Geologists explore the depths of the Pacific and find anomalies in seismic waves

Geophysicists at ETH Zurich (the Swiss Federal Institute of Technology) are using models of the Earth's lower mantle to identify areas where seismic waves behave differently than previously assumed.

Using these models, researchers can see areas of rock that are cooler or have a different composition than the surrounding rocks. This discovery challenges current understanding of Earth's plate tectonics - and presents researchers with a major mystery.

No one can see inside the Earth . Nor can they drill deep enough to sample rock from the mantle, the layer between the Earth's core and the hard outer layer, the lithosphere, or to measure its temperature and pressure. That's why geophysicists use indirect methods to see what's going on beneath our feet .

For example, they use seismograms, or recordings of earthquakes, to determine how fast earthquake waves travel. They then use this information to calculate the Earth's internal structure .

It works like this: when the Earth shakes, seismic waves propagate from the epicenter in all directions. As they travel through the Earth, they are refracted, diffracted or reflected. The speed at which the waves propagate depends on the type of wave, but also on the density and elasticity of the material through which the waves pass.

The seismograph's graph or record is called a seismogram and is used to calculate the earthquake's location and other parameters. (Image created by AI)

Seismographic stations record these different waves and, based on these records, geophysicists can draw conclusions about the structure and composition of the Earth and examine the processes occurring within it.

Using seismic records, geoscientists have determined the position of tectonic plates submerged in the Earth's mantle . They have always found them where they expected them to be: in an area known as a subduction zone, where two plates meet and one subducts beneath the other into the Earth's interior. This has helped scientists investigate the plate tectonic cycle, or the emergence and destruction of plates on the Earth's surface , throughout the history of our planet.

Remains of plates where they shouldn't be

Now, however, a team of geophysicists from ETH Zurich and the California Institute of Technology has made a surprising discovery: using a new high-resolution model, they have discovered other areas deep inside the Earth that appear to be the remains of submerged plates .

However, these are not located where expected ; instead, they are under large oceans or in the interior of continents - far from plate boundaries. There is also no geological evidence of past subduction in these locations.

The most intriguing thing about this discovery is knowing that these remains of plates are found in places where it would be impossible for them to exist. (Image created by AI)

What is new about their modeling approach is that the ETH researchers are not using just one type of seismic wave to study the structure of the Earth’s interior, but all of them . Experts call this procedure full-waveform inversion. This makes the model very computationally intensive, which is why the researchers used the Piz Daint supercomputer at the CSCS in Lugano.

Is there a lost world beneath the Pacific Ocean?

One of the newly discovered zones lies beneath the western Pacific. However, according to current theories and knowledge of plate tectonics, there should be no material from subducted plates there , because it is impossible for subduction zones to have existed nearby in recent geological history.

“These zones in the Earth’s mantle appear to be much more widespread than previously thought. (...) This is our dilemma. With the new high-resolution model, we can see these anomalies everywhere in the Earth’s mantle. But we don’t know exactly what they are or what material is creating the patterns we have discovered.” Thomas Schouten, first author and PhD student at the Geological Institute of ETH Zurich.

Researchers are not sure what material is involved and what it would mean for the Earth's internal dynamics . It's like a doctor who has been examining blood circulation through ultrasound scans for decades and finding arteries where one would expect them, says ETH professor Andreas Fichtner.

“Then, if you give him a new and better scanning tool, he suddenly sees an artery that doesn’t belong there. That’s exactly how we feel about new discoveries ,” explains the wave physicist. He developed the model in his group and wrote the code.

Extract more information from waves

So far, researchers can only speculate. “We think that the anomalies in the lower mantle have a variety of origins,” Schouten says. He says it’s possible that it’s not just material from cold plates that subducted in the last 200 million years , as previously assumed.

“It could be ancient material, rich in silica, that has existed since the formation of the mantle, around 4 billion years ago, and that has survived despite the convective movements of the mantle, or areas where iron-rich rocks have accumulated as a result of these movements of the mantle over billions of years.” Thomas Schouten.

For the PhD student, this means above all that further study with even better models is needed to see more details of the Earth’s interior . “The waves we use for the model essentially represent just one property, namely the speed at which they travel through the Earth’s interior,” says the geoscientist. However, this does not do justice to the Earth’s complex interior.

News reference

Thomas LA Schouten, Lars Gebraad, Sebastian Noe, Anna JP Gülcher, Solvi Thrastarson, Dirk-Philip van Herwaarden, Andreas Fichtner. Full-waveform inversion reveals diverse origins of lower mantle positive wave speed anomalies . Nature Scientific Reports (2024).