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Volcanic Airwave | John Seach
An air wave is seismic energy transmitted through the atmosphere. Air waves can be seen close up to active vents when they disrupt eruption clouds. For large eruptions, airwaves can travel great distances.
For an observer the visual representation of the wave through the cloud precedes both projectiles and sound.
An explosion in an open vent produces a strong shock wave in the air which shows up as a high-frequency air-wave phase on seismographs. Explosions which are contained within the volcano cannot couple with air, so that the radiated energy is transmitted only through seismic waves, producing a low-frequency seismic event. Airwaves may couple to the ground resulting in seismic events which are detected by seismographs.
The eruption of Hunga Tonga-Hunga Ha'apai Volcano, Tonga on 15 Jan 2022 produced an airwave which was detected on barometers on the east coast of Australia, and seismic stations at Cascade volcanoes in the USA, 9000 km NE.
The eruption of Krakatau volcano, Indonesia in 1883, produced low frequency pressure oscillations that circled the Earth seven times and produced intense acoustic waves, including audible sounds of ‘heavy gunfire’, at distances of 5000 km.
Further reading
Johnson, J.B. and Ripepe, M., 2011. Volcano infrasound: A review. Journal of Volcanology and Geothermal Research, 206(3-4), pp.61-69.
Johnson, J.B., Aster, R.C. and Kyle, P.R., 2004. Volcanic eruptions observed with infrasound. Geophysical Research Letters, 31(14).
Johnson, J.B., 2003. Generation and propagation of infrasonic airwaves from volcanic explosions. Journal of Volcanology and Geothermal Research, 121(1-2), pp.1-14.