The reverberation of earthquakes, as they “bounce” off the core, indeed reveals new details regarding the structure of this core, according to a study published on February 21 in Nature Communications. More specifically, it betrays more and more the presence of a second layer at the heart of what is called the inner core: it is something that was suspected twenty years, but which is not easy to “map”.
The difficulty is not only to have devices sensitive enough to detect such distant oscillations. These echoes of an earthquake occurring on a point of the planet will bounce to another point, opposite the planetand return to their starting point by crossing the central part, in a round trip that can last hours: a single “crossing” from one point on Earth to its opposite is estimated at 20 minutes.
The researchers first had to work to distinguish the echoes of 600 major earthquakes of the last decade, and compare how fast their waves are weakening crossing the core, but also to distinguish the impact that has, on this weakening, the inner core – the solid, metallic part, estimated at 1220 km in radius. And once there, try to identify the differences, if there are anyinside the inner core itself.
One of the conclusions of the Australian seismologists in this new study is therefore that the hypothesis of a central layer, distinct from the rest of the inner core, is reinforced, the “transition” of which to the upper layer might be the “fossilized of a cosmic impact that occurred hundreds of millions of years ago, or even longer.
If they are right, we should therefore review school textbooks, and describe our planet in five layers: the earth’s crust, the mantle, the outer core, the inner core, and now the central part of the inner core.