Earthquakes reveal a fifth layer in the Earth: a hot, solid sphere in the inner core

Until now, seismologists have been able to track the waves as they travel to the antipode and back to the earthquake site, but no further.

“By developing a technique to amplify the signals captured by a ‘dense’ seismic network, we were able for the first time to detect seismic waves bouncing up and down the Earth’s diameter up to five times,” said the observed seismologist. Thanh Son Pham of the Australian National University (ANU) in Canberra, first author of the study on the innermost inner core.

“Our study is made possible by the unprecedented expansion of global earthquake networks, especially dense networks in countries bordering the United States, on the Alaskan Peninsula and across the European Alps.”

The researchers studied the anisotropy of the iron-nickel compound that makes up Earth’s inner core. A material is anisotropic if some physical properties, such as its velocity of light propagation, are unequal in different directions.

In this case, anisotropy is used to describe how seismic waves speed up or slow down as they travel through the inner core material, depending on which direction they are traveling. This can be caused by a different arrangement of the iron atoms at high temperatures and pressures or by a preferential orientation of the growth crystals.

The researchers found that the seismic feedback waves repeatedly penetrated points near the center of the Earth from different angles. By analyzing the variation in the transmission time of seismic waves from different earthquakes, they concluded that the crystal structure in the inner core is likely different from the structure in the outer layer.

The researchers say this may explain why the waves speed up or slow down depending on the angle at which they penetrate the innermost inner core.

According to the team at ANU, their findings indicate that at some point in the timeline of Earth’s evolution, a major global event may have occurred that significantly altered the crystal structure of Earth’s inner core.