Tagged: Lithosphere Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 4:54 pm on January 2, 2018 Permalink | Reply
    Tags: , , Lithosphere, , Seismic-wave attenuation studies have great potential to add knowledge in this field, Wave attenuation hints at nature of Earth’s asthenosphere   

    From physicsworld.com: “Wave attenuation hints at nature of Earth’s asthenosphere” 

    physicsworld
    physicsworld.com

    Jan 2, 2018

    1
    Soft on the inside. No image credit.

    Researchers in Japan have used measurements of the aftershocks of the 2011 Tohoku earthquake to gain insight into the dynamics of the Earth’s crust and upper mantle. Nozomu Takeuchi and colleagues at the University of Tokyo, Kobe University, and the Japan Agency for Marine–Earth Science and Technology, analysed the attenuation of seismic waves as they propagated through the rigid lithosphere and the less viscous asthenosphere beneath. The team found that the rate of attenuation in the lithosphere showed a marked frequency dependence, whereas in the asthenosphere the relationship was much weaker. The result demonstrates the possibility of using broadband seismic attenuation data to characterize the properties of the Earth’s subsurface.

    Weak, deep and mysterious

    The lithosphere is the rigid outermost layer of the Earth. It comprises two compositional units – the crust and the upper mantle. The movement of individual fragments of the lithosphere (the tectonic plates) is responsible for the phenomenon of continental drift, and is possible due to the low mechanical strength of the underlying asthenosphere.

    Away from the active mid-ocean ridges, the lithosphere–asthenosphere boundary (LAB) lies at least tens of kilometres below the ocean floor, making direct investigation impossible for now. The LAB is even less accessible beneath the continents, where the lithosphere can be hundreds of kilometres thick. Nevertheless, seismic wave velocities and the way the continents have rebounded after deglaciation have allowed the viscosity of the asthenosphere to be estimated even though the physical cause of the mechanical contrast between the layers remains mysterious. A rise in temperature across the boundary presumably contributes, but probably does not explain the disparity completely; partial melting and differences in water content have also been proposed.

    Complex signal

    To help discriminate between these mechanisms, Takeuchi and collaborators looked to differences in the attenuating effects of the lithosphere and asthenosphere. This is a promising approach, because the process of anelastic attenuation is closely related to a material’s thermomechanical properties. The situation is complicated, however, by the fact that high-frequency seismic waves are also attenuated by scattering from small-scale features, and low-frequency waves are attenuated by geometrical spreading.

    Using a dataset obtained after the 2011 earthquake by an array of ocean-floor seismometers in the northwest Pacific, the group compared actual records of seismic waves with a series of probabilistic models. To isolate the anelastic attenuation signature for high-frequency (>3 Hz) waves, the researchers conducted simulations in which the scattering properties of the lithosphere and asthenosphere were varied. The model that most closely matched observations indicated a rate of attenuation for the asthenosphere 50 times that for the lithosphere, and suggested that this attenuation is not related to frequency. Seismic waves in the lithosphere, in contrast, seem strongly frequency dependent.

    More experiments needed

    Although Takeuchi and colleagues’ research shows that seismic-wave attenuation studies have great potential to add knowledge in this field, the results themselves do not immediately support one model over another. Laboratory experiments reveal that partial melting of a sample can produce a weak frequency dependence similar to that determined by this study for the asthenosphere, which on its own would strongly suggest that as the reason for the layer’s low viscosity. However, a similar effect has been observed for samples below the material’s solidus, undermining that explanation somewhat, and also failing to explain why the same response is not observed in the solid lithosphere. Further experiments involving additional factors will be needed to settle the issue.

    Full details of the research are published in Science. A commentary on the research, written by Colleen Dalton of Brown University in the US, is also published in the same issue.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

     
  • richardmitnick 11:28 am on October 31, 2017 Permalink | Reply
    Tags: , , , , Lithosphere, , Volcanic activity causes the seafloor to spread along oceanic ridges forming new areas of crust and mantle   

    From Eos: “Seafloor Activity Sheds Light on Plate Tectonics” 

    AGU bloc

    AGU
    Eos news bloc

    Eos

    27 October 2017
    Sarah Witman

    1
    Seafloor topography under the Atlantic Ocean. Credit: ETOPO1/NOAA

    Much like the way humans constantly generate new skin cells, the bottom of the ocean regularly forms fresh layers of seafloor. Volcanic activity causes the seafloor to spread along oceanic ridges, forming new areas of crust and mantle. After being generated, this new oceanic lithosphere cools down and contracts by up to 3% of its own volume. This contraction can trigger oceanic earthquakes.

    The basic mechanics of tectonic plates—the massive, constantly shifting puzzle pieces that make up the Earth’s surface—are fairly well understood.

    The tectonic plates of the world were mapped in 1996, USGS.

    However, scientists cannot accurately predict how much the oceanic lithosphere will contract horizontally during the process described above.

    Sasajima and Ito studied this thermal contraction [Tectonics]by examining stress released by oceanic earthquakes over the past 55 years in newly formed sections of oceanic lithosphere (approximately 5–15 million years old). They also simulated this activity using mathematical models.

    The team found a distinct difference in two components of the released stress: one parallel to the ridge and another perpendicular to the ridge (i.e., in the seafloor spreading direction). Namely, the ridge-parallel components experienced 6 times as much extensional stress release, whereas the spreading components endured 8 times as much compressional stress release.

    In their numerical simulation, the researchers found that young oceanic lithosphere hardly ever contracts in the ridge-parallel direction. At most, it would do so only a quarter of the times that it would contract in the spreading direction. They concluded that because the layer of mantle underneath the lithosphere (the asthenosphere) is weak (low viscosity) and also because oceanic ridges are relatively weak, the young oceanic lithosphere is able to contract more freely in the spreading direction.

    This study provides critical insight into the driving and resisting forces underlying plate tectonics, one of the greatest physical phenomena in our world. (Tectonics, https://doi.org/10.1002/2017TC004680, 2017)

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: