When seismic waves travel at different speeds, a one-second discrepancy can reveal a great deal about what's happening deep inside the Earth.

It was the year 1997 when a large earthquake struck the Kermadec Islands region in the South Pacific Ocean. In September 2018, more than 20 years later, another big earthquake struck the same area, with the same seismic waves.

The earthquakes occurred two decades apart, but because they occurred in the same region, they'd send seismic waves through the Earth's layers at the same speed, according to Virginia Tech geoscientist Ying Zhou.

There is an increased seismic speed in the outer core where a low-density flow has moved into a region where a core-penetrating seismic wave is moving. 

A surprising anomaly was discovered in the data recorded at four of 150 Global Seismographic Network stations. SKS waves traveled approximately one second faster during the 2018 earthquake than their predecessors in 1997.

Zhou's findings published recently in Nature Communications Earth & Environment give us a unique insight into what's happening deep inside the Earth's interior, in its outer core, through the one-second discrepancy in SKS wave travel time.

The outer core of Earth is sandwiched between the mantle, a thick layer of rock beneath the crust, and the inner core, the planet's deepest layer. Its main component is liquid iron, which flows as the Earth cools by convection. Resulting from this swirling, a magnetic field is created on Earth, which protects life and the planet from harmful radiation and solar winds.

A lifeless planet would not be able to sustain its magnetosphere, and a magnetic field without liquid metal flowing inside the outer core would not be functional. An associate professor, Zhou, said that scientists understand this dynamic based on simulations. 'We only know in theory that if convection exists in the outer core, a magnetic field can be generated,' she said.

There are blue lines in the outer core, indicating that core-penetrating seismic waves moved through that region more rapidly in 2018 than in 1997. Additionally, scientists can only speculate about what may be causing the gradual changes in magnetic field strength and direction observed, which can be attributed to changing flows in the outer core.

The North geomagnetic pole is moving at a rate of about 50 kilometers per year, according to Zhou. In terms of its movement, it is moving away from Canada and toward Siberia. It is not the same magnetic field every day.

The North geomagnetic pole is moving at a rate of about 50 kilometers per year, according to Zhou. In terms of its movement, it is moving away from Canada and toward Siberia. It is not the same magnetic field every day.

Zhou's next task is to do that. In order to analyze continuous seismic recordings from two stations, her team will use interferometry, a method used to measure wave motion. One of the seismic stations will serve as a 'virtual' earthquake source.

“When relying on earthquake data, we have the limitation of being unable to control the location of the earthquakes,” Zhou said. However, seismic stations can be located anywhere. We can place the stations wherever we want them, with waves traveling through the outer core between each station.

We can observe over time how those core-penetrating seismic waves change between those two stations if we monitor that. This will allow us to see the fluid movement in the outer core over time.'