Evanescent Wave Coupling in a Geophysical System

Abstract:

Earthquakes and explosions generate elastic waves in the solid earth, oceans and atmosphere. Underwater earthquakes are one of the dominant sources of hydro-acoustic waves in the oceans. However, atmospheric low frequency sound, i.e., infrasound, 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 infrasound from a large underwater earthquake (Mw 8.1). Seismic waves coupled to hydro-acoustic waves at the ocean floor, after which the energy entered the SOund Fixing And Ranging (SOFAR) channel. 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 frequent acoustic waves. Current research focuses on the contribution of underwater sources to ambient atmospheric noise field of infrasonic waves. Such infrasonic energy is expected to be partly absorbed in the upper atmosphere, i.e., mesosphere and thermosphere.