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The article focuses on the effective strategy of the U.S. for future engagement with North Korea. Topics include the failed negotiations between former U.S. president Donald Trump and North Korean leader Kim Jong-un towards the denuclearization of North Korea and the stabilization of the Korean Peninsula, at the Singapore Summit declaration, in June 2018; North Korea's growing nuclear-weapons programme and missile advances; and the resumption of diplomatic engagement with South Korea.

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 re‐estimate the explosive yields of the North Korean nuclear tests by incorporating the refined burial depths. Key Points: A 3‐D high‐precision relocation method is explored using multiple phasesAccurate estimates are generated for the burial depths of the North Korean nuclear testsThe yield is estimated to be 225.7 kt for the largest nuclear explosion at the North Korea test site based on a data‐constrained burial depth of 570 m [ABSTRACT FROM AUTHOR]

The small yields of Pyongyang's 2006 and 2009 nuclear tests have left analysts wondering how the North designed its devices. One preeminent Chinese expert has a worrisome theory. [ABSTRACT FROM AUTHOR]

Declared North Korean nuclear tests in 2006, 2009, 2013 and 2016 were observed seismically at regional and teleseismic distances. Waveform similarity allows the events to be located relatively with far greater accuracy than the absolute locations can be determined from seismic data alone. There is now significant redundancy in the data given the large number of regional and teleseismic stations that have recorded multiple events, and relative location estimates can be confirmed independently by performing calculations on many mutually exclusive sets of measurements. Using a 1-D global velocity model, the distances between the events estimated using teleseismic P phases are found to be approximately 25 per cent shorter than the distances between events estimated using regional Pn phases. The 2009, 2013 and 2016 events all take place within 1 km of each other and the discrepancy between the regional and teleseismic relative location estimates is no more than about 150 m. The discrepancy is much more significant when estimating the location of the more distant 2006 event relative to the later explosions with regional and teleseismic estimates varying by many hundreds of metres. The relative location of the 2006 event is challenging given the smaller number of observing stations, the lower signal-to-noise ratio and significant waveformdissimilarity at some regional stations. The 2006 event is however highly significant in constraining the absolute locations in the terrain at the Punggye-ri test-site in relation to observed surface infrastructure. For each seismic arrival used to estimate the relative locations, we define a slowness scaling factorwhich multiplies the gradient of seismic traveltime versus distance, evaluated at the source, relative to the applied 1-D velocity model. A procedure for estimating correction terms which reduce the double-difference time residual vector norms is presented together with a discussion of the associated uncertainty. The modified velocity gradients reduce the residuals, the relative location uncertainties and the sensitivity to the combination of stations used. The traveltime gradients appear to be overestimated for the regional phases, and teleseismic relative location estimates are likely to be more accurate despite an apparent lower precision. Calibrations for regional phases are essential given that smaller magnitude events are likely not to be recorded teleseismically. We discuss the implications for the absolute event locations. Placing the 2006 event under a local maximum of overburden at 41.293°N, 129.105°E would imply a location of 41.299°N, 129.075°E for the January 2016 event, providing almost optimal overburden for the later four events. [ABSTRACT FROM AUTHOR]

When the DPRK conducted its first nuclear test on October 9, 2006, the second Korea nuclear crisis reached a climax, but almost everybody still believed in that North Korea was just playing a political card. After it conducted a second nuclear test on May 25, 2009, most people changed their thoughts and concluded that the Six Party Talks which aimed to solve the crisis are a total failure. This article analyzes the reasons and purposes for the DPRK's nuclear tests, considers the origin and development of the Six Party Talks, and concludes that the talks, while frustrating to various parties, have not been a total failure since they were moving in the right direction. The article examines the changing Sino-DPRK relationship and concludes that China should develop a long-term strategy toward the Korean peninsula and change its diplomatic policy toward the DPRK. [ABSTRACT FROM AUTHOR]