Seismic wave data revealed changes in the Earth’s outer core

Earthquakes send seismic waves through the Earth’s layers at the same speed. The Kermadec Islands region of the South Pacific was hit by a powerful earthquake in May 1997. A second major earthquake hit the same site in September 2018, just over 20 years later, with its seismic energy coming from the same area.

In data recorded at four of the more than 150 Global Seismic Network stations that record seismic vibrations in real time, a new study finds anomalies between the dual events: During the 2018 earthquake, a group of seismic waves known as SKS waves traveled about one second faster than their counterparts in 1997.

This one-second difference in SKS wave travel time provides a crucial and unprecedented glimpse into what is happening inside and outside the Earth.

The Earth could not sustain life without its magnetic field, and the magnetic field would not function without the moving flows of liquid metal in the outer core. However, the scientific understanding of this dynamic is based on simulation.

“We know that only in theory, if you have convection in the outer core, you will be able to generate the magnetic field,” said Ying Zhou, a geologist in the Department of Earth Sciences at Virginia Tech.

In addition, scientists were only able to guess the reason for the gradually observed differences in the strength and direction of the magnetic field, which most likely entailed a change in the fluxes in the outer core.

Zhou said, “If you look at the north geomagnetic pole, it is currently moving at a speed of about 50 kilometers [31 miles] every year. It is moving away from Canada towards Siberia. The magnetic field is not the same every day. It is changing. As it changes, we also speculate that convection in the outer core changes over time, but there is no direct evidence. We’ve never seen her before.”

Chu set out to find this clue. She said, “The changes taking place in the outer core are not dramatic, but they are fundamentally worth emphasizing and understanding.”

In seismic waves and their changes in velocity on a time scale of a decade, Zhou saw a way to “directly sample” the outer core. This is because the SKS waves you studied pass through them.

SKS represents three phases of the wave:

It passes through the mantle as an S wave or shear wave, to the outer core as a compression wave, to exit through the mantle as an S wave.

The speed of travel of these waves depends in part on the density of the outer core in their path. If the thickness is less in an area of ​​the outer core as the wave penetrates, then the wave will travel faster, just as the anomaly SKS waves did in 2018.

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

The difference in wave speed indicates areas of lower intensity formed in the outer core in the 20 years since the 1997 earthquake. The 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 as the Earth cools.”

“The materials that were there 20 years ago no longer exist. This is a new material and it is lighter. These lighter elements will move up and change the density in the area where they are.”

“It is evidence that motion occurs in the core, and that it changes over time, as the scientists hypothesized. We are able to see it now. If we can see it from seismic waves, in the future, we can set up seismic stations and monitor that flow.”

The scientists also plan to analyze continuous seismic recordings from two seismic stations. One of them will act as a “virtual” source of earthquakes.

“We can use earthquakes, but the limitation of relying on earthquake data is that we can’t control the locations of the earthquakes. But we can control the locations of the earthquake stations. We can put the stations anywhere we want, so that the wave path from one station to the other goes through the core,” Zhou said. If we watch that over time, we can see how the underlying penetrating seismic waves change between these two stations. With that, we’ll be better able to see the movement of fluids in the outer core over time.”

Journal reference:

Zhou, Y. Transient variation in seismic wave velocity points to rapid fluid motion in the Earth’s outer core. Commun Earth Environ 3, 97 (2022). DOI: 10.1038 / s43247-022-00432-7

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