2023-10-07 14:41:14
The difference in water content between the lithosphere and the upper layer of the Earth’s mantle can explain the observed seismic changes.
Image by PublicDomainPictures from Pixabay
The difference in water content between the lithosphere and the upper layer of the Earth’s mantle can explain the observed seismic changes.
The oceanic lithosphere, which constitutes the upper layer comprising the Earth’s crust and the mantle under the oceans, has long intrigued scientists because of its particular behavior. This layer appears to slide over a weaker region below called the asthenosphere, characterized by strong seismic attenuation and low shear wave speeds. The asthenosphere has different physical properties, such as lower viscosity than the lithosphere, resulting in a sharp boundary called the lithosphere-asthenosphere boundary (LAB). The exact reasons for these distinct properties and how they allow the lithosphere to move above the asthenosphere are not clearly understood.
Although partial melting occurring near mid-ocean ridges due to high temperatures can produce such anomalous conditions, it does not account for the sharp and large drops in seismic wave speeds observed at LAB away from mid-ocean ridges. . Understanding the origin of this velocity drop and attenuation of seismic waves at the oceanic LAB is crucial to deciphering the low viscosity of the asthenosphere and how it facilitates the movement of tectonic plates on the Earth’s surface, giving rise to processes of mountain formation, earthquakes and volcanism.
Source: pourlascience.fr
In this regard, a team of Japanese researchers, led by Professor Takashi Yoshino from the Institute of Planetary Materials at Okayama University, recently studied the effect of water on the seismic properties of olivine rocks without titanium, similar to those found in the asthenosphere. Their study was published in volume 120, number 32 of the journal Proceedings of the National Academy of Sciences on July 31, 2023.
“We experimentally determined the attenuation characteristics of seismic waves, parameters for determining the smoothness of the lithosphere and asthenosphere, at high temperature and pressure, using our short-period oscillation generation technology ”, explains Professor Yoshino.
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The team studied the anelastic properties of olivine rocks under conditions that resemble the LAB beneath the ancient ocean floor – pressure of 3 GPa and temperatures ranging from 1,223 to 1,373 K. They used ray monitoring X in situ and subjected the olivine rocks to mechanical stress. tests by generating forced vibrations over a wide range of seismic frequencies — 0.5 to 1,000 seconds — using their unique short-period oscillation technology.
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The experiments revealed that water had a significant effect, improving energy dispersion and reducing the elastic moduli of rocks over a wide range of frequencies. Additionally, the researchers observed a peak in seismic attenuation at higher frequencies of 1 to 5 seconds, which became more pronounced with increasing water content. “The presence of water induces attenuation at higher frequencies, leading to a decrease in the speed of seismic waves. The presence of water also weakens the asthenosphere, allowing the lithosphere to move smoothly over it,” explains Professor Yoshino.
These observations suggest that the oceanic asthenosphere must contain water. This difference in water content between the two layers constituting the LAB can explain the strong drops in velocity as well as the almost constant attenuation observed over a wide range of frequencies in the asthenosphere.
Notably, the researchers acknowledge that their conclusion assumes a negligible effect of iron on hydrogen-related defects in rocks, indicating the need for further research to explore the anelastic properties of iron-bearing olivine rocks.
Professor Yoshino highlights the long-term implications of their findings. “The presence of water in the asthenosphere can provide important information regarding volcanic and seismic activities, thus facilitating their prediction and detection,” he speculates.
Overall, this study contributes to our understanding of tectonic plate movement, paving the way for a better understanding of various tectonic activities.
Source : Université d’Okayama/Sciencedaily
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