Seismological discrimination and yield estimation of the 3 September 2017 Democratic People′s Republic of Korea (DPRK) underground nuclear test

Abstract

At 11:30 on September 3, 2017 (Beijing time), a strong earthquake occurred in Democratic People′s Republic of Korea (DPRK). International seismic monitoring agencies, e.g., the China Earthquake Network Center (CENC) and the United States Geological Survey, suspected that it is an explosion. Based on seismic data from the China National Digital Seismic Network (CNDSN) and Global Seismic Network (GSN), we investigated characteristics of this event and an aftershock 8 min after the main event. The P- and S-wave excitation functions of explosion and earthquake sources are scaled differently. Therefore, the P/S-type spectral ratios can be an effective discriminant for separating explosions from earthquakes. Using the P/S spectral ratios Pg/Lg, Pn/Lg and Pn/Sn as discriminants, we confirmed the 3 September 2017 event was an explosion. For the aftershock occurred after the main event, we identified it is a collapse, likely caused by the failure of the explosion generated cavity. Using a pre-calibrated regional seismic network in Northeast China and the Korean peninsula, and the regional Lg-wave attenuation model developed previously, we obtained the Lg wave body wave magnitudes for the 3 September 2017 main event and its aftershock to be m b(Lg)=5.6±0.2, and 3.95±0.04. We used a group of historical events to calibrate the regional network for calculating Rayleigh wave magnitude. After correcting for the site responses, the network averaged surface wave magnitudes for the main shock and the aftershock were obtained to be M s=5.1±0.2 and 3.95±0.08. To test body-wave versus surface-wave magnitude as a potential discriminant, we compared the M s (Rayleigh) and m b (Lg) for all 6 DPRK nuclear explosions and a group of earthquakes in Northeast China and the Korean peninsula. The explosion and earthquake populations were largely overlapped with each other. The above results show that the P/S ratio method is a more effective discriminant than the m b(Lg)- M s criterion in Northeast China and the Korean Peninsula. The seismic yield of an underground nuclear explosion can be estimated from its magnitude using a calibrated empirical magnitude-yield relation. However, the DPRK test site (DPRKTS) is an uncalibrated test site. Considering that the DPRKTS is located at a granite site in a stable geology platform, we adopted the fully-coupled hard-rock site equation used at the Novaya Zemlya test site to calculate the yield at the DPRKTS. The estimated yield for the 3 September 2017 explosion was 56 kt using this relationship and assuming a normally scaled burial depth. Transferring the measurement error of ±0.2 magnitude unit to the yield calculation introduced uncertainties between 30 and 100 kt. However, if the explosion was over buried at depths between 1000 and 2400 m, the yield could be increased to 100–200 kt.