Relative Seismic Source Scaling Based on Pn Observations from the North Korean Underground Nuclear Explosions

Abstract

ABSTRACT We estimate yields and source depths for the six North Korean underground nuclear explosions (UNEs) in 2006, 2009, 2013, 2016 (January and September), and 2017, based on regional seismic observations in South Korea. Spectral ratios of event pairs are calculated using seismograms from the six UNEs observed along the same propagation paths and at the same receivers. These relative seismic source scaling spectra for Pn provide a basis for a grid search source solution that estimates source yields and depths for each event pair based on assumed explosion source models by Mueller and Murphy (1971; MM71), Denny and Johnson (1991; DJ91), and Walter and Ford (2018; WF18). The grid search is used to identify the best fit to the empirical spectral ratios subject to the source models by minimizing the root mean square misfit in the frequency range of 0.2–15 Hz. To address the trade-off between depth and yield, a modified grid search was implemented that includes elastic propagation effects for different source depths using reflectivity Green’s functions, thus modeling slight differences in propagation path based on source depth. This addition reduces trade-offs between depth and yield, and results in better model fits to frequencies as high as 15 Hz compared with cases in which depth effects were not included. The modified grid search results indicate that both MM71 and WF18 models provide comparable source depth and yield estimates with good agreement between theoretical and observed spectral ratios matching both the long-period levels and the corner frequencies, whereas the DJ91 model estimates produce lower yields due to a difference in corner frequency scaling. The best grid search solutions produce yields from ∼0.9 kt for the first UNE and up to ∼290 kt for the explosion in 2017, with depths varying from ∼280 to ∼750 m.