Strange as it may seem, the Earth’s atmosphere rings out in a chorus of frequencies just below the reach of the human ear. Although we cannot hear these “infrasonic” waves — which have frequencies ranging from 0.01–10 Hz — we know they exist from acoustic recordings around the globe. About 10 years ago, however, researchers discovered another type of infrasonic background noise of 3–7 mHz believed to originate from the solid Earth itself.

Scientists have struggled to understand the mechanism of this intriguing phenomenon, known as the Earth’s "hum". Now, a pair of physicists at the University of California claim to have the most convincing physical evidence yet to locate the hum's origin. Using data from a wide-spread array of seismic recorders, Peter Bromirski and Peter Gerstoft have found the Pacific coast of Central America to be the dominate source of background noise, with the western coast of Europe acting as the main secondary supply.

The sound of silence

In the late 1990s, studies of this mysterious sound suggested that it was a direct result of fluctuations in the atmospheric pressure at the Earth’s surface. More recent work, however, suggests that the hum is caused by energy being transferred from winds in the atmosphere to the solid Earth via waves in the ocean.

Within this new view of hum, long-wavelength surface waves — known as ocean swell — act as the mediator between the sky and the sea. As the swell reaches shallower waters, a portion of its energy is then transformed by non-linear processes into infragravity (IG) waves, which have even longer wavelengths than the surface waves. Some of the energy from IG waves can then couple to the solid Earth, registering as blips at seismic recording stations around the world.

In their new paper Bromirski and Gerstoft explain how they study this phenomenon in detail by recording both infragravity wave activity and the seismic waves associated with background hum. They gathered data using the USArray EarthScope transportable array, which monitors seismic activity right across the US and its surrounding waters.

In search of the new sound

The physicists found a close relationship between the Earth’s hum, ocean waves and infragravitational waves, with coastlines being the dominant source of hum. Strong tides along the US Pacific are the biggest contributor to the hum, with the western coast of Europe being the strongest secondary contributor. The IG wave amplitudes are greatest over coastal shelves where water is relatively shallow because wave pressure decays exponentially with depth, say the researchers.

“None of the previous hum studies have included infragravity wave measurements, so that link has not been clearly demonstrated until now,” Bromirski told physicsworld.com. “Because storms generally propagate from west-to-east, long period swell energy is much higher along eastern boundaries of ocean basins, so that's where the "dominant" hum signals are generated,” he says.

Barbara Romanowicz, a geophysicist at Berkeley Seismological Laboratory is impressed by the new work. “This study throws a much larger dataset at the problem giving a much more accurate picture of where the hum is coming from,” she says. She notes that the research does not give a complete explanation of the complex non-linear mechanism of the "Hum" generation, but believes it provides an important step in our understanding. “It strengthens the idea that we have to look for a mechanism involving interactions of ocean waves with shallow coastal seafloor,” she said.

Spahr Webb, a marine seismologist at Columbia University is less convinced by the new findings. He suspects the hum is forced by sources under most of the ocean floor and that these types of studies are biased toward detecting only the local sources. “The waves making up the hum signal are long wavelength with little attenuation so that they travel many times around the Earth before they are significantly attenuated,” he argues.

Bromirski told physicsworld.com that he is going to now develop this research by looking more closely at global hum variability and the distribution of hum source areas across the whole Earth.

This research has been published in Geophysical Research Letters.