Understanding Earth’s structure is key to unlocking its internal dynamics. Seismic tomography shows faster-than-normal wave speeds in the mantle, which match the locations where tectonic plates have been pushed down into the Earth (subducted slabs).
This link is often used in plate reconstructions and geodynamic models. However, global travel-time tomography usually relies on a limited set of easily identified seismic waves, heavily influencing the positioning of the source and receiver.
A team of geophysicists from ETH Zurich and the California Institute of Technology has made a groundbreaking discovery deep within Earth’s mantle that could change our understanding of plate tectonics. Using a new, high-resolution model, the researchers have found evidence of anomalous zones in the Earth’s interior that resemble the remnants of submerged tectonic plates.
However, these zones are not where they were expected — not along the plate boundaries, but instead under vast oceans or within continental interiors, far from any known subduction zones.
The unusual zones show different earthquake wave behavior than previously assumed, suggesting they are composed of colder or differently composed rocks than the surrounding material.
“We’ve discovered that these areas, which look like remains of subducted plates, are much more widespread in the mantle than we ever thought,” says Thomas Schouten, the study’s first author and a doctoral student at ETH Zurich’s Geological Institute.
The key to the discovery lies in a new method called full-waveform inversion. Unlike earlier approaches that used only one type of seismic wave to study the Earth’s structure, this technique incorporates all earthquake waves, providing a much clearer and more detailed view of the mantle’s composition.
This approach is highly computationally intensive, so the researchers ran their models on the Piz Daint supercomputer at the Swiss Center for Scientific Computing (CSCS) in Lugano.
The result is a much finer picture of Earth’s interior, revealing unexpected anomalies, including one under the western Pacific — an area where, according to current tectonic theories, there should be no traces of subducted plates. The team cannot yet identify what material is present in these zones, leaving researchers puzzled about their origin and meaning.
“We see these anomalies everywhere in the mantle, but we still don’t know what material is creating these patterns,” Schouten explains. “That’s our dilemma.”
The new findings raise critical questions about the current understanding of plate tectonics. The presence of anomalous zones far from active subduction zones contradicts the widely accepted theory that subducted plates are the primary source of material anomalies in the mantle. The new zones are found in areas that should have no recent history of subduction, such as the interior of continents and beneath large oceanic regions.
Currently, the researchers can only speculate about the nature of these anomalies. Schouten believes the anomalous zones might not be cold, subducted plate material from the last 200 million years, as previously assumed.
Instead, he proposes that these could be remnants of ancient, silica-rich material that has been in the mantle since Earth’s formation around 4 billion years ago, surviving despite the mantle’s constant convective movements. Another possibility is that these zones are the result of iron-rich rocks accumulating over billions of years due to mantle movements.
“To fully understand these anomalies, we need to develop even better models that can dig deeper into Earth’s interior,” says Schouten. “The waves we currently use to create our models mainly tell us about the speed at which they travel through the Earth. But this doesn’t reveal the full complexity of the mantle. We need to figure out what material properties are responsible for the observed wave speeds — essentially, we need to understand the material behind the waves.”
Journal Reference:
- Schouten TLA, 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: 10.1038/s41598-024-77399-2