Yield and burial depth of the North Korean underground nuclear tests constrained by amplitude envelopes of regional seismic waveforms

Both the yield and burial depth of a nuclear explosion are important parameters of an underground nuclear explosion test. According to the regional seismic stations in northeast China, we investigate these two parameters of the North Korean nuclear tests on 9 October 2006, 25 May 2009, 13 February 2013 and 6 January 2016 by calculating the horizontal component waveforms of coda amplitude envelopes using Pn,Pg,Sn and Lg waves. As an example, we use the seismograms recorded at the Mudanjiang (MDJ) station to show the procedure of calculating amplitude envelopes by applying a bandpass filter to horizontal component waveforms. Thus, we obtain stable estimations of amplitude envelopes at regional seismic stations, from which the regional phases are apparently displayed. The time-domain amplitude envelopes of regional phases can be modeled by using source spectral function, geometric spreading, site response, transfer function and coda shape function. Based on the grid searching method, we simultaneously estimate the yields and burial depths of the North Korean nuclear tests. The results show that the yields increase from 0.6±0.2 kt to 3.0±1.5 kt and then 10.0±2.0 kt, decrease to 8.0±2.0 kt according to their origin time. The first test has a shallow burial depth of 150±100 m, for the second one, the depth is estimated to be 350±100 m, while both the 2013 and 2016 North Korean nuclear tests have very similar depths of 500±200 m. Our results are consistent with the previous findings, which suggest that the yield and burial depth of the underground nuclear explosion tests can be simultaneously constrained by using the time-domain horizontal component of amplitude envelopes at a single station or a few stations.