Regional seismic observation of the October 9, 2006 underground nuclear explosion in North Korea and its interpretation

The recent underground nuclear explosion (UNE) in North Korea on October 9, 2006 leaves many issues to be resolved. One outstanding question is how big the event was. Another question is why the Lg wave, which is typically the most prominent regional phase even for UNEs, is not observed much in regional observations at South Korea. This Lg phase is analyzed importantly both for constraints to the size of detonation and for the discrimination of nuclear explosion from natural earthquake. We find that the Lg from the nuclear explosion was significantly dissipated by energy leakage into the mantle due to the variation in crustal structures along the ray paths. A part of the leaked energy develops in a form of mantle-lid waves, causing large energy addition to the Sn. These features are confirmed with numerical waveform modelings and comparisons with waveforms from natural earthquakes. The discriminative Lg attenuation dependent on ray path causes underestimation of the magnitude. We redetermine the magnitude of the UNE from regional records with pure continental ray paths. The body-wave magnitude of the UNE is estimated to be 4.0±0.2.