Despite the trials and tribulations experienced by humanity, the Earth itself continues to hum along through space — quite literally, as researchers have made new recordings of the faint, yet steady low-level hum the Earth generates, using sensitive seismic instruments placed on the ocean floor.
Although its exact cause is not entirely understood, this "hum" is a low-frequency vibration that is continuously generated by the Earth, even in the absence of earthquakes. It was first recorded in 1998, and has been detected by land-based sensors, but studying the phenomenon from the perspective of the ocean floor has proven problematic, due to the ambient noise generated by waves on the surface, and the continuous passage of ocean currents. This meant that scientists were only getting part of the picture: since the oceans cover 70 percent of the Earth’s surface, that meant there was a corresponding amount of data that researchers didn’t have access to.
To develop this new listening technique, the researchers used the seismic data from two ocean-floor sensors that provided the highest quality recordings, and then systemically filtered out known sources of noise, such as earthquakes, ocean currents, and infragravity waves.
They found that Earth’s vibe peaks at a number of different frequencies between 2.9 and 4.5 millihertz — roughly 10,000 times below the 20 hertz that the average human can hear. This new technique is expected to also allow researchers to generate better models of the interior of the Earth: traditionally, the seismic waves from earthquakes are used to study Earth’s deeper layers, but the technique only works sporadically, when ‘quakes occur. Since the Earth’s hum is continuous, however, this could allow researchers access to data from deep within the planet, to a potential depth of 500 kilometers (310 miles), whenever they wish.