Research progress on monitoring the North Korean underground nuclear tests using regional seismic data

Based on broadband regional seismic data generated by six North Korean underground nuclear tests, recorded by networks in Northeast China and adjacent areas, this paper developed a set of seismological methods to monitor North Korean underground nuclear tests, including magnitude measurement and yield estimation, high-precision relocation, and discrimination between atomic explosions and natural earthquakes. Since the North Korean nuclear test site (NKTS) is uncalibrated, it is crucial to determine the appropriate empirical magnitude-yield equation for the NKTS. This paper gathered ground truth data from chemical explosions used for deep seismic sounding, including their locations, yields, and charge depths, and determined a magnitude-yield relation suitable for the NKTS. A high-precision relative relocation method was applied to the six nuclear explosions at NKTS. To further alleviate the tradeoff between the detonate time and the source depth, according to the fact that the Pn and Pg waves have different incident angles, Pn and Pg waveform correlations are jointly used to increase the accuracy of relative source depth calculation. By analyzing the P/S type spectral amplitude ratios, Pn/Lg, Pg/Lg, and Pn/Sn, for 6 nuclear tests, 4 natural earthquakes, and 3 chemical explosions, this paper found that, at above 2 Hz, the network averaged spectral amplitude ratios could reliably separate the North Korean nuclear tests from the natural earthquakes. The spectral amplitude ratios of 3 chemical explosions are located between those of the nuclear explosions and the natural earthquakes.