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In 2015 and 2016, collapse-induced earthquakes, accompanied by surface subsidence, occurred at two underground mines in South Korea and were followed by impulsive infrasound signals that were detectable at distances greater than 200km. To explain the infrasound generation caused by the collapses, we hypothesize that excess pressure was released by the rapid outflow of an underground air mass into the atmosphere caused by the sudden implosion of earth into the underground space. We tested this idea with a physical model that assumes a displaced air volume as a monopole source with reduced pressure. The estimated volumes of the two collapses were similar to 1.7x10(5) and similar to 7.3x10(5)m(3), respectively, which agrees with the collapse volumes inferred from surface subsidence data. The infrasound generation model we used is applicable to the monitoring of other catastrophic failures within manmade or natural subsurface cavities.

Plain Language Summary Infrasound, a low-frequency atmospheric acoustic wave, was observed following accidental underground mine collapses in South Korea. Examination of the observations showed that underground air abruptly flowed out of the mines. The size of the collapses was inferred based on an infrasound generation model. The results imply that it may be possible to identify failures of large underground spaces, such as manmade tunnels or naturally formed caves, from infrasound.