Wednesday, August 10, 2022

Understanding changes in the Earth’s outer core with the help of seismic waves

The composition of the bulk, Earth’s outer core, is not pure iron, but has a density of about 5–10%, indicating the existence of considerable amounts of light elements in the outer core, possibly including hydrogen, carbon, nitrogen, oxygen, sulfur. and silicone. These are observations of the speed of seismic waves and the density of Earth’s outer core.

As Earth cools, the crystallization of liquid iron releases lighter elements and the solid inner core grows. The time scale of the iron solidification process and the associated convection (geodynamo) are unclear.

If the released light elements are locally focused, they can have a detectable effect on local seismic waves. Seismic waves generated by large earthquakes provide a direct sampling of the Earth’s outer core. The speed at which a seismic wave propagates through the outer core can be used to prevent lateral asymmetries in the outer core.

Two Earthquakes, Same Area, And Surprising Differences

In May 1997, a major earthquake shook the Kermadec Island region in the South Pacific Ocean. Almost 20 years later, in September 2018, a second major earthquake struck at the same location, waves of seismic energy emanating from the same area.

Although the earthquakes were two decades apart because they occurred in the same region, they would be expected to send seismic waves at the same speed through Earth’s crust, said Ying Zhou, a geophysicist in the Department of Geosciences at Virginia Tech College. Told. science.

But in data recorded at four of the more than 150 Global Seismographic Network stations that log seismic vibrations in real time, Zhou found a discrepancy between twin events: during the 2018 earthquake, known as SKS waves. A set of outgoing seismic waves traveled for about a second. Faster than its counterparts in 1997.

According to Zhou, whose findings were recently published in Nature Communications Earth and Environment, that one-second discrepancy in SKS wave travel time gives us an important and unprecedented view of what is happening in Earth’s interior, its outer core. Gives a glimpse

The blue lines are seismic rays in the outer core, where core-penetrating seismic waves moved faster in 2018 than in 1997. Image courtesy of Ying Zhou.

What matters most is the internal structure

The outer core is sandwiched between the mantle, the thick layer of rock beneath Earth’s crust, and the inner core, the planet’s deepest inner layer. It is composed primarily of liquid iron that undergoes convection, or fluid flow, as the Earth cools. The resulting swirl of liquid metal generates electric currents responsible for generating Earth’s magnetic field, which protects the planet and all life on it from harmful radiation and solar winds.

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Without its magnetic field, Earth could not sustain life, and without the dynamic flow of liquid metal in the outer core, the magnetic field would not work. But scientific understanding of this dynamic is based on simulations, said Zhou, an associate professor. “We only know that in theory, if you had convection in the outer core, you would be able to generate magnetic fields,” she said.

Scientists are also only able to speculate about the source of the gradual changes in the strength and direction of the observed magnetic field, with the possibility of changing flux in the outer core.

“If you look at the North Geomagnetic Pole, it’s currently moving at about 50 kilometers [31 miles] per year,” Zhou said. “It’s moving away from Canada toward Siberia. The magnetic field isn’t the same every day. It’s changing. Since it’s changing, we also predict that in the outer core.” Convection is changing over time, but there’s no direct evidence of that. We’ve never seen it.”

Zhou set out to find that evidence. The changes taking place in the outer core are not dramatic, she said, but they are worth confirming and fundamentally understanding. In seismic waves and their changes in motion on a decade time scale, Zhou saw a means for “direct sampling” of the outer core. This is because the SKS waves he studied pass through exactly that.

The “SKS” represents the three phases of the wave: first it travels through the circle as an S wave, or shear wave; then as a compression wave in the outer core; Then exit back through the mantle as an S wave. How fast these waves travel depends on the density of the outer core in their path. If a region of the outer core has a lower density as the wave enters it, the wave will travel faster, as did the anomalous SKS waves in 2018.

“Something has changed in the path of that wave, so it may be fast moving now,” Zhou said.

For Zhou, the difference in wave speed points to areas of low density in the outer core in the 20 years following the 1997 earthquake. The high SKS wave speed during the 2018 earthquake can be attributed to the release of lighter elements such as hydrogen, carbon and oxygen in the outer core during convection, which occurs as Earth cools, she said.

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

For Zhou, this is evidence that the movement is indeed occurring in origin, and that it is changing over time, as scientists have theorized. “We can see it now,” she said. “If we can see it with seismic waves in the future, we can set up seismic stations and monitor that flow.”

what will happen next

Using a method of wave measurement known as interferometry, his team plans to analyze continuous seismic recordings from two seismic stations, one of which will serve as a “virtual” earthquake source, he said. Told.

“We can use earthquakes, but the limitation of relying on earthquake data is that we can’t really control the locations of earthquakes,” Zhou said. “But we can control the locations of the seismic stations. We can place the stations wherever they want, the wave path from one station to the other passes through the outer core. If we monitor this over time, we can see how the core-penetrating seismic waves change between those two stations. This will allow us to better see the movement of the fluid in the outer core over time.”

Futuristic Approach:

Observations in this study suggest that the outer core is not well mixed over a decade time scale, and that lateral asymmetries associated with outer core convection are strong enough to produce seismic wave changes detectable in earthquake recordings. Huh. This opens up the possibility of monitoring temporal changes in the outer core using seismic data.

journal reference

  1. Zhou, Y. Transient variations in seismic wave speed point to rapid fluid motion in the Earth’s outer core. community earth environment 3, 97 (2022). DOI: 10.1038/s43247-022-00432-7
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