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地震产生的地震波揭示了地球外核的变化

一组地震波传播时间的一秒变化让我们对地球深处正在发生的事情有了前所未有的重要了解。

该理论支持我们理解地球外核的对流及其在控制地球磁场中的作用。 科学家们还没有直接观察到对流通量或它们可能如何变化。 弗吉尼亚理工大学地质学家 Ying Zhou 首次提供了证据。

1997 年 5 月,南太平洋的克马德克群岛地区发生大地震。仅仅 20 多年后,即 2018 年 9 月,第二次大地震袭击了同一地点,其地震能量波来自同一地区。

弗吉尼亚理工大学地球科学系的地质学家 Ying Zhou 说,尽管 20 年的地震活动将地震分开,但由于它们发生在同一地区,预计它们会以相同的速度通过地球层发送地震波。 科学。

地震波穿透地核

蓝色路径显示了穿过核心区域的穿透核心的地震波,由于低强度通量传输到该区域,地震速度增加。 图片来源:弗吉尼亚理工大学的 Ying Zhou

然而,在 150 多个实时记录地震振动的全球地震台站中的四个记录的数据中,周发现这两个事件之间存在令人惊讶的异常。 在 2018 年地震期间,一组称为 SKS 波的地震波比 1997 年的地震波移动速度快约一秒。

据周说,他的研究结果最近发表在 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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