Moment Tensor Inversion for Nuclear Explosions: What Can We Learn from the 6 January and 9 September 2016 Nuclear Tests, North Korea?

Abstract

Two nuclear explosions were carried out by the Democratic People's Republic of North Korea in January and September 2016. Epicenters were located close to those of the 2006, 2009, and 2013 previous explosions. We perform a seismological analysis of the 2016 events combining the analysis of full waveforms at regional distances and seismic array beams at teleseismic distances. We estimate the most relevant source parameters, such as source depth, moment release, and full moment tensor (MT). The best MT solution can be decomposed into an isotropic source, directly related with the explosion and an additional deviatoric term, likely due to near-source interactions with topographic and/or underground facilities features. We additionally perform an accurate resolution test to assess source parameters uncertainties and trade-offs. This analysis sheds light on source parameters inconsistencies among studies on previous shallow explosive sources. The resolution of the true MT is hindered by strong source parameters trade-offs, so that a broad range of well-fitting MT solutions can be found, spanning from a dominant positive isotropic term to a dominant negative vertical compensated linear vector dipole. The true mechanism can be discriminated by additionally modeling first-motion polarities at seismic arrays at teleseismic distances. A comparative assessment of the 2016 explosion with earlier nuclear tests documents similar vertical waveforms but a significant increase of amplitude for the 2016 explosions, which proves that the 9 September 2016 was the largest nuclear explosion ever performed in North Korea with a magnitude M w 4.9 and a shallow depth of less than 2 km, although there are no proofs of a fusion explosion. Modeling transversal component waveforms suggests variable size and orientation of the double-couple components of the 2009, 2013, and 2016 sources.Electronic Supplement: Results of simulated annealing moment tensor (MT) inversion using a synthetic dataset, and a table of amplitude correction coefficients for each station used for the MT inversion.

North Korea's 2017 Test and its Nontectonic Aftershock

“…Moment magnitude M w trades off with depth, decreasing from 6 to 5.6 over the examined depth range of 0.5–2.5 km. At the 1.5‐km depth, which might perhaps be preferred, in analogy with the seismic array study of the 2016 test (Cesca et al, ), the M w 5.7 is obtained; the full MT is given in Table S1b. The ISO part (ISO ~55–60%), together with the CLVD (CLVD ~30–45%), clearly dominates over the DC part (DC ~0–10%).…”

“…Before proceeding to aftershocks, which practically cannot be analyzed from more distant stations, we verify the mainshock results at 10 stations in Japan. Lower frequencies (0.02–0.04 Hz), as in Cesca et al (), need to be used due to modeling problems at the considerably greater epicentral distances (750–1,100 km). The full MT derived from the 12 stations was used to forward simulate the observed data in a single 1‐D model derived from Crust2.0 (Bassin et al, ), while assuming the source depth of 1.5 km (i.e., M w 5.7).…”

“…Nonisotropic radiation, including shear motion, was observed in the 2013 test (Barth, ; Vavryčuk & Kim, ). The 2016 tests helped to delimit a significant uncertainty in the source parameters (Cesca et al, ). The uncertainty is due to poor resolution of the depth and certain moment‐tensor components when a shallow nondouble‐couple source is studied at long wavelengths (e.g., Bukchin et al, ; Henry et al, ).…”

Seismological Constraints on the Source Mechanism of the Damaging Seismic Event of 21 August 2017 on Ischia Island (Southern Italy)

“…This factor, in the absence of OBSs, can negatively affect the moment tensor resolution (e.g., Domingues et al, 2013). The second factor is the very shallow depth of the seismic source, which can strongly affect the resolution of certain moment tensor components (Cesca et al, 2017).…”

Yield Estimates for the Six North Korean Nuclear Tests From TeleseismicPWave Modeling and Intercorrelation ofPandPnRecordings

“…A variety of methods have been used to estimate the yields or seismic moments of the six tests, including assumed transportability of body wave, surface wave, or Lg magnitude yield scaling relations (e.g., Chun et al, ; Hartmann et al, ; Yao et al, ; Zhang & Wen, ; Zhao et al, , , , , Zhao, Xie, Wang, et al, ,Zhao, Xie, He, et al, ); modeling of surface deformation (Wang et al, ; Wei, ), regional waveforms and spectra (e.g., Murphy et al, ; Rougier et al, ; Stevens & O'Brien, ; Stroujkova, ; Wang et al, ), regional coda envelopes (e.g., Pasyanos & Myers, ), network‐averaged teleseismic P wave spectra (e.g., Murphy et al, ), and teleseismic P waveforms (e.g., Chaves et al, ; Ni et al, ); and moment tensor inversion for the total and isotropic moment of the explosions (e.g., Alvizuri & Tape, ; Barth, ; Cesca et al, ; Chiang et al, ; Ford et al, ; Ichinose et al, ; Liu, Li, Zahradnik, Sokos, Liu, et al, ,Liu, Li, Zahradník, Sokos, & Plicka, ; Vavrycuk & Kim, ; Wang et al, ; Yao et al, ). Some depth constraints are inferred from infrasound as well (Assink et al, ).…”