Gravity waves as a mechanism of coupling oceanic and atmospheric acoustic waveguides to seismic sources

Direct excitation of acoustic normal modes in horizontally stratified oceanic waveguides is negligible even for shallow earthquakes because of the disparity between velocities of seismic waves and the sound speed in the water column. T-phases, which propagate at the speed of sound in water, are often reported to originate in the open ocean in the vicinity of the epicenter of an underwater earthquake, even in the absence of prominent bathymetric features or significant seafloor roughness. This paper aims to evaluate the contribution of scattering by hydrodynamic waves into generation of abyssal T-waves. Ocean is modeled as a range-independent waveguide with superimposed volume inhomogeneities due to internal gravity waves and surface roughness due to wind waves and sea swell. Guided acoustic waves are excited by volume and surface scattering of ballistic body waves. The surface scattering mechanism is shown to explain key observational features of abyssal T-waves, including their ubiquity, low-frequency cutoff, presence on seafloor sensors, and weak dependence on the earthquake focus depth. On the other hand, volume scattering due to internal gravity waves proves to be ineffective in coupling the seismic sources to T-waves. The theory is extended to explore a possible role that scattering by gravity waves may play in excitation of infrasonic normal modes of tropospheric and stratospheric waveguides by underwater earthquakes. Model predictions are compared to observations [L. G. Evers, D. Brown, K. D. Heaney, J. D. Assink, P. S. M. Smets, and M. Snellen (2014), Geophys. Res. Lett., 41, 1644–1650] of infrasonic signals generated by the 2004 Macquarie Ridge earthquake.