The Earth's Hum: A Decades-Old Puzzle
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. Now researchers say they believe the noise is caused by ocean swells generated in the Southern Ocean, the sound of which echoes across the planet.
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. 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.
The University of Melbourne's Earthquake Research Group hopes to conduct further investigations into the sound. 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.
A Landslide in Greenland
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.
While recording the hum, Byrne's work coincidentally captured another signal: a big earthquake in the South Sandwich Islands. Byrne said the earthquake rattled every seismometer in the world, adding that this highlights how much we still have to learn about the Earth.
Listening to the Earth
Pascale explained that low-frequency waves travel long distances, much like a whale song. 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. The Earth's hum is an infrasound, too low for humans to hear but audible to some animals. Byrne said infrasound could explain why certain houses feel haunted, as windows can make sounds that are inaudible but unnerving.
Byrne said Australia is a good place to listen for such signals because of its wide open spaces and quiet environments. He added that the idea that an ocean wave contributes to a vibration traveling through the entire planet gives a new perspective to the sound of the sea.