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By Donna Lu

Michael Runkel / Alamy

Seismic wave data has revealed giant structures 2900 kilometres beneath the surface of Earth, at the boundary between Earth’s molten core and solid mantle.

Analysing data from hundreds of major earthquakes, Doyeon Kim at the University of Maryland and his colleagues have found a new structure beneath the volcanic Marquesas Islands in the South Pacific Ocean. The structure, known as an ultra-low velocity (ULV) zone, is about 1000 kilometres in diameter and 25 kilometres thick, says Kim.

These structures are called ULV zones because seismic waves pass through them at slower velocities, but what they are made of is still a mystery. They might be chemically distinct from Earth’s iron–nickel alloy core and silicate rock mantle, or have different thermal properties.

The researchers discovered the structure while analysing 7000 records of seismic activity from earthquakes that occurred around the Pacific Ocean basin between 1990 and 2018. The earthquakes all had a magnitude of 6.5 or greater, and were all deeper than 200 kilometres below Earth’s surface.

The team looked at echoes generated by a specific type of seismic wave, known as shear waves, as they travelled along the core-mantle boundary. The shear wave echoes from a single seismogram are difficult to distinguish from random noise, so the researchers used a machine-learning algorithm originally designed for identifying trends in large astronomy data sets.

The simultaneous analysis of thousands of shear waves revealed an enormous structure beneath the Marquesas Islands. The team also found that a previously discovered ULV zone beneath Hawaii is far larger than previously thought.

Studying the mantle is important, says Kim, because it may shed light on how Earth’s structures have developed and changed over time. “Earth’s mantle is where the convection is occurring, and it is actually the driving mechanism for hotspot volcanism as well as plate tectonics,” he says.

Identifying ULV zones may help us to better understand whether some volcanoes have origins deep below Earth’s surface, and the composition of Earth’s lower mantle.

Journal reference: Science, DOI: 10.1126/science.aba8972