What causes the most mysterious earthquakes deep in the Earth?

Photo: This close-up image of an ultra-deep diamond highlights its inclusions, which appear here as black spots. Inclusions like these provide geochemical evidence that a submerged oceanic plate can hold water and … view more

Credit: Photo by Evan Smith / © 2021 GIA

Washington, DC – The cause of the deepest earthquakes on Earth has been a mystery to science for more than a century, but a team of Carnegie scientists may have solved the issue.

New research published in AGU Advances provides evidence that fluids play a major role in deep-focused earthquakes – which occur 300 and 700 km below the planet’s surface. The research team includes Carnegie scientists Stephen Sherry, Lara Wagner, Peter Van Kecken, Michael Walter, as well as Graham Pearson of the University of Alberta.

Most earthquakes occur near the Earth’s surface, even about 70 kilometers. It occurs when pressure builds up at a fracture between two blocks of rocks – known as a rift – causing them to suddenly slide over one another.

However, deep in the ground, extreme stresses create a lot of friction to allow this type of slip to occur and higher temperatures enhance the rock’s ability to deform to accommodate the changing stresses. Although not expected in theory, scientists have been able to identify earthquakes that have arisen more than 300 kilometers below the surface since the 1920s.

“The big problem that seismologists have faced is how we can have these highly concentrated earthquakes at all,” Wagner said. “Once you go down a few dozen kilometers, it becomes very difficult to explain how we slip in error when the friction is so high.”

Ongoing work over the past several decades has shown that water plays a role in medium-depth earthquakes – those that occur between 70 and 300 kilometers below the Earth’s surface. In these cases, the water is released from the minerals, weakening the rocks around the fault and allowing the rock masses to slide off. However, scientists did not believe that this phenomenon could explain deep-focus earthquakes, largely because water and other fluid-forming compounds were thought to be unable to make them far enough into Earth’s interior to provide a similar effect.

This thinking changed for the first time when Sherry and Wagner compared the depths of rare-earth deep diamonds to mysterious deep-focused earthquakes.

“Diamonds form in liquids, if diamonds are present, then liquids are present,” Sherry explained.

The diamonds themselves indicate the presence of fluids, however, they have also brought samples from deep within the Earth to the surface for scientists to study. When diamonds form in the ground, they sometimes pick up metal pieces from the surrounding rock. These minerals are called inclusions and may make your jewelry less expensive, but they are invaluable to earth scientists. It is one of the only ways that scientists can study samples directly from deep within our inner planet.

Diamond inclusions have the distinct chemical signature of similar substances found in the oceanic crust. This means that water and other materials did not somehow form deep underground. Instead, they were carried as part of a submerged oceanic plate.

Wagner said, “The seismology community has moved away from the idea that there could be water that deep. But diamond amputators like Steve were showing us samples and saying, ‘No, no, no. There is definitely water here “so we all had to get together to see how it got there.”

To test the idea, Wagner and Van Keiken built advanced mathematical models to simulate the temperatures of sunken plates at depths much greater than had been tried before. In addition to the modeling, Walter examined the stability of water-bearing minerals to show that under extreme heat and deep-Earth pressures, they would actually be able to hold onto water under certain conditions. The team showed that although warmer plates do not contain water, minerals in cooler oceanic plates could theoretically transport water to depths that we associate with deep-focused earthquakes.

To strengthen the study, the team compared the simulations with real-life earthquake data. They were able to show that panels that could theoretically carry water to these depths were also those that had previously experienced deep, unexplained earthquakes.

This study is unusual in applying four different disciplines – geochemistry, seismology, geodynamics, and petroleum science – to the same question, and they all point to the same conclusion: water and other fluids are an essential component of deep-focused earthquakes.

“The nature of deep earthquakes is one of the big questions in Earth sciences,” said Sherry. “We needed all four of these different disciplines together to make this argument. And it turns out we have it all at the company’s headquarters in Carnegie.”

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The authors thank the Carnegie Institution for Science and the University of Alberta for their continued support. Diamond research is supported by the Carbon Deep Observatory and the US National Science Foundation.

The Carnegie Institution for Science (carnegiescience.edu) is a private, non-profit organization based in Washington, DC, with three research departments on both coasts. Since its founding in 1902, the Carnegie Endowment has been a pioneering force in fundamental scientific research. Carnegie scientists are leaders in the life and environmental sciences, earth and planetary sciences, astronomy and astrophysics.

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