For several decades, researchers have known that Earth emits a constant hum. Even if you took off your headphones and got every creature and machine to be quiet, you still wouldn’t hear it; the planet vibrates 10,000 times lower than human hearing. Now, as Elaina Zachos at National Geographic reports, for the first time, scientists have isolated the hum using instruments at the bottom of the Indian Ocean—and the discovery could help researchers map the depths of our planet.
As Zachos reports, scientists discovered the hum in 1959, finding microseismic activity called “continuous free oscillations,” which vibrates between 2.9 and 4.5 millihertz. The idea was later confirmed in 1998 by a team of Japanese researchers.
Why does our planet hum? In 2004, researchers theorized that the sloshing of ocean waves against the seafloor was responsible for the barely perceptible vibrations. That's at least part of the story. Recent research suggests that the hum is likely a combination of ocean waves rolling over ridges and continental shelves as well as waves bashing into one another in the open ocean.
Most of this information about the hum, however, has been collected on land. Measuring this vibration on the ocean floor seemed impractical—if not impossible—due to the massive amounts of interfering noise underwater. But Martha Deen, geophysicist at the Paris Institute of Earth Physics, decided to try anyway.
In late 2012 and 2013, researchers deployed 57 spherical ocean seismometers in 1,200 square miles of ocean floor around France’s Réunion Island to study volcanic hot spots. And as Mindy Weisberger at LiveScience reports, Deen and her team realized that it indeed was possible to probe the data for evidence of the hum underwater. They analyzed the 11 months of recordings, filtering out interference using algorithms to isolate signals created by the ocean waves and seafloor currents.
What they found is that the waves create oscillating frequencies that correspond with the hum, matching up with measurements of the hum from a land-based station in Algeria. The research appears in Geophysical Research Letters.
The study does not completely solve the mystery of the hum—some researchers still believe it is partially derived from atmospheric turbulence. But being able to record the hum at the bottom of the ocean does offer a new way for scientists to study these noises imperceptible to the human ear, and perhaps more importantly, interior of our planet.
When earthquakes occur it gives researchers the rare opportunity to study the planet’s interior, measuring the vibrations as they rumble through the planet. But unlike quakes, the hum is continuous and appears all around the world, making it a potentially more regular and reliable way to peer deep inside the Earth.
The hum "can be used to map the structure of the Earth,” Spahr Webb of Columbia's Earth Institute, not involved in the study, tells Zachos. “Getting data from new places is going to help.”
According to a press release, Deen says that combining hum data collected on land with readings from the ocean floor, the researchers believe they could map the planet’s interior with a resolution of 310 miles.