Moment Tensor Inversion Analysis of DPRK6 Nuclear Events Using CTBTO/IMS Data

The global verification system established under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) is designed to detect all nuclear explosions on Earth. Seismic monitoring, one of the four verification technologies, relies on the International Monitoring System (IMS), a global network of sensor stations, to identify nuclear explosion signals. This study presents an application of Moment Tensor (MT) inversion analysis to assist individual States Parties through expert technical analysis (ETA) of IMS data and any additional datasets provided by the requesting State Party. MT inversion enables precise determination of parameters such as total seismic moment, focal mechanism, and source depth. 

To evaluate this approach, we analyzed data from declared nuclear events in the Democratic People’s Republic of Korea (DPRK). For the most recent event, DPRK6 (2017/09/03), two methodologies were applied: (1) a regional moment tensor inversion in the time domain (TDMT, Dreger, 2003) and (2) a joint inversion using regional waveforms and teleseismic firstmotion polarities (Nayak and Dreger, 2015; Chi-Durán et al., 2024). The analysis included 4 regional waveforms (filtered between 20–50 s) and 81 teleseismic first-motion polarities from CTBTO stations. Known regional velocity models were used to model the synthetic waveforms (Ford et al., 2010; Dreger et al., 2021).

The TDMT approach achieved a high waveform fit and revealed a predominantly isotropic mechanism with a minor double-couple component. These findings are consistent with previous studies using other station datasets (e.g., Alvizuri and Tape, 2018; Chiang et al., 2018). The joint inversion further improved the waveform fit, with the isotropic component remaining dominant. The source-type lune plot confirmed a mechanism primarily characterized by isotropy. Current efforts aim to incorporate additional data, such as teleseismic waveforms, to refine the depth and other characteristics of the event across all declared DPRK events.