Abstract

In 2006 and 2009, North Korea conducted underground nuclear tests at a remote mountainous site in the northeastern part of the country. Both events were small, leading to uncertainties for location, yield estimation, and discrimination based on seismic observations. The objectives of the studies described in this paper have been to more fully exploit data from the International Monitoring System (IMS) network and other unclassified sources (e.g., Incorporated Research Institutions for Seismology [IRIS], Japanese National Research Institute for Earth Science and Disaster Prevention [NIED]) to analyze these nuclear tests. We have used differential waveform interferometry (DWIF) to refine relative locations for the tests and extended the methodology using observations from regional stations to estimate relative emplacement depths, which are consistent with explosion source model predictions for broadband regional P‐wave spectral ratios. Topographic data from the test site were analyzed to constrain relative locations and depths needed for containment of the two tests based on normal testing practice. Network‐averaged teleseismic P‐wave spectra were inverted using a model‐based approach and provide explosion yield estimates in the range 0.6–1.0 kt for the 2006 test and 2.0–4.8 kt for the 2009 test, depending on assumed emplacement depth. Detailed spectral analysis of broadband regional P observations are most consistent with theoretical explosion models with yields of 0.9 and 4.6 kt for the 2006 and 2009 tests, respectively, with associated source depths of 200 and 550 m. Consistent with previous studies, the surface‐wave MS magnitudes for the 2006 and 2009 tests provide anomalously large MS yield estimates and problematic MS/mb identification characteristics. Finite‐difference simulations of long‐period surface‐wave signals incorporating tensile prestress, consistent with the regional tectonics, lead to predicted MS values that agree much better with the observed values.

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