The majority of what we know about our planet's core comes from the study of seismic waves emitted by earthquakes. Analyzing these waves carefully can reveal the nature of rocks and metals under the Earth's surface.
A new study of seismic waves propagating from two independent earthquakes in comparable places but separated by 20 years has shown changes in Earth's outer core, the whirling layer of liquid iron and nickel between the mantle (the rock beneath the surface) and the inner core (the deepest layer).
The outer core and the iron stored inside it have a direct impact on our planet's magnetic field, which protects us from space and solar radiation, which would otherwise make life impossible on Earth.
Understanding the outer core and its change through time is therefore critical. Data from four seismic wave monitors located across both earthquakes revealed that waves from the latter event moved approximately one second quicker while passing through the identical location of the outer core.
"Something has changed along the path of that wave, so it can go faster now," explains Virginia Tech geoscientist Ying Zhou. "The material that was there 20 years ago is no longer there."
"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."
The waves studied here are SKS waves, which move through the mantle as shear waves (the S), then into the outer core as compressional waves (the K), then out the other side as additional shear waves (the second S). The timing of that journey might be telling.
Both earthquakes occurred near the Kermadec Islands in the South Pacific Ocean, the first in May 1997 and the second in September 2018, providing researchers with a rare chance to see how the Earth's core may have evolved over time.
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| How seismic waves travel through the outer core. (Ying Zhou) |
Convection in Earth's outer core's liquid iron as it crystalizes onto the inner core generates flowing electrical currents, which govern the magnetic field around us. The link between the outer core and the Earth's magnetic field, on the other hand, is not entirely known; most of it is based on speculative modeling.
"If you look at the north geomagnetic pole, it's currently moving at a speed of about 50 kilometers [31 miles] per year," Zhou explains. "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."
This current research, and maybe future ones like it, could give helpful insights into how the outer core and its convection are evolving. While the adjustments described here are minor, the more we know, the better.
Zhou claims that lighter elements including hydrogen, carbon, and oxygen have been released in the outer core since 1997. According to the published article, that equates to a density drop of roughly 2-3% and a convection flow speed of about 40 kilometers (25 miles) per hour.
There are presently 152 Global Seismographic Network stations measuring seismic waves in real time across the world. While we cannot control the location or timing of earthquakes, we can ensure that as much data as possible about them is recorded.
"We're able to see it now," Zhou adds. "If we're able to see it from seismic waves, in the future, we could set up seismic stations and monitor that flow."
The research has been published in Nature Communications Earth & Environment.
