How Big Was North Korea’s Bomb ?

My good friend Alec wrote on hearing about the DPRK nuclear test:

One presumes that there is a small chance it’ll have been staged with conventionals;

That got me thinking – how large a bomb was it ? We know the USGS detected a mag 4.2 shock so I went hunting around to see if there was an algorithm for converting magnitudes on the Richter Scale into energy, and, hopefully, into kilotons or megatons. It turns out J.C. Lahr wrote up a method for the “Comparison of earthquake energy to nuclear explosion energy” and helpfully included a piece of Fortran code to create a table of comparisons.

A quick “apt-get install gfortran” and a bit of mucking around with the code and I had an approximate answer:

Mag.   Energy      Energy      TNT         TNT         TNT         Hiroshima
       Joules      ft-lbs      tons        megatons   equiv. tons  bombs
4.2   0.126E+12   0.929E+11   0.301E+02   0.301E-04   0.201E+04   0.134E+00

So a magnitude 4.2 earthquake is (roughly) equivalent to a 2 kiloton device, less than one fifth of the size of Hiroshima bomb. This means it’s probably unlikely to have been a conventional device.

So what North Korea tested was fairly small in these days of megaton devices but certainly nothing you’d want to be anywhere near..