A large earthquake shook the Kermadec Islands in 1997. In September of last year, a second big earthquake hit the same location, its waves of seismic energy coming from the same area.
Because the earthquakes occurred in the same region, they would be expected to send waves through the earth's layers at the same speed.
A set of seismic waves known as the SKS waves traveled about one second faster than their counterparts, according to Zhou, who analyzed the data at four of the Global Seismographic Network stations.
According to Zhou, whose findings were recently published in Communications Earth & Environment, that one-second discrepancy in wave travel time gives us an important and unprecedented glimpse of what's happening deeper in the Earth's interior.
What is inside?
The outer core is sandwiched between the mantle and the deep inside of the planet. As the Earth cools, the liquid iron undergoes fluid flow. The swirling of liquid metal creates electrical currents that protect the planet from harmful radiation and solar winds.
Without the magnetic field, the Earth could not sustain life and the magnetic field wouldn't work. Zhou said that the understanding of this dynamic is based on simulations. She said that the magnetic field could be generated if the outer core has convection.
Scientists have only been able to theorize about the source of gradual changes in strength and direction of the magnetic field that have been observed.
The north geomagnetic pole is moving at a speed of about 50 kilometers per year. It is moving away from Canada. The field is not always the same. The thing is changing. We think that the outer core is changing, but there's no proof. We haven't seen it yet.
Zhou wanted to find that proof. She said that the changes happening in the outer core are worth knowing. Zhou was able to see a way for "direct sampling" of the outer core from the waves that hit. She studied the waves and they passed through it.
First it goes through the mantle as an S wave, then into the outer core as a compressional wave, and finally back out through the mantle as an S wave. The density of the outer core is what determines how fast these waves travel. The wave will travel quicker if the density is lower in a region of the outer core.
Zhou said that something had changed along the path of the wave.
Zhou thinks the difference in wave speed points to the low density regions that formed in the outer core after the 1997 earthquake. She said that the release of light elements such as hydrogen, carbon, and oxygen in the outer core can be attributed to the cooling of the earth.
Zhou said that the material from 20 years ago isn't there anymore. The material is new and lighter. The density in the region where these light elements are located will be changed by their movement upward.
Scientists theorize that movement is happening in the core and that it is changing over time. She said they are able to see it now. If we can see it from the waves, we could set up stations to watch it.
What are we going to do next?
Zhou's next attempt is that. She said that her team will use a method of wave measurement called interferometry to analyze the recordings from the two stations.
The limitation of relying on earthquake data is that we can't really control the location of the earthquakes. We have the ability to control the location of the stations. The stations can be placed anywhere we want them to be, with the wave path from one station to the other station going through the outer core. We can see how the waves between those two stations change over time. We will be able to see the movement of fluid with time.
More information: Ying Zhou, Transient variation in seismic wave speed points to fast fluid movement in the Earth's outer core, Communications Earth & Environment (2022). DOI: 10.1038/s43247-022-00432-7
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