The reverberations from the earthquakes at the center of the Earth have revealed new details about the structure of the inner core of the planet, according to a study published in Nature Communications.

For several decades, evidence has been mounting to suggest that the planet’s solid inner core consists of distinct layerswhose properties remained mysterious.

To better understand the structure of the inner core, the researchers used multiple seismometers to examine how seismic waves are distorted as they pass through the solid iron-nickel sphere at the heart of the Earth. “The Earth wobbles like a bell after a big earthquake, and not just for hours, but for days,” says Hrvoje Tkalčić, a geophysicist at the Australian National University.

To detect these oscillations, the researchers recorded them waveforms near the earthquake’s original location and the antipodes—the exact opposite location on Earth’s surface. “It’s like a ping pong ball bouncing back and forth“, he says Thanh-Son Pham, a postdoctoral fellow at the Australian National University. Each reverberation lasts approx twenty minutesto pass from one side of the planet to the other and seismometers recorded up to five bounces from a single event.

Stacked metrics

The initial earthquakes each reached a magnitude greater than 6 Richter, but the waves progressively weakened as they passed through the Earth’s core. The researchers used a technique called “stacking,” in which they combined waveforms from a single event to create a more detailed picture of the deformation from the innermost core.

They found that the waves traveled differently through the core – which they estimate is about a thickness 650 kilometers – rather than through the external department. Waves passing through the inner part of the core were slowed down in one direction, while waves passing through the outer layer were slowed down in another direction.

The technique is usually used for mineral exploration and not so much in geophysics.

The latest find will help understand how Earth’s solid inner core formed – a process believed to have started somewhere between 600 million and 1.5 billion years before – and what role it might have had in shaping the magnetic field.