Planet Earth hums and now scientists know why – ocean waves colliding

The Earth has a low-frequency hum that has baffled scientists for many years. We cannot hear it, but seismic instruments have been detecting it for decades – so we know the Earth hums. A team of French researchers says the continual background hum is caused by ocean waves crashing into each other.

They published their study findings in the Journal of Geophysical Research Letters.

Earth scientists specializing in the propagation of seismic waves (seismologists) discovered in the 1990s that the Earth is continually vibrating at a very low frequency, which persists even when no earthquake is present.

The micro-seismic activity is every so slight, but some animals, such as whales and elephants might be able to sense it.

Earth’s persistent hum sound comes from ocean waves crashing into each other.

Some people claim to hear the hum, and complain of sleepless nights, nosebleeds and stress from what they describe as a relentless ‘kind of torture’. They say it sounds like a a “diesel car idling in the distance.” There is wide skepticism regarding these claims.

Over the years, seismologists have offered several suggestions for the cause of the hum, from submarine communications, mating fish, to electromagnetic radiation.

Lay people, including conspiracy theorists have added their own theories, including secret military activity, beings from another Universe trying to contact us, and aliens emitting a controlling tone to manipulate us.

 

The culprit appears to be something much simpler and harmless – ocean waves.

Eléonore Stutzmann and Lucia Gualtieri, from the Institut de Physique du Globe, and Fabrice Ardhuin from CNRS-Ifremer-UBO-IRD (all in France), used computer models of the ocean, wind and seafloor and found that ocean waves could generate mini-seismic waves when they collided.

Ocean waves can generate seismic waves with a frequency from 13 to 300 seconds, which is the length of the ripple as it penetrates the Earth’s mantle before dispersing.

Hum comes from longer waves

Most of the Earth’s mysterious hum sound comes from the longer waves, say the authors.

Ardhuin said in an interview with Live Science:

“I think our result is an important step in the transformation of mysterious noise into an understood signal.”

Now that they know where the hum comes from, Ardhuin and colleagues say they will be able to generate more comprehensive maps of the interior of the Earth.

They have not yet determined how far into the Earth’s mantle the seismic waves penetrate. It is possible they make their way right into the core.

By tracking the paths of these waves more closely, we may be able to chart a more comprehensive 3-D picture of Earth’s structure, the authors said.

The hum could also be coming from other sources, such as waves crashing along the shore, as well as some type of water movement inside mountains or at the bottom of the sea.

Reference: Fabrice Ardhuin, Eléonore Stutzmann and Lucia Gualtieri. “How ocean waves rock the Earth: Two mechanisms explain microseisms with periods 3 to 300 s.” Geophysical Research Letters. Published 10 February 2015. DOI: 10.1002/2014GL062782.