According to a new study published in Nature, a layer of fluid rock could be at the bottom of the upper mantle that circles the Earth. The discovery was made by measuring the lingering movement detected by GPS sensors on islands in the Pacific Ocean near Fiji.

This is the first solid observation of deformation following deep quakes, which could explain some other seismologists’ suggestions that there may exist “stagnant” rock slabs at the bottom of the upper mantle that doesn’t move much. “It has been hard to reproduce those features with models, but the weak layer found in this study makes it easier to do so,” said Sunyoung Park, a geophysicist at the University of Chicago and the study’s lead author.

The mantle is the layer beneath the Earth’s crust made of rock, but at those extreme temperatures and pressures, the rock becomes viscous, flowing slowly like honey or tar. The properties of the Earth’s mantle are critical to its evolution, influencing how much heat the planet retains for how long and how its materials cycle over time. However, our current understanding is minimal and is based on numerous assumptions.

Park reasoned that studying the aftermath of intense earthquakes might provide a novel way to measure the properties of the mantle. These earthquakes reach deep into the mantle, and Park believed they could help him understand how it works. The scientists examined data from GPS sensors on several nearby islands. They discovered that the Earth continued to move in the months following the 2018 Fiji earthquake, eventually settling in its wake.

The researchers discovered evidence of a layer about 50 mi (80.46 km)  thick that is less viscous than the rest of the mantle and sits at the bottom of the upper mantle layer by examining how the Earth deformed over time. They believe this layer may extend around the world. This low-viscosity layer may affect how the Earth transports heat, cycles materials, and mixes them over time between the crust, core, and mantle.

References

• Lerner, L. (2023, February 22). Deep earthquakes could reveal secrets of the Earth’s mantle. University of Chicago News; The University of Chicago. https://news.uchicago.edu/story/deep-earthquakes-could-reveal-secrets-earths-mantle
• Park, S., Avouac, J.-P., Zhan, Z., & Gualandi, A. (2023). Weak upper-mantle base revealed by postseismic deformation of a deep earthquake. Nature, 1–6. https://doi.org/10.1038/s41586-022-05689-8