The waveform from the USGS seismometer that detected the North Korean "earthquake" that happened Monday near Sungjibaegam, off of Nuclear Test Road, looks very similar to the waveform of the 2009
That test registered a 4.5 magnitude, shown in blue, where today's registered a 4.9, the red waveform. The South Korean estimate is that the bomb may have been between 6 and 7 kilotons. Estimates for the 2009 test indicated the blast was about 2.5 kilotons.
By comparison, the nuclear bombs dropped by the US on Hiroshima produced a 16 kiloton explosion.
The graph was tweeted out by Andy Frassetto (@drrocks1982), a geoscientist at the University of Arizona.
Comparison for the nearest real-time broadband seismometer. As promised @cppgeophysics twitter.com/drrocks1982/st…
— Andy Frassetto (@drrocks1982) February 12, 2013