There are a number of locations around the world where so-called “Earth hums” are reported. Among the most famous locations where this low, droning or rumbling sound is reported are Taos, New Mexico, and Bristol in Southwest Wales.
For years, Geophysicists have also sought to understand the cause behind a mysterious, bell-like hum, often likened to ringing or droning that reverberates throughout the earth. While unnoticeable to humans, seismic equipment can spot it easily, and at almost any given time. In other words, it never stops… but what causes it?
According to a study recently featured in Geophysical Research Letters, this previously mysterious phenomenon, classified as microseismic activity caused by ocean waves, may have a lot to do with the movement of deep waves moving across ridges and other geological formations along the ocean floor. (1)
In truth, there are actually two phenomenon that are believed to be contributing to the mystery: massive ocean waves deep beneath the surface that comb along bottom slopes, producing seismic waves in the process, paired with collisions between ocean waves that occur elsewhere. The resulting microseisms can range from 3 to 10 seconds; while these are often reported, seismic periodicity lasting up to nearly 300 seconds has also been documented. The authors of the study, however, note that these longer seismic features have been “strongly debated.” (1)
It was previously suspected that ocean waves might play a role in the mystery of Earth ‘humming’, but neither separate oceanic phenomenon could account for the mystery entirely. The study’s abstract can be viewed here.
“The secondary mechanism only explains seismic motions with periods shorter than 13 s,” as noted in the abstract. “Our results build on a quantitative numerical model that gives access to time-varying maps of seismic noise sources.”
While imperceptible to most, the supposed “mystery hums” that are reported at various localities have often been likened to droning, comparable at times to a diesel engine idling in the distance. Any link between these noises and various oceanic phenomena remains inconclusive; however, a previous, though disputed, explanation for similar hums reported in Seattle, Washington, as well as part of California and the United Kingdom involves the mating call of certain species of fish. (2)
Others have explained the hums as possible cases of otoacoustic emissions produced by the human ear, which may be experienced by up to 60% of adults, according to some studies. (3) Could a geophysical explanation present a more likely source for some of these noises?
- Ardhuin, Fabrice; Gualtieri, Lucia and Stutzmann, Eléonore. “How ocean waves rock the Earth: Two mechanisms explain microseisms with periods 3 to 300 s” Geophysical Research Letters, February 10, 2015. http://onlinelibrary.wiley.com/doi/10.1002/2014GL062782/abstract
- “Seattle ‘Hum’ May Be Due To Midshipman Fish That Produce Sound For Mating”. The Huffington Post. September 7, 2012. http://www.huffingtonpost.com/2012/09/07/seattle-hum-fish-midshipman_n_1865742.html
- “In Taos, Researchers Can Hum It, but They Can’t Name That Sound”. LA Times. September 1, 1993. http://articles.latimes.com/1993-09-01/news/mn-30113_1_taos-hum