Zemestrīču seismiskos viļņus atklāja izmaiņas Zemes ārējā kodolā

Seismisko viļņu grupas ceļošanas laika atšķirība vienas sekundes laikā sniedz mums svarīgu un nepieredzētu ieskatu tajā, kas notiek dziļi Zemes iekšienē.

Teorija atbalsta mūsu izpratni par konvekciju Zemes ārējā kodolā un tās funkciju planētas magnētiskā lauka kontrolē. Zinātnieki nav tieši novērojuši konvekcijas plūsmas vai to, kā tās varētu mainīties. Virdžīnijas tehnoloģiju ģeologs Ying Zhou pirmo reizi sniedz pierādījumus.

Spēcīga zemestrīce satricināja Klusā okeāna dienvidu daļas Kermadekas salu reģionu 1997. gada maijā. Nedaudz vairāk nekā 20 gadus vēlāk, 2018. gada septembrī, to pašu vietu skāra otra liela zemestrīce, kuras seismiskās enerģijas viļņi izplūda no tā paša reģiona.

Lai gan zemestrīces šķīra divas desmitgades seismiskuma, jo tās notika vienā reģionā, sagaidāms, ka tās sūtīs seismiskos viļņus cauri Zemes slāņiem ar tādu pašu ātrumu, sacīja Virdžīnijas Tehniskās universitātes Zemes zinātņu nodaļas ģeologs Ying Zhou. Zinātnes.

seismiskais vilnis, kas iekļūst zemes kodolā

Zilajā ceļā ir redzams kodolā iekļūstošs seismiskais vilnis, kas pārvietojas pa reģionu ārējā kodolā, kur seismiskais ātrums palielinājās, jo reģionā tika pārnesta zemas intensitātes plūsma. Kredīts: Ying Zhou no Virginia Tech

Tomēr datos, kas reģistrēti četrās no vairāk nekā 150 globālo seismisko tīklu stacijām, kas reāllaikā reģistrē seismiskās vibrācijas, Džou atklāja pārsteidzošas anomālijas starp dvīņu notikumiem. 2018. gada zemestrīces laikā seismisko viļņu grupa, ko sauc par SKS viļņiem, pārvietojās aptuveni vienu sekundi ātrāk nekā to 1997. gada kolēģi.

Saskaņā ar Džou teikto, kura rezultāti nesen tika publicēti Seismic Rays Earth's Outer Core

Blue lines are seismic rays in the outer core, where core-penetrating seismic waves moved through that region faster in 2018 than in 1997. Credit: Image courtesy of Ying Zhou

Scientists also have only been able to speculate about the source of gradual changes in strength and direction of the magnetic field that have been observed, which likely involves changing flows in the outer core.

“If you look at the north geomagnetic pole, it’s currently moving at a speed of about 50 kilometers (31 miles) per year,” Zhou said. “It’s moving away from Canada and toward Siberia. The magnetic field is not the same every day. It’s changing. Since it’s changing, we also speculate that convection in the outer core is changing with time, but there’s no direct evidence. We’ve never seen it.”

Zhou set out to find that evidence. The changes happening in the outer core aren’t dramatic, she said, but they’re worth confirming and fundamentally understanding. In seismic waves and their changes in speed on a decade time scale, Zhou saw a means for “direct sampling” of the outer core. That’s because the SKS waves she studied pass right through it.

“SKS” represents three phases of the wave: First it goes through the mantle as an S wave, or shear wave; then into the outer core as a compressional wave; then back out through the mantle as an S wave. How fast these waves travel depend in part on the density of the outer core that’s in their path. If the density is lower in a region of the outer core as the wave penetrates it, the wave will travel faster, just as the anomalous SKS waves did in 2018.

“Something has changed along the path of that wave, so it can go faster now,” Zhou said.

Ying Zhou

Ying Zhou of the Virginia Tech Department of Geosciences. Credit: Photo courtesy of Ying Zhou

To Zhou, the difference in wave speed points to low-density regions forming in the outer core in the 20 years since the 1997 earthquake. That higher SKS wave speed during the 2018 earthquake can be attributed to the release of light elements such as hydrogen, carbon, and oxygen in the outer core during convection that takes place as the Earth cools, she said.

“The material that was there 20 years ago is no longer there,” Zhou said. “This is new material, and it’s lighter. These light elements will move upward and change the density in the region where they’re located.”

To Zhou, it’s evidence that movement really is happening in the core, and it’s changing over time, as scientists have theorized. “We’re able to see it now,” she said. “If we’re able to see it from seismic waves, in the future, we could set up seismic stations and monitor that flow.”

What’s next

That’s Zhou’s next effort. Using a method of wave measurement known as interferometry, her team plans to analyze continuous seismic recordings from two seismic stations, one of which will serve as a “virtual” earthquake source, she said.

“We can use earthquakes, but the limitation of relying on earthquake data is that we can’t really control the locations of the earthquakes,” Zhou said. “But we can control the locations of seismic stations. We can put the stations anywhere we want them to be, with the wave path from one station to the other station going through the outer core. If we monitor that over time, then we can see how core-penetrating seismic waves between those two stations change. With that, we will be better able to see the movement of fluid in the outer core with time.”

Reference: “Transient variation in seismic wave speed points to fast fluid movement in the Earth’s outer core” by Ying Zhou, 25 April 2022, Communications Earth & Environment.
DOI: 10.1038/s43247-022-00432-7

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