Under the background of a never-ending racket bellowed by animals, human machines, and natural forces, the Earth itself is humming to a tune that only the sharpest mechanical ‘ears’ can pick up. For years, scientists have known that our planet produces a low-frequency drone. These subtle oscillations or seismic movements are far too minute to be called an earthquake, but they’re still measurable. Now, researchers report recording this distinct ‘hum’ for the first time underwater.

Earth sunrise

Credit: Pixabay.

Will you sing along?

People are justifiably terrified of earthquakes but the truth is the vast majority of tremors are totally harmless. Of the half a million or so earthquakes estimated to occur on a yearly basis, only 100,000 are strong enough to be felt and it’s just about 100 of these that can actually cause damage. 

There are murmurs that are even quieter. A ground-breaking paper published in the 1990s reported that the Earth is constantly busy with microseismic activity known as “free oscillation”. This constant vibration produces a hum that has been detected on land by special equipment known as seismometers. 

Scientists had been debating the source of this unusual and perpetual hum for some time until a 2015 paper determined that two marine factors are primarily responsible for the imperceptible planetary drone. The ebb and flow of ocean waves reaching the seafloor, and the vibrations caused by the collision of ocean waves add up to produce the hum.  The result is a strange ultralow frequency that resonates almost identically all over the planet.

Now, this hum has been recorded underwater for the first time. This is a great achievement, as three-quarters of the planet is covered in water and up until recently, we had no idea whether or not there was a difference. It turns out that the ultralow frequencies are the same on land as they are at the bottom of the ocean.

The findings were reported in the Geophysical Research Letters by a team led by Martha Deen, a geophysicist at the Paris Institute of Earth Physics. Her team employed a dataset collected from 57 free-fall seismometers deployed around La Réunion Island to the east of Madagascar. The instruments covered an area measuring about 772 square miles (2,000 square kilometers) and were deployed there to study volcanic hotspots. Deen and colleagues found they could study the planetary hum with the same data after cleaning it of noise like ocean currents or waves.

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Impressively, the scientists “were able to reduce the noise level to approximately the same level as a quiet land station.”

The study determined Earth’s natural vibration peaks at several frequencies between 2.9 and 4.5 millihertz. That’s about 10,000 times smaller than the lower hearing threshold of the human ear, which is 20 hertz. When the signal was compared to measurements taken by land stations in Algeria, researchers found the two signals’ amplitudes were similar.

This planetary hum might actually prove practical in some endeavors. Scientists have often been able to exploit earthquakes to plot subterranean maps based on the predictable effect seismic waves have in the crust. Some believe this ultralow hum extends all the way to the core and could be used for similar purposes. Others have even wilder ideas like using an equivalent hum on alien planets to map their subterranean structure.

“Earth is constantly in movement, and we wanted to observe these movements because the field could benefit from having more data,” Deen told the AGU blog.

Let’s not get too carried away, though. We’re only beginning to understand what the deal is with Earth’s hum and how it could impact our lives. It’s, nevertheless, intriguing news to learn that this planet is never quiet. I like it restless.

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