A Norwegian seismic tracking organization named NORSAR yesterday released a chart showing what they call a "fingerprint" of North Korea's last three nuclear detonations. Their fingerprint works because North Korea used roughly the same position to "test" all three nukes.
“Look at the three lines showing the test blasts of 2006, 2009 and 2013,” Steven J. Gibbons, senior research geophysicist at NORSAR, told IEEE Spectrum. “The ripples on the seismograms look identical, except for the difference in amplitude. That’s because the seismic waves have traveled through exactly the same rock, the same rock boundaries.”
“If you increase the yield by a factor of 10, you increase the amplitude on the seismograph by a factor of log 10,” Gibbons explained to IEEE. “We think North Korea’s yields have increased tenfold--from 1 kiloton in 2006 to 5 kilotons in 2009 and to 10, in 2013.”