Evanescent wave coupling in a geophysical system: Airborne acoustic signals from the Mw 8.1 Macquarie Ridge earthquake

Atmospheric low-frequency sound, i.e., infra-sound, from underwater events has not been considered thus far, due to the high impedance contrast of the water-air interface making it almost fully reflective. Here we report for the first time on atmospheric infra-sound from a large underwater earthquake (Mw 8.1) near the Macquarie Ridge, which was recorded at 1325 km from the epicenter. Seismic waves coupled to hydro acoustic waves at the ocean floor, after which the energy entered the Sound Fixing and Ranging channel and was detected on a hydro-phone array. The energy was diffracted by a sea-mount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves. (authors)