Infrasound Generation from the 2017 DPRK Underground Nuclear Explosion

Abstract:

We use the Rayleigh Integral (Pierce 1981) with numerical results of Stevens and O’Brien (2018) to compute far-field excitation of infrasound from the September 3, 2017 DPRK underground explosion conducted under Mt. Mantap. Stevens and O’Brien (2018) performed a three-dimensional non-linear simulation including the mountain topography, non-linear deformation, and complex dynamic free-surface interactions including spall. Using their derived surface accelerations, a(r,t) normal to the ground surface, S(r), we compute a synthetic far-field pressure, P(r,t), at location r and t in a homogeneous atmosphere.

We define a far-field upper-hemisphere equivalent point source radiation pattern as a function of time/frequency and direction/slowness, P₀(r0,t) with respect to a reference distance,|r0|. Source slowness vectors are then defined, s = r0/|r0|/c₀, where c₀ is the local sound speed. This equivalent point source may be used to drive numerical synthetics using geometrical ray theory, parabolic equation (PE), or modal summation with integration over source slowness. NCPAProp synthetics define transmission loss with respect to amplitudes at a reference 1 km from the source. This far-field source representation captures the spatial-temporal effects of the surface acceleration as well as the spatial-temporal delays associated with propagation from the surface topography.

We present examples of the temporal source function P₀(s,t). Our results predict substantial frequency-dependent infrasound directivity. The 3-D non-linear simulations predict enhanced spall on the southern flank of Mt. Mantap. The asymmetry in the surface spall combined with the inclination of the southern flank contribute to southerly enhanced/focused infrasound radiation with a distinctive frequency dependence. We compare infrasound data from this event with these results.

Pierce, A. D. (1981) Acoustics, An Introduction to Its Physical Principals and Applications, McGraw Hill, 534 pages.

Stevens, J. L. and M. O’Brien (2018), “3D Nonlinear Calculation of the 2017 North Korean Nuclear Test,” Seismological Research Letters, 89, 2068-2077. doi: 10.1785/0220180099.

(CENTER) Upper hemispheric projection of the radiation pattern at 0.67 Hz. Equivalent source functions at 20-degree takeoff to YPDAR (LEFT) and IS45 (RIGHT).