For the past few decades, something has been becoming increasingly clear: Earth constantly hums, even though we can’t hear it.
The first attempt to detect this hum was made in 1959, but it wasn’t until 1998 that a paper was finally published proving it.
Earth expands and contracts constantly, ever so slightly. This is known as “free oscillations”, and they register as a background vibrational signal – or hum – in the absence of any other seismic activity.
Now, for the first time, scientists have been able to record our planet’s hum from the bottom of the ocean.
There have been many observations made of Earth’s hum since that study in 1998. They confirm that the signal is real – but they’ve all been taken using seismometers on land.
Taking measurements on the bottom of the ocean is an important piece of the puzzle, not least because 70 percent of our planets surface is covered by water.
And having measurements from deep under the sea could help us figure out what’s really causing the hum.
A longstanding hypothesis is that Earth’s free oscillations are caused by the constant pounding of waves on the ocean floor, and there have been a number of studies demonstrating just how this could be the case.
Another version is that the hum is partially affected by atmospheric turbulence, since it is stronger in the northern hemisphere’s Pacific Ocean during the northern winter, and southern oceans during southern hemisphere winter, associated with winter storms.
In recent years, seismometer stations have been installed on the seafloor around the world, designed to capture seismic and acoustic signals.
But seismometers measure movement, and there’s a lot of that under the sea. Meanwhile, the signal of Earth’s permanent vibrations is very low frequency and very slight.
To find the signal of the hum, the researchers, led by Martha Deen at the Paris Institute of Earth Physics, first gathered 11 months worth of observation data from 57 seismometer stations on the seafloor in the Indian Ocean east of Madagascar, installed in 2012-2013 to study volcanic events.
They then selected the two stations that had the highest data quality, and painstakingly removed any sources of interference from identified sources.
After removing the signals caused by ocean infragravity waves, seafloor currents and electronic glitches, the noise level was equivalent to that of a terrestrial station.
In other words, what they had left was Earth’s hum.
They then crossreferenced the signal they had with observations of the hum from terrestrial stations. They had a match.
Based on the signal they found, the researchers determined that Earth’s natural vibration peaks at frequencies between 2.9 and 4.5 millihertz, whether on sea or land – around 10,000 times below the threshold of human hearing, which starts at around 20 hertz.
They also found that, while the amplitude of the hum varied over time, this variation did not correlate, as previous researchers had found, with the seasons.
These previous studies had been based on a wider frequency band, and saw variations at higher frequencies, but not the lower frequencies to which Deen and her team narrowed down the signal.
The effect of atmospheric turbulence, they concluded, could only explain part of the vibration.
The team’s research demonstrates that using ocean-bottom seismometers helps us study Earth’s permanent free oscillations because we suddenly have so many more data points to work with.
They also believe this could help map Earth’s interior, which is usually done using seismic data from earthquakes. Because the hum is constant, geologists wouldn’t need to wait for earthquakes for data.
The team’s research has been published in the journal Geophysical Research Letters.