To better understand the inner core’s structure, researchers used multiple seismometers to examine how seismic waves are distorted as they pass through the solid ball of iron nickel at Earth’s heart. “Earth oscillates like a bell after a large earthquake, and not just for hours, but days,” says co-author Hrvoje Tkalčić, a geophysicist at the Australian National University in Canberra, Australia.
To detect these oscillations, researchers recorded the waveforms at close to the original site of the earthquake and at the antipode —the direct opposite position on the surface of Earth. This enabled them to look at the multiple journeys through Earth’s centre. “It’s like a ping-pong ball that’s bouncing back and forth,” says co-author Thanh-Son Pham, a postdoctoral fellow at the Australian National University. Each reverberation takes around twenty minutes to cross from one side of the planet to the other, and the seismometers recorded up to five bounces from a single event.