An analysis of the propagation of underground-explosion-generated infrasonic waves based on the equations of fluid dynamics: signatures at ground and in the thermosphere.

Underground tests induce infrasonic acoustic waves in the Earth's atmosphere, which can travel over hundreds or thousands of kilometers (Assink et al., GRL {43}, 2016) and up to the thermosphere-ionosphere (Rudenko & Uralov, J. Atmos. Sol.-Terr. Phys. {57}, 1995; Yang et al., GRL {39}, 2012; Park et al., Radio Sci {48}, 2016). In this work, the atmospheric propagation of underground-explosion-generated signals is investigated via numerical simulations of the equations of fluid dynamics. The radiation of acoustic energy by the ground motion caused by underground tests is initiated by enforcing the equality, at ground level, between the air velocity normal to the Earth's surface and the normal velocity of the soil layer. The ground velocity is defined semi-empirically as a function of the depth of burial and of the yield (Rudenko & Uralov, J. Atmospheric Sol.-Terr. Phys. {57}, 1995). The effects of the depth and of the source energy on the perturbations recorded in the epicentral zone are first discussed. The phases traveling at long range and up to the ionosphere are then analyzed. Synthesized waveforms are finally discussed and compared to the ground and ionospheric total electron content (TEC) data available in the literature respectively for the 2013 DPRK test (Assink et al., GRL {43}(7), 2016) and for the 2009 DPRK underground explosion (Yang et al., GRL {39}, 2012). Good agreement is found between numerical results and experimental ground signals, which motivates the use of infrasound technologies, alongside with seismic techniques, for the characterization of underground tests. Due to the severe thermospheric absorption induced by viscous stresses and thermal conduction, the synthesized thermospheric velocity perturbations for the 2009 event are quite small ( ∼few m/s). This suggests the need for deeper analyses of the TEC waveforms observed after the 2009 test, to confirm consistency and establish quantitative thresholds for detectability.