They are located more than 2,900 kilometres below the Earth’s surface.
Based on a new method of analyzing seismic data, this research reveals that the large blocks of dense material located at the boundary between the core and the mantle of the planet are much more widespread than scientists have previously believed.
AN ALGORITHM DEDICATED TO THE STUDY OF THE STARS TO PROBE THE BOWELS OF THE EARTH
With the Structure of Earth’s core some 2,900 kilometers below the surface, scientists are forced to be creative in probing the bowels of our planet. By studying how earthquakes and tremors propagate through different types of materials, seismologists have been able to map its structure globally, revealing the presence of large blocks of burning material, up to the size of a continent, at the boundary between Structure of Earth’s core and mantle.
Because heat causes higher degrees of melting, these zones slow the speed of propagation of seismic waves towards the Earth’s core, and are therefore known as “ultra-low velocity zones”. Although the nature of these structures is not yet known (they could be magma or molten iron escaping from the core), better knowledge of their location would allow scientists to better understand the geological processes occurring at such depths.
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However, the conventional approach to studying earthquakes provides only snippets of information: each earthquake probes only a narrow area, and the weakest signals can get lost in the wider noise. To get around this problem, an international team of researchers, whose work was presented in the journal Science, relied on Sequencer, an algorithm designed for the study of stars and capable of browsing large astronomical data sets to derive scientific models.
After modifying it, researchers fed it with nearly three decades of seismic data, about 7,000 seismograms of a particular wave type, to identify seismic echoes indicating the presence of ultra-low velocity zones that may not have been detected by previous research.
“By looking at thousands of echoes at the boundary between core and mantle at the same time, rather than focusing on a few at a time, as we usually do, we got a whole new perspective,” says University of Maryland geologist Doyeon Kim. “This analysis showed that the region at the boundary between the Earth’s core and mantle is home to many structures that can produce these echoes, something we hadn’t done before because we had limited vision. »
The results returned by the algorithm revealed subtle changes in seismic waveforms from earthquakes in Asia and Oceania, suggesting an area of ultra-low velocity under the Marquesas Islands in the South Pacific that had never been identified before.
“We were surprised to discover such an important structure under the Marquesas Islands that we didn’t know existed before,” says geologist Vedran Lekić from the University of Maryland. “It’s really exciting because it shows how the Sequencer algorithm can help us contextualize seismogram data around the world better than we could before. »
“WE EXPECTED THEM TO BE MUCH RARER.”
In addition, the previously identified ultra-low velocity zone under Hawaii was found to produce much more intense seismic echoes than scientists previously estimated. This suggests that the latter is much more extensive than previous studies have estimated. More generally, this new research suggests that these structures are much more widespread than previously thought.
“We’ve identified these types of echoes for about 40% of all seismic wave paths,” says Lekić. ” Which was surprising, because we expected them to be much rarer. »
Having demonstrated the efficiency of the algorithm for this type of wave, the team’s approach can now be applied to other types of waves and frequencies. This could lead to a new high-resolution map of the bowels of the Earth. In turn, the physical properties thus revealed could help geologists deduce the chemistry and temperature of these structures, and thus determine the causes of their presence.
Source : Science Alert