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

Abstract

S U M M A R YThe 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.

Significant release of shear energy of the North Korean nuclear test on February 12, 2013

“…Nevertheless, this study shows that only the nuclear explosion in 2013 did release a major part of shear energy, while, corresponding to the results of Shin et al (2010), NK2009 did not. Moreover, the obtained strike of the potential fault planes of the shear energy release for both tests correlates well with the observed orientation of fault systems in the region, which are dominantly NE-SW to NNE-SSW (Şengör & Natal'in 1996).…”

“…Dreger & Woods 2002, Shin et al 2010. Figure 4 compares synthetic waveforms calculated for a pure isotropic (explosive) source for a 1D and 3D global earth model.…”

“…For an explosion, the Pg/Lg ratio is expected to be significantly higher than for a tectonic event, as the compressional nature of an explosion ideally does not radiate shear energy. Recent studies analysed those kind of data for the previous two nuclear tests in North Korea (NK2006 and NK2009) and determined an explosive character (Kim & Richards 2007;Zhao et al 2008;Shin et al 2010;Hong 2013;Ford et al 2009;Ford et al 2012).…”

A Catalog of Regional Moment Tensors in Utah from 1998 to 2011

“…This method is well established and has been widely used in regional moment tensor studies worldwide, including South Africa (Brandt and Saunders, 2011), New Zealand (Ristau, 2008), Japan (Kubo et al, 2002), and Brazil (Assumpção et al, 2011). The full moment tensor capability of this method has been utilized to determine whether certain events have isotropic components, such as known and suspected nuclear explosions (Dreger and Woods, 2002;Ford et al, 2009;Shin et al, 2010), suspected mine-collapse events (Ford et al, 2008), source processes of volcanic events (Dreger et al, 2000;Templeton and Dreger, 2006;Cannata et al, 2009), and glacial icequakes (Walter et al, , 2010. This ability to determine a volume-change component is especially important for the mining areas in Utah, where implosive events can occur (Fletcher and McGarr, 2005;Ford et al, 2008).…”

Comment on "Advanced Seismic Analyses of the Source Characteristics of the 2006 and 2009 North Korean Nuclear Tests" by J. R. Murphy, J. L. Stevens, B. C. Kohl, and T. J. Bennett

“…Adopting values of 4.62 and 3.58 for m b and M s (Selby et al, 2012), respectively, and 4.15 for M w based on the average of isotropic moments for a pure explosion source (Ford et al, 2009;Shin et al, 2010), the TOCs for the 2009 DPRK test are plotted in Figure 4. The results show reasonably good agreement for TOCs based on M s and M w .…”