Scientists Capture Earth’s “Hum” on Ocean Floor

An image of the Earth constructed from NASA’s Terra satellite.

Image credit: NASA/Goddard

Now, using seismic instruments on the bottom of the ocean, researchers have successfully quantified Earth’s vibrational “hum”. A new study published in Geophysical Research Letters, a journal of the American Geophysical Union, determined at the ocean bottom the frequencies at which the Earth naturally vibrates, and confirmed the viability of using ocean instruments to study the hum.

Capturing the hum at the ocean bottom could provide new insights into the source magnitude, according

to Martha Deen, a geophysicist at the Paris Institute of Earth Physics in Paris, France and lead author of the new study.

Additionally, the new findings could be used to map the interior of Earth with more detail and accuracy. Including the hum from seismometers on the ocean sea floor can give a better overall picture than using land seismometers alone by increasing data coverage in large uncovered areas, Deen said.

Image credit: RHUM-RUM experiment/Meteor Cruise

In the new study, the researchers 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.

Because Deen and her colleagues were able to account for these sources of interference, the researchers were able to successfully capture the hum using seismometers at the bottom of the ocean.

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.

Deen and her co-authors believe other researchers can apply their findings to better model the structure of the Earth’s interior. 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, according to Deen. 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), she said.

Contacts and sources:
Joseph Cariz

American Geophysical Union