New Structures Detected Near Earth's Core Using Seismic Wave Recordings

James Marshall
June 13, 2020

This discovery comes after researchers from the University of Maryland in the U.S. analyzed data from 7000 recorded earthquakes, including major earthquakes, recorded in the Pacific Ocean region between 1990 and 2018.

In a similar fashion, the team used their Sequencer algorithm to analyse thousands of seismograms, pulling out echoes that let it create a new map showing the details of the Earth's mantle, just above the liquid iron core, at a depth of 1,860 miles.

The earthquakes that were of magnitude 6.5 or higher and deeper than 200 kilometers below the surface of the Earth generated echoes as they traveled across the previously undiscovered structures. Researchers are hoping that Sequencer will allow them to use all of the datasets and bring them together to take a better look for lower mantle structures.

Doyen Kim, a seismologist at the University of Maryland and co-author on the paper, said, "Imagine you're outside in the dark".

'This is how bats echolocate their surroundings'.

In almost half of the waves that were diffracted, the researchers found three-dimensional structures near the core-mantle boundary.

However, their task is hard: they need to wait for an natural disaster to record data, and when this happens, it only provides information in a piecemeal manner; the data is restricted to a tiny region and most of the time, it's impossible to distinguish weaker echoes from noise.

As many as 7,000 records of seismic activity around the Pacific Ocean basin between 1990 and 2018 was analysed.

Earthquakes, seen as yellow stars here, send sound waves through the Earth. Seismograms record the echoes as those waves travel along the core-mantle boundary, diffracting and bending around dense rock structures.

'We were finally able to identify the seismic echoes and use them to create a map'.

Researchers found echoes along about 40% of all seismic wave paths which was surprising as Co-author of the study, Vedran Lekić explains, "We were expecting them to be more rare, and what that means is the anomalous structures at the core-mantle boundary are much more widespread than previously thought".

The scientists found that the large patch of very dense, hot material at the core-mantle boundary beneath Hawaii produced uniquely loud echoes, indicating that it is even larger than previous estimates. There were found two "mega-ULVZs" zones that span for at least 1,000 kilometers (621 miles). "This is really exciting, because it shows how the Sequencer algorithm can help us to contextualize seismogram data across the globe in a way we couldn't before".

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