ASU scientists identify unexpected energy in seismic data

About 2,000 miles below the surface of our planet, Earth's rocky mantle meets the molten, metallic outer core. The changes in physical properties across this boundary are more significant than those between the solid rock we stand upon and the air we breathe. 

Using seismic data recorded by sensors deployed in Antarctica, the core-mantle boundary, or CMB, beneath a large portion of the Southern Hemisphere has been probed in high resolution for the first time. The precise method examined sound wave echoes from the CMB. 

Arizona State University scientists Edward Garnero, Mingming Li and Sang-Heon (Dan) Shim of the School of Earth and Space Exploration, with a team of international researchers, identified unexpected energy in the seismic data that arrives within several seconds of the CMB-reflected wave. The scientists' work appears in the April issue of Science Advances.  

“Analyzing thousands of seismic recordings from Antarctica, our high-definition imaging method found thin anomalous zones of material at the CMB everywhere we probed," Garnero said. "The material’s thickness varies from a few kilometers to tens of kilometers. This suggests we are seeing mountains on the core, in some places up to five times taller than Mount Everest."

One benefit of having seismic sensors in Antarctica is that earthquakes come from many directions, which enables seismic analyses of a large swath of the CMB in the Southern Hemisphere.

Subtle signaling 

Earth is layered like an onion, with a crust, mantle, outer core and inner core. Deep below Earth's surface, the team used subtle signals to map a very thin (tens of kilometers) variable layer across the study region. The properties of the strange CMB coating include substantial wave speed reductions — termed an ultra-low velocity zone, or ULVZ. 

“Seismic investigations, such as ours, provide the highest-resolution imaging of the interior structure of our planet, and we are finding that this structure is vastly more complicated than once thought,” said Samantha Hansen, the George Lindahl III Endowed Professor in geological sciences at University of Alabama and lead author of the study. “Our research provides important connections between shallow and deep Earth structure and the overall processes driving our planet.”

While ULVZs have been previously detected in isolated spots, the current study implies that such a strange structure is a global phenomenon. ULVZs are well explained by the former oceanic seafloor that has sunk to the CMB. The oceanic tectonic plate is carried into the planet's interior at subduction zones. Subducted oceanic material accumulates along the CMB, pushed by the slowly flowing rock in the mantle. The distribution and variability of such material explain the range of observed ULVZ properties. 

“A popular hypothesis about the origin of ULVZs is that they are remnants of the Earth's early magma ocean. However, dynamic simulations have shown that old, dense molten materials could be pushed around and build larger structures over geological time. In this new paper, we show that mantle convective flow can spread subducted surface materials globally at the core-mantle boundary,” Shim said. “Because subducted materials are more likely to be in a molten state at the core-mantle boundary, our study offers an intriguing possibility that some ULVZs may result from surface materials and may be much younger than previously thought.”

The thin and highly variable ULVZ material appears denser than the surrounding mantle rock and is predicted to be everywhere along the CMB. 

"The diverse morphology of ULVZ mountains on the CMB reflects fluctuations in subduction history and deep mantle flow in both space and time,” Li said.

Global mountain ranges of the ULVZ material at the bottom of Earth's mantle may play an essential role in how heat escapes from the core, the portion of our planet that powers our magnetic field. Some specific chemistries related to the outer core and the deepest mantle become entrained in plumes that eventually reach Earth's surface as volcanic eruptions.

Written with contributions from Samantha Hansen and Adam Jones.