The Coupled Location/Depth/Yield Problem for North Korea's Declared Nuclear Tests

On 3 September 2017, the Democratic People's Republic of Korea (DPRK) conducted its sixth and largest declared nuclear test at the Punggye-ri test site. Recently, we have been using regional waveform envelopes to estimate the explosive yield and overburden of chemical and nuclear explosions by coupling explosion source models to propagation parameters. Similar to most yield determination methods, there can be trade-offs between yield and depth, leading to uncertainties in both parameters. The relative locations are well constrained by small timing differences in seismic phase arrivals at stations that recorded multiple events, but there are potential uncertainties on the absolute locations. Depths are poorly constrained by the relative arrival times. In this study, we performed a coupled location and yield analysis of the DPRK nuclear tests. We obtain highly accurate travel times using correlation methods and relative locations using a Bayesian location method. Then, while keeping the relative locations of the six tests constant, we consider the consequences of shifts to the absolute locations on the resulting overburden for each event. Given that overburden, we determine the yield that minimizes the waveform misfit. We also test and compare a number of explosion source models. By considering the coupled location/depth/yield problem, we reduce uncertainties in the absolute locations, and determine yield and depth estimates of the events. Based on statistical analysis, we estimate that the 2017 test has a yield of 125 kt (equivalent trinitrotoluene [TNT]) with a 1 sigma uncertainty range of 103-150 kt at about 600 m of overburden.