When North Korea next tests the Disco Ball of Death or the Peanut, you can use the USGS Earthquake Map, or other data source to get a rough estimate on how large a yield North Korea got from the test.
m is the Lg wave magnitude of the seismic event
h is the depth of burial at which the test occurred
Y is the yield in kilotons
Be aware that while Wolfram Alpha will give you a precise answer, the answer is pseudo precise. In actuality, it has a ± 30% error bar, so a 100 kiloton yield could be as low as 70 kt or as high as 130 kt or somewhere in between. To date, I have been unable to get the PlusMinus operator to work with a percentage. I think this is because Wolfram Alpha interprets the input as an integer or float etc and percentages are operators, so when the calc is done, WA does its best to interpret what it was given.
I would also note that there are other methods, for example, fellow Wonk Jim Kiessling has his own scaling method on his LinkedIn and it's about as accurate as Zhang and Wen's, without the need for a PhD in geology. And Ben Muller at CNS is working on an amazing calculator that is much more applicable to nuclear tests from a much wider selection of sites in a number of other countries such as Russia and India
Thanks to Aminal for adding variables to the calculation and Nathan for helping me update the description of what the calc does