In a study published this week in “Science”, Princeton geophysicists Jessica Irving and Wenbo Wu, in collaboration with Sidao Ni of the Institute of Geodesy and Geophysics in China, used data from a huge earthquake in Bolivia to find mountains and other layer topography. located 660 kilometers (410 miles) straight down, which separates the upper and lower mantle. (Lacking an official name for this layer, researchers simply call it the “660 km limit.”)
To peek deep into Earth, scientists use the most powerful waves on the planet, which are generated by massive earthquakes. “You want a big, deep earthquake to shake the whole planet,” said Irving, a geoscience assistant.
Large earthquakes are significantly more powerful than small ones – the energy increases 30 times with each step up the Richter scale – and deep earthquakes, “instead of scattering their energy in the crust, can move the entire mantle,” Irving said. It gets its best data from earthquakes of magnitude 7.0 or higher, she said, because shock waves that send in all directions can travel through the core to the other side of the planet – and vice versa. For this study, key data come from waves captured after a magnitude 8.2 earthquake – the second largest deep earthquake ever recorded – that shook Bolivia in 1994.
“Such large earthquakes do not happen very often. We are lucky now that we have so many more seismometers than 20 years ago. Seismology is a different field than it was 20 years ago between instruments and computing resources. “
Seismologists and data researchers are using powerful computers, including Princeton’s supercomputer cluster, to simulate the complex behavior of scattering waves in the deep Earth.
The technology depends on a basic property of waves: their ability to bend and bounce. Just as light waves can bounce off a mirror or bend when refracted through a prism, earthquake waves travel directly through homogeneous rocks, but reflect or refract when they encounter a boundary or roughness.
“We know that almost all objects have a surface roughness and therefore scatter light,” said Wu, the lead author of the new paper, which has just completed a doctorate in geosciences. and a PhD student at the California Institute of Technology. “That’s why we can see these objects – scattering waves carry information about surface roughness. In this study, we examined scattered seismic waves traveling inside the Earth to limit the roughness of the Earth’s boundary to 660 km.”
Researchers were surprised at how rough this boundary is – rougher than the surface layer we all live on. “In other words, a stronger topography than the Rockies or the Appalachians is present at the 660-kilometer border,” Wu said. Their statistical model does not allow accurate determination of altitude, but there is a chance that these mountains are larger than anything on the Earth’s surface. The roughness was also not evenly distributed; just as the surface of the crust has smooth ocean bottoms and massive mountains, the 660 km boundary has rough areas and smooth spots. Researchers also examined a layer 410 kilometers (255 miles) down, at the top of the “transition zone” in the middle of the mantle, and found no such roughness.
“They found that the deep layers of the Earth were as complex as what we observed on the surface,” said seismologist Christine Hauser, an assistant professor at the Tokyo Institute of Technology who was not involved in the study. “Detecting changes in elevation of 2 miles (1-3 km) at a boundary that is over 400 miles (660 km) deep, using waves that travel across the Earth and back, is an inspiring feat.” Findings show that as earthquakes occur and seismic instruments become more complex and expand into new areas, we will continue to discover new small-scale signals that reveal new properties of the earth’s layers. “
The presence of a roughness of 660 km is important for understanding how our planet was formed and continues to function. This layer divides the mantle, which represents about 84 percent of the Earth’s volume, at the top and bottom. For years, geologists have debated how important this boundary is. In particular, they studied how heat passes through the mantle – whether hot rocks are transferred smoothly from the core-mantle boundary (almost 2,000 miles down) to the top of the mantle, or whether this transfer is interrupted at this layer. Some geochemical and mineralogical evidence suggests that the upper and lower mantles are chemically different, which supports the idea that the two sections do not mix thermally or physically. Other observations show that there is no chemical difference between the upper and lower mantle, which leads some to argue for what is called a “well-mixed mantle”, with the upper and lower mantle participating in the same heat transfer cycle.
In addition, the roughness discovered by researchers, which exists on a large, moderate and small scale, could theoretically be caused by thermal anomalies or chemical heterogeneities. But because of the way heat is transported in the mantle, Wu explained, any small-scale thermal anomaly will be smoothed out within a million years. This leaves only chemical differences to explain the small roughness they found.
“It’s easy to assume, since we can detect seismic waves traveling through the Earth in its current state, that seismologists can’t help understand how the Earth’s interior has changed in the last 4.5 billion years,” Irving said. . “The exciting thing about these results is that they give us new information to understand the fate of the ancient tectonic plates that descended into the mantle and where the ancient mantle material may still be located.”