Odbijające się fale sejsmiczne ujawniają tajemnice wewnętrznego jądra planety
Trzęsienie ziemi na Alasce powoduje, że fale sejsmiczne penetrują najgłębsze wewnętrzne jądro Ziemi. Źródło: Drew Whitehouse, Son Phạm i Hrvoje Tkalčic.
Według sejsmologów z Australian National University (ANU), dane zebrane z fal sejsmicznych wywołanych przez trzęsienia ziemi rzuciły nowe światło na najgłębsze części wewnętrznego jądra Ziemi.
Mierząc różne prędkości, z jakimi te fale penetrują i przechodzą przez wewnętrzne jądro Ziemi, naukowcy są przekonani, że udokumentowali istnienie odrębnej warstwy wewnątrz Ziemi, znanej jako najbardziej wewnętrzne jądro – litej „metalowej kuli”, która znajduje się w środku rdzenia wewnętrznego.
Nie tak dawno temu sądzono, że struktura Ziemi składa się z czterech odrębnych warstw: skorupy, płaszcza, jądra zewnętrznego i jądra wewnętrznego. Odkrycia, opublikowane w
“This inner core is like a time capsule of Earth’s evolutionary history – it’s a fossilized record that serves as a gateway into the events of our planet’s past. Events that happened on Earth hundreds of millions to billions of years ago,” he said.
The researchers analyzed seismic waves that travel directly through the Earth’s center and “spit out” at the opposite side of the globe to where the earthquake was triggered, also known as the antipode. The waves then travel back to the source of the quake.
The ANU scientists describe this process as similar to a ping pong ball bouncing back and forth.
“By developing a technique to boost the signals recorded by densely populated seismograph networks, we observed, for the first time, seismic waves that bounce back and forth up to five times along the Earth’s diameter. Previous studies have documented only a single antipodal bounce,” Dr. Phạm said.
“The findings are exciting because they provide a new way to probe the Earth’s inner core and its centremost region.”
One of the earthquakes the scientists studied originated in Alaska. The seismic waves triggered by this quake “bounced off” somewhere in the south Atlantic, before traveling back to Alaska.
The researchers studied the anisotropy of the iron-nickel alloy that comprises the inside of the Earth’s inner core. Anisotropy is used to describe how seismic waves speed up or slow down through the material of the Earth’s inner core depending on the direction in which they travel. It could be caused by different arrangement of iron atoms at high temperatures and pressures or the preferred alignment of growing crystals.
They found the bouncing seismic waves repeatedly probed spots near the Earth’s center from different angles. By analyzing the variation of travel times of seismic waves for different earthquakes, the scientists infer the crystallized structure within the inner core’s innermost region is likely different to the outer layer.
They say it might explain why the waves speed up or slow down depending on their angle of entry as they penetrate the innermost inner core.
According to the ANU team, the findings suggest there could have been a major global event at some point during Earth’s evolutionary timeline that led to a “significant” change in the crystal structure or texture of the Earth’s inner core.
“There are still many unanswered questions about the Earth’s innermost inner core, which could hold the secrets to piecing together the mystery of our planet’s formation,” Professor Tkalčić said.
The researchers analyzed data from about 200 magnitude-6 and above earthquakes from the last decade.
Reference: “Up-to-fivefold reverberating waves through the Earth’s center and distinctly anisotropic innermost inner core” by Thanh-Son Phạm and Hrvoje Tkalčić, 21 February 2023, Nature Communications.
DOI: 10.1038/s41467-023-36074-2
