Earthquakes reveal odd 'fluid' rock layer in Earth's solid mantle

An 'unexpected' fluid-rock layer may be ringing the Earth at the very bottom of the upper mantle, according to a study published in Nature on February 22 by a scientist from the University of Chicago.

The discovery was made by analyzing the lingering movement that GPS sensors on islands in the Pacific Ocean near Fiji detected after a strong earthquake. Significantly, the research may lead to the development of a brand-new technique for determining the fluidity of the Earth's mantle.

What more do we now know of Earth's mantle?

A precise measurement of the mantle layer's viscosity (or thickness), the layer beneath the Earth's crust, has proven to be a challenge for scientists.

Sunyoung Park, a geophysicist with the University of Chicago and the study's lead author, demonstrates that there might be a unique way to measure the mantle's properties. That is, studying the aftermath of intense earthquakes.

Park and her team examined one such earthquake in detail, which took place in 2018 off the coast of Fiji. The quake had a magnitude of 8.2; however, because it occurred 350 miles below the surface, it did not significantly harm or kill anyone.

When the scientists carefully analyzed the data from GPS sensors on several nearby islands, they found the Earth continued to move even after the earthquake had passed. Even years later, Tonga is still moving slowly, at a rate of roughly 1 centimeter each year.

"You can think of it like a jar of honey that slowly comes back to level after you dip a spoon in it—except this takes years instead of minutes," explained Park in a press release. 

While the phenomenon had previously been noted for shallow earthquakes, scientists believed the effect would be too tiny to be noticed for deep earthquakes. Therefore, Park's observation is the first to solidly detect the deformation following deep earthquakes.

Crucially, this observation was used by Park and her associates to determine the mantle's viscosity.

By looking at how the Earth deformed over time, they discovered evidence of a layer, about 50 miles thick, at the bottom of the upper mantle layer that is less viscous than the rest of the mantle. They believe that this layer may cover the entire planet.

This low-viscosity layer could explain some other observations by seismologists. For example, it has been previously suggested that there are "stagnant" slabs of rock that don't move very much; these are located around the same depth at the bottom of the upper mantle. 

"It has been hard to reproduce those features with models, but the weak layer found in this study makes it easier to do so," Park stated.