Geophysicists from Princeton University in the US and the Chinese Academy of Sciences used data from that massive earthquake to reveal the topography of our planet's interior. The scientists found mountains and smooth plains on a boundary layer deep within the Earth's mantle.
Jules Verne, the father of science fiction, once imagined an entire world located beneath the one we live on. He envisioned a world unlike anything we’ve ever seen, complete with prehistoric species and never-before-seen plant life. Verne has been the second most-translated author in the world since 1979, ranking between Agatha Christie and William Shakespeare. Now, researchers studying seismic waves have discovered that Verne would have been proud of. Scientists have recently revealed they’ve discovered mind-bending features deep inside our planet that resemble structures located on the surface.
Scientists say that there are mountains deep below rivaling anything up here on Earth’s surface. In June of 1994, a powerful 8.2 magnitude earthquake rocked Bolivia. The tremor was really strong. Experts revealed at the time that nothing this powerful had been seen in decades, with shocks being felt as far away as Canada. Geophysicists from Princeton University in the US and the Chinese Academy of Sciences used data from that massive earthquake to reveal the topography of our planet’s interior.
Massive Mountains and Smooth Plains
“Earthquakes this big don’t come along very often,” explained geoscientist Jessica Irving. “We’re lucky now that we have so many more seismometers than we did even 20 years ago. Seismology is a different field than it was 20 years ago, between instruments and computational resources.” As explained by Princeton seismologists and data scientists, with the help of powerful computers, including Princeton’s Tiger supercomputer cluster, experts could simulate the complicated behavior of scattering waves in the deep Earth. The technology depends on a fundamental property of waves: their ability to bend and bounce. Just as light waves can bounce (reflect) off a mirror or bend (refract) when passing through a prism, earthquake waves travel straight through homogenous rocks but reflect or refract when they encounter any boundary or roughness.
“We know that almost all objects have surface roughness and therefore scatter light,” said Wu, the lead author of the new paper, who just completed his geosciences Ph.D. and is now a postdoctoral researcher at the California Institute of Technology. “That’s why we can see these objects — the scattering waves carry the information about the surface’s roughness. In this study, we investigated scattered seismic waves traveling inside the Earth to constrain the roughness of the Earth’s 660-km boundary.” The researchers were surprised by how rough that boundary is — rougher than the surface layer we live on.
“In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary,” said Wu. Their statistical model didn’t allow for precise height determinations, but there’s a chance that these mountains are bigger than anything on the surface of the Earth. The roughness wasn’t equally distributed, either; just as the crust’s surface has smooth ocean floors and massive mountains, the 660-km boundary has rough areas and smooth patches. The researchers also examined a layer 410 kilometers (255 miles) down, at the top of the mid-mantle “transition zone.” They did not find similar roughness.
“They find that Earth’s deep layers are just as complicated as what we observe at the surface,” said seismologist Christine Houser, an assistant professor at the Tokyo Institute of Technology who was not involved in this research. “To find 2-mile (1-3 km) elevation changes on a boundary that is over 400 miles (660 km) deep using waves that travel through the entire Earth and back is an inspiring feat. Their findings suggest that as earthquakes occur and seismic instruments become more sophisticated and expand into new areas, we will continue to detect new small-scale signals that reveal Earth’s layers’ new properties.”
What it means
The presence of roughness on the 660-km boundary has significant implications for understanding how our planet formed and continues to function. That layer divides the mantle, which makes up about 84 percent of the Earth’s volume, into its upper and lower sections. For years, geoscientists have debated just how important that boundary is. In particular, they have investigated how heat travels through the mantle. Whether hot rocks are carried smoothly from the core-mantle boundary (almost 2,000 miles down) all the way up to the top of the mantle or whether that transfer is interrupted at this layer. Some geochemical and mineralogical evidence suggests that the upper and lower mantle are chemically different, supporting the idea that the two sections don’t mix thermally or physically.
Other observations suggest no chemical difference between the upper and lower mantle, leading some to argue for what’s called a “well-mixed mantle,” with both the upper and lower mantle participating in the same heat-transfer cycle. “Our findings provide insight into this question,” explained Wu.
Their data suggests that both groups might be partially right. The smoother areas of the 660-km boundary could result from more thorough vertical mixing, while the rougher, mountainous areas may have formed where the upper and lower mantle don’t mix as well. Besides, the roughness the researchers found, which existed at large, moderate, and small scales, could theoretically be caused by heat anomalies or chemical heterogeneities. But because of how heat is transported within the mantle, Wu explained that any small-scale thermal anomaly would be smoothed out within a few million years because of how heat is transported within the mantle. That leaves only chemical differences to explain the small-scale roughness they found.
What could cause significant chemical differences? The introduction of rocks that used to belong to the crust now rests quietly in the mantle. Scientists have long debated the fate of seafloor slabs that get pushed into the mantle at subduction zones, the collisions happening all around the Pacific Ocean and elsewhere. Wu and Irving suggest that these slabs’ remnants may now be just above or below the 660 km boundary. “It’s easy to assume, given we can only detect seismic waves traveling through the Earth in its current state, that seismologists can’t help understand how Earth’s interior has changed over the past 4.5 billion years,” said Irving.
“What’s exciting about these results is that they give us new information to understand the fate of ancient tectonic plates which have descended into the mantle and where ancient mantle material might still reside.” She added: “Seismology is most exciting when it lets us better understand our planet’s interior in both space and time.”