Music of the sphere: from surf beats to Earth's hum... now explained

The solid Earth is ringing with incessant oscillations caused by ocean waves, that take the form of seismic waves. These "noises" range over 8 octaves, just like a piano keyboard, but with much lower and inaudible tones, with periods of 0.5 to 500 s. The faintest of these signals are found in the lowest tones, with periods 50 to 300 s, and amplitudes of a few tens of nanometers. These weak signals were only discovered in 1998. A team of French researchers has now established that the amplitude of the vertical component of these motions can be predicted from the known properties of ocean waves. Their paper just appeared in the Journal of Geophysical Research and provides a detailed quantitative theory for how much ocean waves, from very long ones to short ones across 6 octaves, interact with the ocean floor to produce seismic waves. This research opens the way to a better use of seismometer data from all around the globe, for the analysis of ocean storm properties and properties of the solid Earth.

The amplitude of the ground motion excited by ocean waves rarely exceeds a few microns, which is why they are usually called "microseims". The strongest oscillations are in the mid-frequency range, with periods around 5 s, and were discovered in the late 19th century when the first seismometers were built. Their connection to ocean waves was established in the 1930s, with a detailed explanation in 1950. The weaker components, around 15 s period have been tentatively explained in 1963 by Klaus Hasselmann who proposed a general theory of how ocean waves feed energy into seismic waves with two main mechanisms.