Associate Professor Hrvoje Tkalcic from the ANU College of Physical and Mathematical Sciences and his team used earthquake doublets to measure the rotation speed of Earth’s inner core over the last 50 years and discovered that not only did the inner core rotate at a different rate to the mantle – the layer between the core and the crust that makes up most of the planet’s interior – but its rotation speed was variable.
Scientists assumed the rotation rate of the inner core to be constant because they lacked adequate mathematical methods for interpreting the data, says Tkalcic. A new method applied to earthquake doublets – pairs of almost identical earthquakes that can occur a couple of weeks to 30 or 40 years apart – has provided the solution.
“This is the first experimental evidence that the inner core has rotated at a variety of different speeds,” Tkalcic said in their statement. “We found that, compared with the mantle, the inner core was rotating more quickly in the 1970s and 1990s, but slowed down in the 80s. The most dramatic acceleration has possibly occurred in the last few years, although further tests are needed to confirm that observation. Interestingly, Edmund Halley, namesake of Halley’s Comet, speculated that the inner shells of the Earth rotate with a different speed back in 1692.”
This new method could help us understand the role of the inner core in creating the magnetic field that allowed life to evolve on Earth by acting as a shield from cosmic radiation.
“It’s stunning to see that even 10, 20 or 30 years apart, these earthquakes look so similar. But each pair differs very slightly, and that difference corresponds to the inner core. We have been able to use that small difference to reconstruct a history of how the inner core has rotated over the last 50 years,” Tkalcic said. “What we have developed is a very powerful way to understand the internal structure and dynamics of our planet.”