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Geophysics

Earth's Good Vibrations

Our planet is humming, and this hum relates to the Earth's structure below its crust. This finding,1 that also promises to help delve into the structures of planets beyond our own, was revealed by a collaboration between teams from the University of Tokyo's Earthquake Research Institute and the Institute of Earth Physics of Paris (IPGP).2
In 1998, Japanese scientists discovered that vibrations coursing through the Earth generated a hum, said to result from the interaction between solid landmasses and fluid layers of air and water. Since then, researchers have been listening very attentively to our planet's song, trying to read more into the melody.
For though this low-frequency3 hum may elude human ears, it nevertheless speaks volumes—literally.
Until now, the Earth's subsurface has been explored by tomographic methods, where section images are compiled from earthquake data. Images can be created on the basis that seismic waves evolve as they travel through various geological formations. For example, they travel more quickly through cold, dense volumes than through hot, less dense ones.
While traditional tomography can only use data from strong earthquakes to generate images, the Franco-Japanese team has come up with an alternative that “could work without quakes, relying instead on the Earth's perpetual and ubiquitous hum,” says Jean-Paul Montagner of the IPGP.
Using data from 1986 to 2003, the team analyzed the hum's sound waves gathered by 54 stations around the globe. As hum variations caused by atmospheric disturbances or ocean swells occur randomly, it is possible to track the speed and path of specific sounds passing through the Earth's crust and thus establish phase-velocity maps.
By associating wave velocity fluctuations with variations in the mantle's physical and chemical composition, the team successfully derived a 3D model of the upper mantle.4 The feasibility of this approach, capable of plunging to depths of 500 km, is confirmed by its correlation with mantle models established from seismic data.
Where the team's method promises to go even further is in the exploration of other terrestrial planets. Since the existence of quakes on such planets is not demonstrated, the team posits that their mantles may instead be modeled by tapping into their hums. This solution, Montagner believes, could work “on Mars and Venus, planets with atmospheres dense enough to excite their own hums.” Ideally, this possibility would be tested by setting up a network of stations on the planets and operating them for several years.

Fui Lee Luk

Notes :

1. K. Nishida et al., “Global Surface Wave Tomography Using Seismic Hum,” Science, 2009. 326: 112.
2. Institut de physique du globe de Paris (CNRS / Université Paris Diderot / IPG).
3. Frequencies of 2-10 mHz.
4. Indicating the speed at which a given phase of a wave travels.

Contacts :

Jean-Paul Montagner
IPGP, Paris. jpm@ipgp.jussieu.fr


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