea, at shortly before 2am UK time yesterday had been of 10 to 20 kiloton magnitude. The "Little Boy" A-bomb which wrecked Hiroshima in 1945 was in this class.
However, the Comprehensive Test Ban Treaty Organisation (CTBTO) preparatory commission says that the magnitude of the explosion on the Richter seismological scale was 4.52. According to the British Geological Survey, this indicates that the Nork explosion was equivalent to approximately six kilotons of TNT exploding (or around 700 GBU-43B "Mother of All Bombs" conventional blockbuster bombs).
The Federation of American Scientists has suggested that the explosion was a "fizzle", in the same class as the relatively feeble explosion produced by North Korea in 2006. That test was put at 4.1 on the Richter scale by the CTBTO, for a rough yield of 1.4 kilotons.
However, the Pyongyang regime said that "we have successfully conducted another nuclear test on 25 May as part of the republic's measures to strengthen its nuclear deterrent". North Korea went on to test-fire short-ranged ballistic missiles yesterday, according to reports.
Apart from short-ranged missiles, North Korea is known to have a heavy multistage rocket design - the so-called "Taepodong-2" - which could put smaller satellites into orbit or potentially deliver heavier payloads as far as the continental USA. However, the prevailing view is that while North Korea has the materials and expertise to cause nuclear explosions, it is not yet able to make an atomic warhead small enough to be fitted to a Taepodong-2.
Furthermore, the Taepodong-2 has been tested only twice. On the first occasion in 2005 it blew up less than a minute after launch. On the second try last month, the upper stages failed to separate and the payload - said by Pyongyang to be a satellite - fell into the Pacific.
Nonetheless the weekend's moves by North Korea were greeted with alarm by politicians worldwide. US President Barack Obama said there had been "a blatant violation of international law". Russia and China also condemned the test firmly, leading to some hope of concerted action by the UN.
Some analysts are concerned that recent weapons-development moves by Pyongyang signal a change of strategy. The North Korean regime might be hoping not for big aid handouts to shut down its programmes, as has happened in the past with temporary effect, but rather to sell its technology to other aspiring nuclear powers.
North Korea is thought by Western intelligence agencies to have enough weapons-grade material for several nuclear devices on hand, and to be engaged in making more in its nuclear facilities. ®
The US Geological Survey records the explosion as having a magnitude of 4.7, with other sources ranging from 4.5 to 5.0. However these are body wave magnitudes, not Richter-scale. Scientists using these numbers have said they indicate a range of 2 to 8 kilotons for the Nork explosion, with results centring around 4. Even the rule of thumb relating Richter numbers to kilotons is subject to dispute, however, and it's difficult at this stage to say a lot more than "it was a good bit less powerful than the Hiroshima bomb".
More precise assessments will be forthcoming once atmospheric tests for gases generated by the explosion can be carried out.