Regional observations of the second North Korean nuclear test on 2009 May 25

P>The suspicious seismic event that occurred in the northern Korean Peninsula on 2009 May 25 was declared to be the second underground nuclear test (NK2ND) by North Korea. We investigated the characteristics of NK2ND using seismic signals recorded at regional-distance stations in South Korea and China. The Pn/Lg ratios of NK2ND definitely discriminate this event from two nearby natural earthquakes at frequencies above 4 Hz. Full moment tensor inversion of full waveform data shows that NK2ND had a very large isotropic component. Pure isotropic moment tensor inversion also resulted in good recovery of observed waveforms, with clear indication that NK2ND was explosive in origin. The moment magnitude (M(w)) from the full moment tensor inversion was estimated to be 4.5 and network-averaged values of 4.6 and 3.6 were calculated for rms m(b)(Lg) and M(s)(VMAX), respectively. Although m(b) - M(s) signature has been considered one of the most reliable discriminants for separating explosions and earthquakes, this signature showed poor discrimination in the case of NK2ND. The Pn/Lg ratios and moment tensor inversion give more reliable evidence than does the m(b) - M(s) for classifying the suspicious event in the northern Korean Peninsula as a possible explosion. The characteristics of NK2ND are also quite similar to those of the first North Korean nuclear test on 2006 October 9.