This bright green peridotite is a mantle xenolith, a piece of mantle magma swept away and carried to the surface by a lava flow. Credit: James St. John, CC BY 2.0
Source: Geochemistry, Geophysics, Geosystems

By studying seismic waves, scientists can essentially peel back the thin layer of the Earth’s crust and peer inside the planet. Seismology is arguably the backbone of our understanding of the planet’s interior, and it’s surprisingly simple. When earthquakes travel through dense, hot spots, they slow down; if a seismic wave reaches a sensor later than expected, geologists know that it traveled through one of these so-called low-velocity regions hidden deep within the Earth. Researchers can track multiple seismic waves to pinpoint the exact locations of mantle plumes or subducted tectonic plates.

To simplify many seismic measurements, geologists assume that the Earth’s mantle is relatively isotropic, meaning that the materials within the layer will affect seismic waves in the same way regardless of the wave’s direction. Yet mantle flow near subduction zones is anything but isotropic. Instead, the anisotropic, or directionally dependent, characteristics of the mantle can be seen on a macroscopic level—in melt-filled cracks, for example—or on a microscopic level, in the alignment of crystals within mantle rocks. This disconnect between models and reality is a problem for seismology given that seismic waves will travel at different velocities throughout such a heterogeneous medium.

Here Bezada et al. assessed just how much this assumption leads to disparities between predictions and observations. To begin, the researchers modeled the anisotropic characteristics of the mantle at a hypothetical subduction zone using a previously established petrological-thermomechanical technique and simulated seismic waves propagating throughout the region. The team measured the travel times of those seismic waves under both isotropic and anisotropic conditions and compared the results.

The team found that the composed images of seismic wave speeds at the subduction zone appeared to have a number of false readings—artifacts that could be incorrectly interpreted by a geologist.

But the researchers also found that their results improved if they first included an a priori constraint on the anisotropy of the field. This constraint can be found from geodynamic modeling—and it need not be perfect to dramatically improve the final seismic image so that geologists see many fewer of those artifacts in the first place. This new method could help researchers see the Earth beneath our feet even better than before (Geochemistry, Geophysics, Geosystems, doi:10.1002/2016GC006507, 2016)

—Shannon Hall, Freelance Writer


Hall, S. (2016), Seismological models are biased, but scientists have a solution, Eos, 97, Published on 26 September 2016.

Text © 2016. The authors. CC BY-NC-ND 3.0
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