High-Precision Relocation With the Burial Depths of the North Korean Underground Nuclear Explosions by Combining Pn and Pg Differential Traveltimes

In seismic nuclear monitoring, an accurate source depth is an important prerequisite for reliably estimating the explosive yield. Relative location methods are often used for this purpose. However, the conventional method based on regional Pn waves usually fails to give satisfactory constraints on the source depth, mainly due to the strong trade-off between the burial depth and origin time. This study explores a high-precision relative location method to simultaneously determine the relative epicenter and source elevation by using differential traveltimes from both downward-takeoff Pn waves and horizontal-takeoff Pg waves. The properties of both Pg and Pn waves, including the consistency between waveforms, the reliability of differential traveltime measurements, and their sensitivities to epicenter and depth variations, are investigated. By jointly applying both types of waveform data, the proposed method significantly enhances the constraint on the source depth variation. This method is applied to regional seismic data collected from China, South Korea, and Japan to determine the relative epicenters, origin times and relative burial depths of six North Korean underground nuclear explosions. The source depths are then used to provide burial-depth corrections to estimate the explosive yields. The depths of the six North Korean nuclear explosions detonated on October 2006, May 2009, February 2013, January 2016, September 2016, and September 2017 are determined to be 330, 540, 506, 468, 521, and 570 m, respectively, and their yields after burial depth corrections are 1.6, 5.7, 13.4, 12.6, 21.7, and 225.7 kt, respectively. Plain Language Summary For many years, the issue of nuclear testing has been a concern throughout the international community. The 1996 Comprehensive Nuclear-Test-Ban Treaty stipulated that radioactive, infrasound, seismic wave and underwater acoustic detection techniques can be used to monitor nuclear testing. From October 9, 2006 to September 3, 2017, the Democratic People's Republic of Korea conducted six underground nuclear tests whose locations, origin times and explosive yields have been estimated by numerous studies. Among these parameters, the determination of the burial depth of a nuclear test is always particularly difficult yet nevertheless crucial, being a prerequisite to reasonably estimate the explosive yield. Seismological relative location methods determine locations of earthquakes relative to a master reference event by comparing differences in seismic wave propagation times between earthquakes to be determined and the master event. In a large number of previous studies, relative location methods based on one kind of seismic wave often encounter difficulties to determine the burial depth. Therefore, in this study, we explore a multiple seismic wave relocation method to simultaneously determine the surface location and burial depth of underground nuclear explosions. Additionally, we reestimate the explosive yields of the North Korean nuclear tests by incorporating the refined burial depths.

Automated summary

In this study, a high-precision relative location method was explored to simultaneously determine the relative epicenter and source elevation by using differential traveltimes from both downward-takeoff Pn waves and horizontal-takeoff Pg waves. By jointly applying both types of waveform data, the proposed method significantly enhances the constraint on the source depth variation, and successfully applied to regional seismic data from China, South Korea, and Japan.