The Earth's constant hum comes from the ocean floor

For more than fifty years, scientists have known that the Earth hums. We can't hear the sound as it's at a frequency 10,000 times lower than our hearing threshold but new research suggests that it's coming from the ocean floor. Scientists from the Paris Institute of Global Physics analyzed data from earthquake sensors on the Indian Ocean floor and found the familiar and constant oscillations of between 2.9 and 4.5 millihertz. From National Geographic:

"To better understand where the signal comes from, we believe that observing oscillations from the ocean bottom can help," says co-author Martha Deen...

Since early observations, a number of studies have hypothesized that the Earth's free oscillations are a side-effect of the pounding of ocean waves. Other research suggests the hum could originate from atmospheric turbulence, or the wind motions around the globe, cued by storms. The current study says turbulence could account for part of the vibration, leading the rest to be fueled by ocean waves...

By studying the Earth's hum signal from ocean-bottom stations, scientists can map out a detailed landscape of the Earth's interior. Currently, they can only look at the inside of the planet during earthquakes, which limits studies to certain times and areas. And when looking at seismic activity from land monitors, researchers can't chart places far removed from islands and land masses. But the hum signal, droning and constant, can be detected across the world.

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