My good friend Alec wrote on hearing about the DPRK nuclear test [1]:
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 [2] so I went hunting around [3] 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 [4] wrote up a method for the “Comparison of earthquake energy to nuclear explosion energy [5]” 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..
