So why was the North Korean blast so small? There are three possibilities- It was a failed test, it was deliberately kept small due to the absence of a large desert to test it under or it was a hoax designed to intimidate the country's enemies, ie everyone.
The arguments that it was an unsuccessful test for what was intended to be a larger explosion is set out in a series of posts at Defense Tech. Essentially the argument is that it produced an energy yield of only around ten percent of what it should have been expected to produce. These devices designed to be attached to Kim Jong Il's Dong missiles explode too soon. Kim's Dong goes off prematurely.
The possibility that shows North Korea's scientists in the best light is that they kept the explosion small deliberately either to avoid upsetting its neighbours too much or for enviromental reasons. As this article notes:
carrying out nuclear tests inside North Korea would be an extremely sticky action. That is because this kind of nuclear testing could only be carried out underground. There is absolutely no way they could do in the air or above ground. Even with underground nuclear testing, you normally need a fifty to sixty kilometer square of desert for a nuclear test. In the U.S., this would be something like the Nevada desert. Unless you have the kind they have in India or Pakistan, you cannot do it. The reason for this is that the underground water system gets damaged. North Korea has a very abundant flow of underground water, and if you carry out an underground nuclear test in this kind of place, radioactive materials would get into the water supply for the whole of the Korean peninsulaWhilst the hermit nation isn't known for its devotion to its citizen's welfare even a hellhole wants to keep some of its own people safe from radiation poisoning.
The arguments in favour of the proposition that it is a hoax by the DPRK are that the size of explosion can easily be reproduced by other means, the fact that NK has been trying to get the respect of its neighbours for a couple of years and the fact that they insist that there has been no radiation leakage which is highly unusual although it does have the convenient result that air samples can't confirm or disprove the claims of a successful blast. It is certainly the case that the possibility of it being a hoax is being discussed more widely since yesterday when I was worried that I had become the classic internet conspiracy crank for voicing my scepticism:
U.S. intelligence agencies say, based on preliminary indications, that North Korea did not produce its first nuclear blast yesterday.The consensus now appears to be that North Korea's military conducted an unsuccessful test and are probably going to behave even more crazily to regain some respect. For what it's worth I still believe the whole thing is a fraud perpetrated by Pyongyang.
U.S. officials, speaking on the condition of anonymity, said that seismic readings show that the conventional high explosives used to create a chain reaction in a plutonium-based device went off, but that the blast's readings were shy of a typical nuclear detonation.