AYBABTU, baby!
Even if you don’t get the nerdy reference in the title or the line above, you probably have heard that North Korea claims to have detonated a nuclear weapon. I’m inclined to believe it, despite the fact that the estimates of the total yield of Kim Jong-Il’s bomb are all over the map. In a single CNN article, China says they have detected a half kiloton yield, while Russia says that it was as large as 15 kilotons.
As I understand it, nuclear weapons with yields under a kiloton are very hard to detect, and pretty hard to build. It’s actually easier to make a 20 kiloton weapon than a tiny one.
Considering the modest size and the short time it has taken North Korea to go from threating to build it to actually testing, it’s clear that they must have built a fairly primitive atomic bomb rather than a hydrogen bomb, which is much harder to assemble in part because it needs a plain old atomic bomb to set it off. See the differences between the two nuclear weapon types here.
If you are both patient and determined to learn everything about nuclear test monitoring, check out the book Technical Issues Related to the Comprehensive Nuclear Test Ban Treaty by the Committee on International Security and Arms Control. It’s free to read online, but a bit dense.
A more readable summary of nuclear test monitoring issues is available through Geotimes which is published by the American Geological Institute.
For the most part, monitoring nuclear tests is the domain of seismologists. CNN published this seismograph data from the North Korean test.