The South Pacific Ocean's Kermadec Islands region experienced a strong earthquake in May 1997. A second large earthquake struck the same site in September 2018, just over 20 years later, with its seismic energy coming from the same area.
Ying Zhou, a geoscientist with the Department of Geosciences in the Virginia Tech College of Science, explained that even though the earthquakes occurred two decades apart, they would be expected to send seismic waves through the Earth's layers at the same speed because they occurred in the same region.
However, Zhou discovered an abnormality between the twin occurrences in data collected at four of the more than 150 Global Seismographic Network stations that capture seismic vibrations in real time: SKS waves, which were produced after the 2018 earthquake, moved around one second more quickly than their counterparts from 1997.
The one-second difference in SKS wave travel time, according to Zhou, who just published his findings in Communications Earth & Environment, gives us a vital and unheard-of sight of what's going further inside the Earth, in its outer core.
What's inside counts:
The inner core, the planet's deepest interior layer, and the mantle, the thick layer of rock beneath the surface of the Earth, are sandwiched between the outer core. It is mostly made of liquid iron that flows or convects as the Earth cools. The resultant spinning of the liquid metal creates electrical currents that create the Earth's magnetic field, shielding the planet and all life within it from dangerous solar winds and radiation.
The Earth couldn't support life without its magnetic field, and the magnetic field wouldn't function without the flowing liquid metal in the outer core. Simulations, however, are the foundation of science's knowledge of this dynamic, according to assistant professor Zhou. We only understand in principle that if there is convection.
Scientists also have only been able to speculate about the source of gradual changes in strength and direction of the magnetic field that have been observed, which likely involves changing flows in the outer core.
The north geomagnetic pole, according to Zhou, is currently travelling at a speed of around 50 kilometres (31 miles) per year. "It is heading toward Siberia and away from Canada. Every day the magnetic field is different. It is altering. We also hypothesise that convection in the outer core is altering over time because it is changing, although there is no concrete evidence for this. Never have we seen it."
Zhou set out to locate that proof. She claimed that while the changes occurring in the outer core are not dramatic, they are nonetheless important to confirm and fully comprehend. Zhou recognised a method for "direct sampling" of the outer core in seismic waves and changes in speed on a ten-year time frame. the SKS waves she investigated pass as a result.
What's next:
Zhou's next endeavour is that. Her team intends to examine continuous seismic recordings from two seismic stations, one of which will act as a "virtual" earthquake source, using a wave measurement technique called interferometry, she said.
We can use earthquakes, but using earthquake data has its drawbacks because we can't truly control where the earthquakes happen, according to Zhou. "But we have some control over where seismic stations are placed. The stations can be placed wherever we want them to be, with the outer core serving as the wave channel between them. Core-penetrating seismic waves between the two sites can be observed over time if we keep an eye on that.
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