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 ten 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 pinpoint the exact origin and 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. Using data from a wide-spread array of seismic recorders, Peter Bromirski and Peter Gerstoft have located the Pacific coast of Central America as the dominate source of background noise, with the western coast of Europe acting as the main secondary supply.
Hum beam power observations using the USArray EarthScope transportable array, combined with infragravity wave observations, show that the dominant source area of the Earth’s hum over the 120–400 s period band during winter months is the Pacific coast of North America, with the western coast of Europe a secondary source region.
Correlation of hum with model ocean wave heights indicates that the Pacific coast of Central America is an important hum source region when impacted by austral storm waves. Hum is excited by relatively local infragravity wave forcing as ocean swell propagates along coasts, with no indication of significant deep-ocean hum generation.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. Source
