Exploitation of the IMS and Other Data for a Comprehensive Advanced Analysis of the North Korean Nuclear Tests

On May 25, 2009, the North Koreans conducted a second underground nuclear test in the same area as their initial 2006 test. Preliminary analysis indicated that the explosive yield of the second test was roughly 5 times that of the first, and it was found to have been well-recorded by a variety of globally distributed seismic networks. We carefully analyzed the available short-period seismic data in an attempt to define accurate locations, depths and yields for these two North Korean nuclear tests. In order to determine accurate relative locations for the two events, we made very precise arrival time measurements at 35 stations that recorded both explosions with good signal to noise ratios. The location of the 2006 explosion was then held fixed at the previously determined preferred location and the relative location of the 2009 explosion was estimated using the Joint Hypocenter Determination (JHD), Double Difference (DD) and Differential Waveform Interferometry (DWIF) location algorithms. All of these relative location techniques yielded very similar results, indicating that the 2009 test was conducted about 2.5 km west-northwest of the 2006 test. These relative seismic locations were subsequently integrated with the local topographic data and satellite imagery to define what we believe to be very reasonable and accurate locations for these two explosions. The corresponding source depths can not be reliably determined using the currently available arrival time data or the observed, narrowband network-averaged teleseismic P wave spectral data. Consequently, we implemented a new approach using broadband P wave spectral ratios of the two explosions at common regional stations to obtain estimates of the corresponding broadband source spectral ratios. The resulting network-averaged source spectral ratio was then compared with theoretical Mueller/Murphy based source spectral ratios to estimate best-fitting source depths and associated yields. The results of this analysis indicated that the two explosions could not have been detonated at any common depth in the plausible 100 to 800 m depth range and, in fact, the observed spectral ratio data are best fit by source depths of about 200 m for the 2006 test and 550 m for the 2009 test. The corresponding yield estimate for the May 25, 2009, explosion was then found to be about 4.6 kt. The long-period surface wave Ms magnitudes for both the 2006 and 2009 tests appear to be anomalously large relative to historical experience, producing unreasonably large Ms yield estimates and problematic Ms/mb identification characteristics. A formal moment tensor inversion analysis of the available data indicated that release of tectonic strain energy by the explosion may have contributed somewhat to the observed anomaly. However, current estimates of the likely strength of this tectonic release are not large enough to fully explain the observed anomaly. Additional research will be required to determine whether unresolved CLVD secondary sources may account for the discrepancy. Identification of the 2009 and 2006 events as explosions based on high-frequency Pn/Lg ratios measured at regional stations are unambiguous; however, results for discrimination based on Ms-versus-mb are inconclusive (again probably due to secondary source contamination to Ms).