A strange bell-like ring emitted by the Earth has left oceanographers confused for decades, but a new study may have found an explanation.
The low-frequency sound, which is inaudible to human ears but can be detected by seismic instruments, has frustrated scientists since the 90s. Theories suggested everything from electromagnetic radiation to earthquakes and secret military operations might be to blame. Although the sound is almost certainly too faint for humans to hear, some people claim to be plagued by a “tinnitus”-like noise — including many residents of Bristol in the 70s, who said the sound caused headaches and even nosebleeds.
However, a new study published in the online journal Geophysical Research Letters has proposed a new cause behind the “ringing”: ocean waves.
According to research carried out by oceanographer Fabrice Ardhuin, of the National Center for Scientific Research based in Brest, France, the microseismic activity is caused by the collision of ocean waves, which in turn create mini seismic waves.
Ardhuin, along with his colleagues Lucia Gualtieri and Eléonore Stutzmann, of the Paris Institute of Earth Physics, used computer models of the wind, ocean and seafloor to pinpoint the exact type of waves causing the incessant “hum”. Although the researchers found that colliding ocean waves create some seismic activity, it was mostly the movement and pressure of giant, slow-moving waves that lay behind the Earth’s constant vibrations.
The study found that these waves could generate seismic waves with a frequency of 13 to 300 seconds: the time it takes for the ripple to travel all the way down to the seafloor. The findings indicate that the waves travel deep into the planet’s mantle — possibly even going as far as the Earth’s core.
In a written statement, Ardhuin said: “We have made a big step in explaining this. Now we know where this ringing comes from, and the next question is: what can we do with it.”
Ardhuin and his team said there could still be additional causes behind the sound, such as ocean waves travelling along shorelines or down underwater mountains. Nevertheless, the team believe that the findings could help to create a more accurate map of the Earth’s interior, paving the way for a clearer understanding of the planet’s structure.