Infragravity waves in the deep ocean: Generation, propagation, and seismic hum excitation

Abstract

Oceanic infragravity waves are investigated as a possible source of seismic free oscillations, often referred to as the “hum” of the Earth, using a numerical model of depth‐independent, nondispersive, long‐wave dynamics with a forcing from nonlinear interactions among the primary wind waves (including swell). Because of near‐resonant amplification, the structure of the primary‐wave forcing field in shallow water, and an edge‐trapping mechanism, infragravity waves are excited very effectively near the coasts. Deep‐water infragravity waves are significantly influenced both by offshore leakage and propagation of the coastally generated free waves and by deep‐water primary‐wave forcing. With the inclusion of “mesoscale” variability on top of the more slowly varying primary waves generated in synoptic storms, the deep‐water infragravity waves are found to have an amplitude on the order of a millimeter in height, which is consistent with field observations and considered to be sufficient to account for local hum excitation in the middle of the basin.