The solid inner core of the earth itself has a core

An earthquake in Alaska produced seismic waves that passed through the Earth’s inner core (red) four times, revealing there was another innermost one (light red sphere). Bild: Drew Whitehouse, Son Phạm and Hrvoje Tkalčic

Researchers have tracked earthquake waves traveling across the planet up to five times. As they show, the solid core of the Earth itself has a core.

Roughly speaking, our earth is structured like a Mozartkugel: the chocolate coating corresponds to a brittle stone crust, the nougat to the earth’s mantle made of partially ductile silicate, and its core is made of iron with an admixture of maybe ten percent nickel instead of green marzipan. In fact, of course, geological confectionery is more complicated.

In 1936, the Danish geophysicist Inge Lehmann (1888 to 1993) discovered that two zones can be distinguished in the Earth’s core. She concluded this from recordings of seismic waves from a powerful earthquake in New Zealand’s South Island traveling across the planet. As was subsequently shown, the outer of these two zones of the earth’s core is liquid, which is why the earth’s rotation causes eddies in it, which in turn generate the earth’s magnetic field, which is so important for the earth’s biosphere. The inner core of the earth, on the other hand, is solid.

Earthquake waves are the only way to study the deep interior of the Earth for yourself, but such studies are not easy. As a rule, sufficiently strong ones only occur in the vicinity of the so-called subduction zones – boundaries of tectonic plates along which ocean crust is pulled into the earth’s mantle earthquake on whose waves can run through the entire planet without attenuating themselves below the detection limit.

However, in order to then record them, seismic measuring stations in the opposite regions of the world from the earthquake source in question must be on reception. But in the oceans or other remote regions, the network of such stations is correspondingly extensive. However, it has become considerably narrower and more sensitive in the last two decades. This means that waves can now also be detected that have crossed the central region of the earth more than once.

Stack earthquake waves

Now the two geophysicists Thanh-Son Pham and Hrvoje Tkalčić from the Australian National University in Canberra report in one technical article in Nature Communications, how they managed to study the center of the earth by analyzing seismic waves that passed through the solid iron sphere at the center of the earth up to five times. According to the two authors, this is a new record.

“As far as we know,” they write, “there are no reports of multiple reflections with more than two passes in the seismological literature.” The researchers evaluated data from sixteen strong earthquakes from the years 2010 to 2019, including Example of one of magnitude 7.9 that rocked the Solomon Islands archipelago in the southwest Pacific on January 22, 2017. Waves radiated from there were recorded by seismometers in East Asia and Australia, but also on both sides of the Atlantic down to Antarctica.

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By “stacking” such wave data from many different seismological measurement stations, Pham and Tkalčić were able to amplify the signals of the waves traveling through the inner core and suppress disturbing noise. This allowed them to conclude that the solid inner core discovered by Inge Lehmann does in fact split into two zones.

First indications of such an “innermost inner core” were first published in 2002 and were supported by subsequent investigations. It was estimated that this innermost region of our planet must have a radius of regarding 300 kilometers – compared to a radius of regarding 1220 kilometers for the entire inner solid core of the earth.

According to the results of the two geophysicists from Canberra, the size of the earth’s core was clearly underestimated. “The properties of the zone of the innermost 650 kilometers of the Earth’s inner core differ significantly from those of the outer zone,” the researchers write.

The difference consists in a significantly higher anisotropy, which means that the propagation of seismic waves there depends to a much greater extent on their direction of incidence. The two authors speculate that this transition between two zones of different anisotropy may be the “fossil” of a change in the growth of the inner core over the course of Earth’s history.

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