Tiny Seismic Signals Offer a Glimpse of Earth’s Textured Inner Core

1 May 2024–Tiny seismic signals captured around the globe can be used to build a picture of the Earth’s inner core, its fabric made up of differences in the size, shape and orientation of iron grains.

This glimpse at the inner core also offers a window into the evolution of the core over Earth’s history, Sin-Mei Wu, a postdoctoral researcher at Lawrence Berkeley National Laboratory, said at the Seismological Society of America (SSA)’s 2024 Annual Meeting.

By examining the scattering of seismic signals reflected off the core, Wu and colleagues found that the inner core contains many small-scale—less than 10 kilometers in size—heterogeneities that are ubiquitous throughout the inner core.

These heterogeneities increase substantially around 500 to 800 kilometers beneath the inner core boundary, which could suggest that the solidification history of the inner core may have been a start-and-stop or variable process, the researchers said.

Modeling shows that “when the inner core solidifies or grows over time, it tends to form this kind of texture,” said Wu. “Seeing heterogeneities deeper in the core gives us the information that growth or solidification history is not regular in Earth’s core.”

The data are consistent with a scenario of rapid solidification from the fluid outer core at first, then a slowdown of solidification for the shallow part of the inner core, she added.

The seismic signals that Wu, Cornell University postdoctoral researcher Guanning Pang, University of Utah professor Keith Koper and their colleagues used to image the inner core are very short wavelength waves that travel from the surface of the Earth, bounce off the boundary of the inner core and reflect back to the surface at a different location. These tiny waves are difficult to detect, which can be a problem for seismologists studying Earth’s depths. “When we don’t see them, we don’t know if that’s because they’re not there, or because we cannot see them,” Wu explained.

In their study, the research team used data from seismic arrays used to detect high-frequency, tiny signals from underground nuclear explosions, which are similar to the signals used to probe the core.

“For our work here, we are trying to push what we can observe for high frequency or short wavelength signals associated with the deep earth structure,” Wu said, “so that’s why we are using these small aperture arrays, for this purpose.”

They analyzed signals from earthquakes larger than magnitude 5.7 over the past three decades that were recorded by the arrays, applying processing techniques to detect and extract the seismic signals that were scattered by the inner core.

To learn even more about the evolution of the inner core, researchers need to more precisely identify the location of heterogeneities in the inner core fabric, Wu suggested, perhaps using data from newly-deployed ocean bottom seismometers.