Infrasound from the North Korea underground explosion and subsequent collapse on 3 September 2017

Strong ground motions induced by North Korea’s declared underground nuclear test in September 2017 and a subsequent subsurface collapse excited substantial and characteristic atmospheric acoustic waves (infrasound) that were detected by multiple stations at regional distances. Back-projection method is applied to the detected long-lasting coherent infrasound wavetrains related to the nuclear test. This allows to reconstruct source locations and reveals ground-to-air coupling in a large area over the northeast Korean Peninsula. To understand the excitation of atmospheric acoustic phases from the underground sources, full 3-D seismo-acoustic simulations are performed with pre-defined seismic moment tensor solutions of the underground sources. The simulations quantitatively predict the excitation of epicentral and diffracted acoustic phases developed by direct vertical ground motion at the immediate epicenter and by seismic surface waves propagating through high mountainous regions, respectively. In the atmosphere, the direct acoustic phases propagate spherically at the speed of sound, but the diffracted phases form inclined wavefronts in the atmosphere as the surface wave moves away from the epicenter. On a broad scale, the simulated acoustic coupling shows good agreement with the infrasound radiation patterns determined from the infrasound observations. Additional simulations for the subsequent subsurface collapse event show that an underground cavity collapse can be a potential mechanism for the production of low-frequency acoustic energy that is also detectable at regional distances. Finally, this study highlights the link between ground motions caused by underground sources and infrasound detection, further enabling infrasound as a depth discriminant for subsurface sources.