Seismic waves reveal giant structures beneath the surface of the Earth

Donna Lu

Under the depths of the Marquesas Islands in the South Pacific is a giant structure near the Earth’s core

Michael Runkel / Alami

Seismic wave data revealed giant structures at an altitude of 2,900 kilometers below the surface of the Earth, at the boundary between the molten Earth’s core and the solid cover.

Analyzing data from hundreds of major earthquakes, Doyon Kim at the University of Maryland in the United States, and his colleagues found a new structure beneath the volcanic islands of Marquesas in the South Pacific. Kim says the structure, known as the extremely low speed zone, is about 1,000 km in diameter and 25 km thick.

These structures are called ULV regions because seismic waves pass through them at slower speeds, but what they are made of is still a mystery. They may be chemically distinct from the core of the iron and nickel alloys in the ground and the silicate rock cover, or have different thermal properties.

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The researchers discovered the structure while analyzing 7,000 records of seismic activity from earthquakes that occurred around the Pacific basin between 1990 and 2018. All earthquakes were 6.5 or larger, and all were deeper than 200 km below the surface of the Earth.

The team looked at echoes from a specific type of seismic wave, known as shear waves, as it traveled along the boundaries of the basic mantle. It is difficult to distinguish the shear wave echoes from an individual seismic diagram from random noise, so researchers used a machine learning algorithm originally designed to map trends in large astronomical data sets.

Simultaneous analysis of thousands of shear waves revealed a massive structure under the Marquesas Islands. The team also found that the previously discovered ULV area under the Hawaiian Islands is much larger than previously thought.

Kim says studying the mantle is important because it might shed light on how Earth’s structures have evolved and changed over time. “The mantle of the Earth is where the convection occurs, and it is in fact the driving mechanism for hotspot volcanoes as well as plate tectonics,” he says.

Defining ULV regions may help us better understand whether some volcanoes have deep origins below the Earth’s surface, and the formation of the Earth’s mantle.

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

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