Could ocean waves be making planet Earth hum?
Amid the constant noise of traffic and suburbia, Earth is quietly humming.
The planet's ultra-low-frequency vibration was first detected in the 1960s, but its origin has puzzled scientists for decades.
Seismology Research Centre chief scientist Adam Pascale said the scientific community believes the hum is caused by the pressure of ocean swells generated in the Southern Ocean.
"It's a seismic signal, which is a very small vibration that gets picked up by seismometers and it seems to be very regular," he said.
Adam Pascale is the Chief Scientist at the Seismology Research Centre in Melbourne. (Supplied: Adam Pascale)
Mr Pascale said the pressure of the swell interacts with the fluid within the Earth's crust that is then amplified to generate microseismic signals.
"In the Gulf of Guinea, just off the west coast of Africa, those swells generate tiny vibrations that travel through the entire Earth," he said.
A fjord landslide
Mr Pascale said it was not the first-time scientists had detected strange, persistent vibrations travelling through the planet.
"There was this mysterious signal that was generated in 2023," he said.
"It was a very regular pulse that was occurring, and nobody knew what the source was as it didn't look like an earthquake, but it had the same sort of decay and signal."
It turned out to be a landslide in a Greenland fjord, which created a 200-metre high wave that sloshed back and forth like water in a bathtub.
"Low-frequency waves travel long distances, much like a whale song," Mr Pascale said.
He said most earthquakes were below the range of human hearing, but by speeding up the signal, it can be brought into an audible range.
A look inside a seismometer, traditionally used to record shifting plates, these instruments are now also being used to listen to the movement of the oceans. (Supplied: Adam Pascale | ABC Midwest and Wheatbelt: Chris Lewis)
Melbourne scientist Rohan Byrne recorded the mysterious hum for the ABC by gathering recordings from 30 seismic stations around the world and speeding them up.
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As he did so, he unknowingly captured another signal at the same time — the rumble of an earthquake thousands of kilometres away.
This spectrogram (frequency vs time) diagram illustrates the microseism that is the earth's continuous hum, with an earthquake near the front. (Supplied: Rohan Byrne)
"Entirely coincidentally there was a pretty big earthquake somewhere in the South Sandwich Islands, it rattled every seismometer in the world," Mr Byrne, who is from the University of Melbourne's Earthquake Research Group, said.
"It highlights how much we still have to learn about how the Earth works," he said.
Another graphic of the same microseism showing the hum and earthquake as a waveform. (Supplied: Rohan Byrne)
Earth's natural tuning fork
So how can the ocean make a hum?
Mr Byrne said just like an orchestra warming up, each musician playing their own notes or frequency, so do the sea waves as they generate many frequencies.
Planetary scientist Rohan Byrne holds a seismometer that records seismic activity in real time. (Supplied: Rohan Byrne)
This falls under the field of infrasound — sounds that are too low for humans to hear, but can be heard by some animals.
Mr Byrne said infrasound could explain why certain houses feel "haunted".
"There may be something about the way the windows are organised, and when the wind blows in such a certain direction, it makes a sound that we can't quite hear, but we find it very unnerving," he said.
He said Australia was good place to listen out for these signals.
Scientists think the hum is coming from continuous ocean waves across the planet. (ABC Midwest and Wheatbelt: Chris Lewis)
"Because we have so many wide open spaces that are a long way from cities and traffic and noises, the same reason why we do so much good radio astronomy here," he said.
"We have so much pristine environments that are very quiet."
A map illustrating the spatially filtered, random selection of seismic stations utilised by Rohan Byrne to record the Earth's continuous hum. (Supplied: Rohan Byrne)
He said the idea that something as familiar as an ocean wave contributes to a vibration travelling through the entire planet gives a new perspective to the sound of the sea.
"Earth is, in effect, a vast and incredibly complex instrument, constantly being struck, shaken and set ringing by the forces acting upon it," he said.
His team is so intrigued by the sound, they plan to investigate it further.
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