Strange Earthquake
Earthquake, or something else? No significant nearby historical seismicity, and depth pretty much at ground level. Brushing off Geodynamics and my rusty math skills yields an explosion of around 51T. This calculation pretty accurate when used against the seismic events resulting from the nuclear tests in Pakistan. To verify my numbers, energy (E) in Joules given by: log10E = 1.44M + 5.24 where M is the magnitude (4.2 in this case). From there, calculate Kilojoules, then work out the volume of TNT by the arms control definition.