Understanding the inner workings of our planet has always been a fascinating challenge for scientists. Now, a team of researchers has employed an innovative high-resolution model to study the Earth’s mantle structure, uncovering unexpected zones that could change our perception of the Earth’s internal processes. This advancement brings us a step closer to unraveling the secrets buried thousands of kilometers beneath our feet.

The Earth’s mantle, located between the core and the crust, has traditionally been inaccessible to direct exploration. There are no technologies capable of drilling to such depths or methods to obtain rock samples from this internal layer. For this reason, geophysicists rely on indirect methods like the analysis of seismic waves generated by earthquakes. This procedure, similar to using ultrasounds in medicine, allows scientists to study how waves propagate through the Earth and reveal details about its density and composition.

When an earthquake occurs, seismic waves travel in all directions from the epicenter. Along their journey, these waves refract, diffract, or reflect, depending on the material they pass through. Data collected by seismographic stations enable scientists to build a map of the planet’s internal structure. So far, this technique has been fundamental in identifying submerged tectonic plates in so-called subduction zones, where one plate sinks beneath another into the mantle.

tectonics pacific
Global distribution of seismic stations, receiver locations, and seismic wave speed anomalies used to construct the FWI model REVEAL. Credit: T.L.A. Schouten et al.

The recent study, led by researchers from the California Institute of Technology and ETH Zurich, has revealed an unexpected discovery: zones in the Earth’s mantle beneath the western Pacific that exhibit characteristics similar to remnants of submerged tectonic plates. These areas, however, are located far from plate boundaries where subduction typically occurs, and there is no geological evidence of subduction in these locations. This finding, published in the journal Scientific Reports, challenges current plate tectonic theories and raises new questions about the Earth’s internal dynamics.

What sets this breakthrough apart from previous studies is the use of a technique known as full-waveform inversion. This method analyzes all types of seismic waves to create a more detailed model of the Earth’s mantle. While highly precise, this approach is also extremely demanding in terms of computational power, requiring researchers to use the Piz Daint supercomputer in Lugano.

One of the most intriguing anomalies lies just beneath the western Pacific. According to Thomas Schouten, the study’s lead author, these zones might consist of material that doesn’t align with current expectations about the tectonic cycle. They could be iron-rich materials accumulated through mantle movements over billions of years or even fragments of ancient material that have remained intact since the mantle’s formation 4 billion years ago.

This discovery suggests that mantle anomalies are more common than previously thought, opening new possibilities for understanding our planet’s internal dynamics. However, it also underscores the need for even more advanced models to interpret the properties of the material behind seismic wave velocities. According to Schouten, the challenge now is to break down these signals and relate them to specific material parameters, such as chemical composition or elasticity.

What lies beneath the western Pacific may be much more than a geological enigma—it could be the key to deciphering the deepest secrets of our planet.

SOURCES

ETH Zürich

Schouten, T.L.A., Gebraad, L., Noe, S. et al. Full-waveform inversion reveals diverse origins of lower mantle positive wave speed anomalies. Sci Rep 14, 26708 (2024). doi.org/10.1038/s41598-024-77399-2

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