Wind, sea swell, and excitation of atmospheric waveguides by underwater earthquakes

Abstract

Powerful underwater sources, such as volcano eruptions and large explosions, can generate infrasonic waves, which remain detectable at distant receivers on shore. Evers et al. [Geophys. Res. Lett. 41, 1644–1650 (2014)] described observations of acoustic signals in air from the 2004 Macquarie Ridge submarine earthquake, by the infrasonic array IS05AU of the International Monitoring System. Infrasound propagated in the tropospheric and stratospheric waveguides and was received 1325 km from the epicenter. Specific physical mechanisms of the excitation of guided infrasonic waves by underwater sources are not fully understood. Here, we emphasize the potential role of local meteorological conditions. Scattering of ballistic waves from the earthquake epicenter by wind waves and sea swell on the ocean surface is a potent mechanism of excitation of normal modes in hydroacoustic waveguides (T-waves) [Godin, J. Acoust. Soc. Am. 150, 3999–4017 (2021)]. We investigate the hypothesis that the scattering of ballistic waves at the ocean surface also explains the earthquake excitation of acoustic normal modes in the tropospheric and stratospheric waveguides. Relative significance of the surface scattering and the previously proposed evanescent coupling mechanism is studied as a function of the wind speed, wave frequency, and the source depth.