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Ever since the late 1990s, seismologists have been aware that our very own planet Earth produces an unmistakable humming noise. This sound, created by microseismic activity, is too faint to be heard or felt by humans. The humming stands apart from the more pronounced vibrations created by earthquakes, which cause the Earth to emit a bell-like sound for several days (or even months). In the case of the massive 2004 earthquake that devastated Indonesia, the bell sound continued for over five months. In contrast, this milder hum is ever-present and never-ending, and its existence has puzzled scientists ever since the discovery.
Researchers believe they have figured out what creates this mysterious humming. Two previous theories — which weren’t off base — fingered ocean waves as the source of the vibrations: (1) One theory involved huge ocean waves shaking the Earth as they hit the ocean floor and collided with seafloor ridges and continental shelves; (2) Another theory pointed towards massive waves colliding with one another. Both of these theories were equally plausible, but neither explained the impressive range of vibrations observed by seismologists upon earthquake sensors.
The new study, led by the French Research Institute for Exploitation of the Sea, buckled down to explain the vibration range. Study author Fabrice Ardhuin led a team that studied computer models of the ocean and its floor. These oceanographers incorporated both of the previous theories (mentioned above) and found that colliding waves create seismic waves that are short in duration. They also observed slower waves, which “could generate seismic waves with a frequency of 13 to 300 seconds” across the sea floor. The end result was that both the shorter and longer waves generated vibrations through the Earth’s crust, but the longer waves carried much more seismic impact.
The scientists concluded that these slower, longer waves hitting the seafloor was what caused (most of) the Earth’s hum. This result may not seem Earth-shattering (har har), but it carries far-reaching implications. These seismic waves “penetrate deep into the planet’s mantle and potentially all the way to Earth’s core,” so they could potentially provide invaluable knowledge of what lies within the planet. Ardhuin and his team plan to use this new knowledge to generate detailed maps of the Earth’s interior.