Earthquakes generate only a small fraction of the seismic energy that travels through Earth and on its surface. Most seismic activity arises from wind-driven ocean waves interacting with the solid ground. The background vibrations carry useful information about the exchange of energy between the ocean, the atmosphere, and Earth’s subsurface, and seismologists extract that information to image the planet’s internal structure and to conduct other geophysical investigations.
The strongest of those vibrations are known as secondary microseisms and are produced by pressure sources at the ocean surface that don’t attenuate with the depth of water. Such fields arise when ocean waves interact nonlinearly with each other.1 Most of the secondary microseisms, known as Rayleigh waves, oscillate in an up–down direction as they propagate across a surface. But some of the seismic energy takes the form of Love waves, which oscillate side to side.
Despite about a century of observations,...
