Magnitude Estimation and Source Discrimination: A Close Look at the 2006 and 2009 North Korean Underground Nuclear Explosions

North Korea detonated its first known nuclear device, a moderate-sized event in the northeastern corner of the Korean Peninsula, on 9 October 2006. A second one, several times more powerful, was set off nearby on 25 May 2009. Both were recorded at high signal-to-noise ratio (SNR) by a modern broadband (0.03-30 Hz) network of seismographs deployed since 2004 along the Sino-Korean border, and by station MDJ due north of ground zero. Spanning a wide range of station azimuth (259 degrees-11 degrees), the near-regional (1.3 degrees-3.3 degrees) paths are all purely continental and away from continental margins, making the resulting data uniquely suited for assessing the capabilities of an out-country network to verify the compliance by North Korea with a future comprehensive nuclear test ban treaty. The m(b)(Lg) magnitudes for the 2006 and 2009 nuclear explosions were determined anew, giving 4.32 +/- 0.13 and 4.86 +/- 0.13, respectively. We show that the M-S-m(b)(Lg) method correctly differentiates these nuclear explosions from natural earthquakes, although the differentiation was not achieved in studies based on published m(b)(Lg) values. An analysis of the Pg:Lg ratio, derived from recordings of the two nuclear tests and of four regional earthquakes selected for their comparable magnitudes (4.2 <= m(b) <= 4.8), reveals that the ratios associated with the explosion and earthquake populations showed surprisingly little overlap for a broad frequency range of 3 to 11 Hz. In principle, M-S-m(b) can also be used to correctly identify the nuclear tests as explosions, but the m(b) measurement for such moderate-sized events is not as robust as the m(b) (Lg) measurement, rendering this traditionally favored method less practical.