cmns.umd.edu: Geophysicists at the University of Maryland analyzed thousands of records of seismic waves passing through the Earth to reveal echoes from the boundary between the molten core of the Earth and the solid mantle above it. Echo analysis revealed extensive heterogeneous structures floating on top of the core - regions of unusually dense, hot rocks at the core-mantle interface.
Earthquakes generate seismic waves below the Earth's surface that travel thousands of miles. When waves collide with changes in rock density, they alter velocity, bend, or scatter, creating an echo that can be detected. Therefore, by measuring the travel time and amplitude of these echoes as they reach seismometers at different locations, scientists can develop models of the physical properties of rocks hidden beneath the surface. This process is similar to how bats use echolocation.
Scientists are unsure of the composition of these structures, as previous studies of this kind have provided only a limited understanding of them. A better understanding of their shape and degree of compaction can help reveal geological processes deep within the Earth. This knowledge could provide a clue to understanding how plate tectonics works and how our planet evolves.
The new study provides the first complete picture of the core-mantle boundary over a vast area with such detailed resolution. The study was published in the June 12, 2020 issue of Science.
The researchers focused on the reflection of seismic waves propagating under the Pacific Ocean basin. Their analysis revealed a previously unknown structure beneath the Marquesas Islands in the South Pacific and showed that the structure beneath Hawaii is much larger than previously thought.
“By looking at thousands of echoes from the core-mantle boundary simultaneously, instead of focusing on a few at a time, as is usually done, we got a completely new global picture,” says Doyoung Kim, PhD in Geology, UMD Fellow and Lead Author articles. “This shows us that there are many structures in the core-mantle boundary that can create these echoes. And before we did not understand this, because we had only a very narrow view.
For this study, Dr. Doyoung Kim and his colleagues looked for echoes generated by a special type of wave called shear wave that occurs when it travels along the core-mantle boundary. In a single earthquake recording, echoes from diffracted shear waves can be difficult to distinguish from random noise. But examining multiple seismograms from multiple earthquakes simultaneously can reveal similarities and patterns that identify the echoes hidden in the data.
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Using the AI Sequencer algorithm, the researchers analyzed 7,000 seismograms from hundreds of earthquakes of magnitude 6.5 or more that occurred in the Pacific basin between 1990 and 2018. The Sequencer was developed by co-authors of the new study from Johns Hopkins University and Tel Aviv University. For earthquake seismograms, the algorithm detected a large number of shear wave echoes.
“The programmatic approach to geology using AI is growing at a tremendous pace and a new method like the Sequencer allows us to systematically detect seismic echoes and gain new insights into structures at the base of the mantle that remain largely mysterious,” says Dr. Kim.
“We found echoes in about 40% of all seismic wave traces,” says Vedran Lekic, assistant professor of geology at UMD and co-author of the study. “It was amazing as we expected them to be more rare. But now we understand that anomalous structures at the core-mantle boundary are much more widespread than previously thought."
The scientists found that a large patch of very dense, hot material at the core-mantle boundary beneath Hawaii produced exceptionally loud echoes, indicating that it was even larger than previous estimates. Such zones, called ultra-low velocity zones (ULVZs), are found in the roots of volcanic plumes, where hot rocks rise from the core-mantle boundary zone to form volcanic islands. ULVZ near Hawaii is the largest known.
This survey also found a previously unknown ULVZ under the Marquesas Islands.
“We were surprised to find such an important feature under the Marquesas Islands that we did not even know about,” Lekic said. "It's really exciting because it shows how the Sequencer algorithm can help us contextualize seismic data around the world in a way that we couldn't previously."
The team of researchers says they have used advanced imaging techniques to detect huge, "unexpected" structures near the Earth's core - a startling reminder of how little we know about the interior of the planet where we live.
Editorial comment
Key points of this study are the theses "We found echoes on about 40% of all seismic wave traces" and "how little we know about the inner part of the planet where we live."
When geologists, physiologists, or some other "ologues" talk about one or another anomaly, it means that the area of the anomaly leaves, at most, a couple of percent of the rest of the studied area. However, when an anomaly affects 40% of the area of interest, it is no longer an anomaly. This is the norm, the natural state of affairs, which the official geology does not explain.
Moreover, what is also interesting: if at the base of each mantle plume there is a certain giant structure (and it is there, if it was found in the two most famous plumes) - then how are these structures connected through the core? Geological antipodes are a known fact, each plume has a mirror-symmetrical twin brother bulging from the core on the other side of the Earth. If the core were solid, then such superstructures penetrating it could somehow be explained, but after all, according to the official dogma, the core is liquid and there are even some vortices there.
In general, it is really, as it is written: "how little we know about the inner part of the planet where we live." To be more precise, we know absolutely nothing, since no one dived into the well. More precisely, I did not dive deep enough, since there are promising technologies:
And in this situation, when science knows little about the world around it and everything is built solely on "generally accepted theories", it would probably be correct to take into account all theories of the world order, including, so to speak, "not generally accepted". For example - the theory that the Earth stands on pillars.
From the point of view of the modern man in the street, who has been hammered into the head of various “scientific dogmas” since childhood, the ancient Greeks who talked about the Flat Earth and the crystal heavenly spheres to which the planets were attached were completely wild and underdeveloped idiots. However, all these idiots studied in the city of Alexandria, Egypt, which was the center of all knowledge about the world.
And the Egyptian astronomers knew a lot. In particular, they calculated the motion of stars for periods of 25,000 years or more. It is possible that they received this knowledge themselves, it is possible that aliens taught them something, but the main thing is that the Egyptians knew a lot. And how did it happen that the people who built the pyramids and communicated with some kind of "gods from space" were so stupid that they considered the Earth flat and thought that the Sun revolves around the Earth, and not vice versa?
It is possible, of course, that in all these tales about elephants and turtle some kind of analogy is hidden that we simply cannot decipher, but it is also possible that some “brothers in mind” simply explained to the ancient Egyptians the structure of the world to the extent of understanding the poor. Therefore, in order not to become like the officials and not to write in a scientific journal about “we don’t know anything” - probably you should not discard any theories and consider all of them. And until the scientific world arrives at this principle, there are still many “wonderful discoveries” awaiting it, so we are following the development of events.