A new study has led scientists to the bottom of the ocean to figure out what causes the Earth to hum – the low-frequency vibrational signal that is generated by our planet even in the absence of earthquakes.
Researchers used seismic instruments on the bottom of the ocean to successfully quantify Earth’s vibrational “hum” and have published their findings in Geophysical Research Letters. The study confirms it is possible to capture Earth’s ‘hum’ at the ocean bottom and this could provide new insights into the source magnitude. Further, the new findings could be used to map the interior of Earth with more detail and accuracy.
In the new study scientists examined Earth’s permanent free oscillations: low-frequency seismic signals that can only be measured with sensitive instruments. The vibration caused by these signals is constantly present in the ground, and is observable in the absence of earthquakes.
Scientists first gathered seismic data from 57 seismometer stations located at the bottom of the Indian Ocean east of Madagascar. These stations were deployed from 2012 to 2013 as part of an earlier study published in Eos, which was designed to image volcanic intraplate hot spots.
The authors selected seismic data from the two stations with the highest data quality, and made sure to correct for the signal generated by any earthquakes. They then applied a combination of techniques to remove interference from ocean infragravity waves, currents and electronic glitches, and were able to reduce the noise level to approximately the same level as a quiet land station.
The study determined Earth’s natural vibration peaks at several frequencies between 2.9 and 4.5 millihertz. These vibrations can’t be heard by people because they are approximately 10,000 times smaller than the lower hearing threshold of the human ear, which is 20 hertz.
The authors also compared the magnitude of their observed hum signal to measurements from a land station in Algeria, and found both signals have a similar amplitude.
Scientists traditionally examine the interior using seismic waves generated from earthquakes, but this only works at specific times and in areas where quakes commonly occur. Using the hum signal as a source of seismic waves would avoid this problem because the hum is generated continuously in many ocean-continent areas at the planet’s surface.
Combining data from both land and ocean bottom seismometers gives seismologists a more complete picture of the entire hum signal compared to using land stations alone. The increased density of possible data points would improve image resolution and could help scientists better map the Earth’s interior down to 500 kilometers (310 miles).