J waves confirm that the inner core of the Earth is solid


J waves confirm that the inner core of the Earth is solid

Researchers at the Australian National University (ANU) believe they have now obtained direct evidence that the inner core of the Earth is solid and shares some similar elastic properties with gold and platinum. To discover it, they devised a way to detect cutting waves, or “J waves” in the inner core, a type of wave that can only travel through solid objects. The research has been published in Science.

The shear waves of the inner core are so small and weak that they can not be observed directly. In fact, detecting them has been considered the “Holy Grail” of global seismology since scientists predicted for the first time that the inner core was solid 80 years ago. “The inner core is like a time capsule, if we understand it, we will understand how the planet was formed and how it evolves,” says one of the authors of the work, Hrvoje Tkalcic.

The so-called correlation wave field method used by these scientists analyzes the similarities between signals in two receivers after a large earthquake, rather than direct wave arrivals. A similar technique has been used by the same team to measure the thickness of ice in Antarctica.

“We found that the inner core is really solid, but we also found that it is softer than previously thought, and if our results are correct, the inner core shares some similar elastic properties with gold and platinum,” he says. scientific. 

Using a global network of stations, they took each pair of receivers and each major earthquake and measured the similarity between the seismograms, in a technique called cross-correlation, or the measure of similarity. From these similarities, the scientists constructed a global correlogram, a kind of fingerprint of the Earth.

“We are discarding the first three hours of the seismogram and what we are seeing is between three and 10 hours after a major earthquake occurs, we want to get rid of the big signals,” said Tkalcic.

The study shows that these results can be used to demonstrate the existence of J waves and infer the cutting wave velocity in the inner core. While this specific information on shear waves is important, for Tkalcic what this research tells us about the inner core is even more exciting:

“For example, we still do not know what the exact temperature of the inner core is, the age of the nucleus internal or the speed with which it solidifies, but with these new advances in global seismology, we are arriving little by little “.

Understanding the inner core of the Earth has direct consequences for the generation and maintenance of the geomagnetic field, without which there would be no life on the surface of the Earth.