Like a planet-sized stacking doll, Earth’s core has its own hidden core, a new study suggests.
Focusing on earthquakes of magnitude 6 or larger that struck in the last decade, the researchers combined data on these quakes that were collected at seismic stations around the world. Combining these signals made it possible to detect even very faint reflections of the seismic waves. Of the 200 or so quakes analyzed, 16 events spawned seismic waves that detectably bounced through the inner core multiple times.
“Understanding how the magnetic field evolves is extremely important for the life on Earth’s surface,” says Hrvoje Tkalčić, a seismologist at the Australian National University in Canberra.. As iron-rich fluid circulates in the outer core, some of the material cools and crystallizes, sinking to form a solid center. That interplay generates Earth’s magnetic field., just a fraction of Earth’s 4.6-billion-year-long life span .
Different types and structures of minerals, as well as different amounts of liquid in the subsurface, can change the speed of seismic waves traveling through Earth, offering clues to the makeup of the interior. In 2002, researchers noted that seismic waves traveling through the innermost part of Earthrelative to the planet’s poles than in other directions. That suggests there’s some oddity there — a difference in crystal structure, perhaps.
The exact source of the wave slowdown isn’t clear, Tkalčić says. The phenomenon might be related to the structure of the iron crystals, which may be packed together differently farther into the center. Or it could be from a different crystal alignment caused by some long-ago global event that changed how inner core crystals solidified out of the outer core.
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