By Suzanne Irby
In May 1997, a large earthquake shook the Kermadec Islands region in the South Pacific Ocean. A little over 20 years later, in September 2018, a second big earthquake hit the same location, its waves of seismic energy emanating from the same region.
Though the earthquakes occurred two decades apart, because they occurred in the same region, they’d be expected to send seismic waves through the Earth’s layers at the same speed, said Ying Zhou, a geoscientist with the Department of Geosciences in the Virginia Tech College of Science.
But in data recorded at four of more than 150 Global Seismographic Network stations that log seismic vibrations in real time, Zhou found an anomaly among the twin events: During the 2018 earthquake, a set of seismic waves known as SKS waves traveled about one second faster than their counterparts had in 1997.
According to Zhou, whose findings were recently published in Nature Communications Earth & Environment, that one-second discrepancy in SKS wave travel time gives us an important and unprecedented glimpse of what’s happening deeper in the Earth’s interior, in its outer core.
The outer core is sandwiched between the mantle, the thick layer of rock underneath the Earth’s crust, and the inner core, the planet’s deepest interior layer. It’s composed mainly of liquid iron that undergoes convection, or fluid flow, as the Earth cools. This resulting swirling of liquid metal produces electrical currents responsible for generating the Earth’s magnetic field, which protects the planet and all life on it from harmful radiation and solar winds.
Without its magnetic field, the Earth could not sustain life, and without the moving flows of liquid metal in the outer core, the magnetic field wouldn’t work. But scientific understanding of this dynamic is based on simulations, said Zhou, an associate professor. “We only know that in theory, if you have convection in the outer core, you’ll be able to generate the magnetic field,” she said.
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.
“If you look at the north geomagnetic pole, it’s currently moving at a speed of about 50 kilometers [31 miles] per year,” Zhou said. “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.”
