With all the talk about the North Korea nuclear test program, and of a planned 6th nuclear test, many are bound to wonder at the power of their nuclear weapons, as so far tested. Well indeed, the answer is very much on the record, on account of the seismic signatures of their previous 5 tests.
Monitoring stations all around the world are constantly alert to the presence of notable seismic events, generating a signature indicative of a nuclear blast, as opposed to a natural earthquake.
Certainly, based upon a multitude of underground tests, as conducted by such as the United States at their Nevada test site from the 1960s onwards, a great deal of data exists to allow one to assess the yield of a nuclear weapon, based upon its seismic signature.
With regard to North Korea then, on the official USGS website (US Geological Survey), as of the present time, all 5 of their tests so far conducted are listed as seismic events. They are given as follows:
Now one can see that the first one was conducted in 2006. And indeed, that their nuclear tests have been steadily getting more powerful. Moreover, with their last test, as conducted 9 September 2016, they achieved their greatest seismic signature to date.
In 2006, with the very first North Korean test, scientists conducted a detailed evaluation of its seismic energy signature, using a special formula unique to the composition of the ground about their test site. This allowed them to determine the TNT explosive yield value of the test.
Now this very same formula is indeed applicable to all of their 5 nuclear tests to date, due to the fact that they were all done at the same test site, in the north eastern part of the country.
The table below specifies the formula in question, as used to derive here the TNT yield values of all of their nuclear weapons tests to date. Further to this, one can also see for comparison, the nuclear yield values of the first 3 devices used by the United States in 1945. They include, the initial Trinity test in New Mexico, and the devices used to bomb Hiroshima and Nagasaki:
Here it can be seen then from the above table that with their last nuclear test, North Korea would appear to have achieved close to parity with the early US devices. As here, their 5th test, with a seismic signature of 5.3, generated a nuclear blast the equivalent of 11662 tonnes of TNT.
Further to this, one may also consider visually the scale of their devices, were they used atmospherically, above ground. The following image compares the Nagasaki blast to the massive US 15,000,000 tonne TNT test in 1954, codenamed: Bravo.
A North Korean nuclear weapon would be just to the left of the Nagasaki Blast; slightly smaller in scale. Far more so that the Bravo test.