On the role of volcanic plume heights in excitation of free oscillations of the solid Earth and atmosphere: Case study

Abstract

Volcanic eruptions are potential sources for generating the Earth’s free oscillations at spheroidal modes of ₀S ₂₉ and ₀S ₃₇ through acoustic resonant coupling between the Earth and its atmosphere. Then, does every volcano produce Earth’s free oscillations? We answer this question by analyzing eight large volcanic eruption events, (i) 15 January 2022 Hunga Tonga-Hunga Ha apai (HTHH), (ii) 13–14 January 2022, HTHH, (iii) 13 August 2021 Fukutoku-Okanoba, (iv) 22 April 2015 Calbuco, (v) 13 February 2014 Kelud, (vi) 4 November 2010 Merapi, (vii) 15 June 1991 Mt Pinatubo, and (viii) 4 April 1982 El Chichon. Our analysis revealed that the 15 January HTHH, Mt. Pinatubo, and El Chichon eruptions excited the Earth’s free oscillations globally at spheroidal modes of ₀S ₂₉ and/or ₀S ₃₇ but no such modes were excited during other eruptions. However, harmonic oscillations in the ionosphere were seen during all recent eruption events. We demonstrate that stronger eruptions excite stronger atmospheric modes as the plume arrives at higher atmospheric heights which enable efficient ground-atmosphere acoustic resonant coupling for the excitation of spheroidal modes. Further, we hypothesize that relatively weaker eruptions/lower plume height perhaps caused weakly confined atmospheric modes thus failure of excitation of the spheroidal modes. A quantitative analysis of source efficiency versus source heights provides more insights into our hypothesis.