The Earth’s core is not rotating in sync with the rest of our planet, but instead is sometimes rotating faster and sometimes reversing direction.
That’s according to new research published in the journal on Friday scientific advances by Southern California seismologists John Vidale and Wei Wang.
“The inner core is not fixed,” said Dr. Vidale in a statement. “It moves under our feet and seems to go back and forth a few kilometers every six years.”
This finding could help scientists better understand how Earth’s core creates the geomagnetic field that blocks radiation from space and enables life, and could explain why the length of the day fluctuates over time.
From the inside out, the Earth consists of a solid inner core surrounded by an outer core of liquid iron, a thick rocky mantle, and then the thin crust and atmosphere where life exists.
More than 6,300 kilometers below the surface, the inner core is inaccessible, and geoscientists must infer its properties and behavior through indirect measurements and its effects. For example, you know the spins of the inner core because the solid metal spinning inside the liquid metal outer core is the geodynamo that creates the planet’s magnetic field.
But for decades, scientists have puzzled over data that suggests the inner core could change its rotational speed compared to the surface.
“From our results, we can see the shifts in the Earth’s surface relative to its inner core that people have been claiming for the past 20 years,” said Dr. Vidale in a statement. “However, our most recent observations show that the inner core rotated slightly more slowly from 1969 to 1971, and then moved in the opposite direction from 1971 to 1974.”
From 1968 to 1971, the planet’s inner core rotated westward relative to Earth’s mantle and crust. From 1972 to 1974, the inner core superrotated, spinning eastward faster than the crust and mantle.
dr Vidale added that the change in inner nuclear spin correlates with a millisecond-scale change in day length that researchers independently observed over a six-year period. The day shortens by 0.01 milliseconds during inner core sub-rotation and lengthens by up to 0.12 milliseconds during super-rotation.
Since the inner core is not accessible for direct measurement, Dr. Vidale and Wang captured seismic waves bouncing off the inner core to measure its speed and direction of rotation. In particular, they examined records of seismic wave reflections produced by atomic bomb tests in the former Soviet Union in 1971-1974 and two tests conducted by the US in Alaska in 1969 and 1971.
This historical data was recorded with the Large Aperture Seismic Array in Montana, more than 300 geophones buried 60 meters deep in the ground and used to listen to seismic waves to determine if they were earthquakes or nuclear explosions.
Detonations create waves that are scattered off the massive inner core and reflected back, with waves scattered from part of the core turning away from the receiving geophone arriving later than waves scattered from parts of the core turning on rotate the geophone. This enabled Dr. Wang and Vidale to calculate the direction and speed of rotation of the inner core.
It was a surprising result, said Dr. Vidale, the idea that the inner core could oscillate in its rotation, just one theory among many ahead of their new study.
“The idea of the inner core resonating was a model that was out there, but the community was divided as to whether it was viable,” he said in a statement. “We expected to see the same rotation direction and speed in the previous two nuclear tests, but instead we saw the opposite. We were quite surprised to find it moving the other way.”
More research is needed but could be hampered by the lack of precisely localized seismic waves since the US and Russia stopped testing nuclear weapons in the 1990s. But the study shows that historical data can still shed light on the inner workings of the Earth.
“We’re trying to understand how the inner core formed and how it moves over time,” said Dr. Vidale in a statement. “This is an important step to better understand this process.”