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The Earth is moving more than 3,000 miles below our feet

A researcher said: "One of the questions we tried to answer is, does the inner core progressively move or is it mostly locked compared to everything else in the long term?"

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Image of earth planet. Elements of this image are furnished by NASA
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By Mark Waghorn via SWNS

The Earth is moving...more than 3,000 miles under our feet, according to new research.

Its inner core goes back and forth over a mile - every six years.

The cycle explains variations in the length of days - which have oscillated for decades.

Lead author Professor John Vidale, of the University of Southern California, said: "From our findings, we can see the Earth's surface shifts compared to its inner core, as people have asserted for 20 years."

The huge iron ball is the size of Pluto - and as hot as our sun. The US team used seismic data from 1969 to 1974 to create a computer model of it.

Simulations contradicted previous theories suggesting consistent rotation faster than at the planet's surface.

Prof Vidale said: "However, our latest observations show the inner core spun slightly slower from 1969-71 and then moved the other direction from 1971-74.

"We also note that the length of day grew and shrank as would be predicted. The coincidence of those two observations makes oscillation the likely interpretation."

When Jules Verne wrote "A Journey to the Center of the Earth" over 150 years ago he imagined a land of glowing crystals, turbulent seas, prehistoric animals and giant mushrooms.

What was actually beneath our feet was a complete mystery. Even today we know more about the rings of Saturn than the interior of our planet.

But that is beginning to change thanks to a golden age of discovery. Our understanding has expanded dramatically in the past 30 years.

It has been shown to move and change over decades. It is impossible to observe directly.

Seismologists rely on indirect measurements to explain pattern, speed and cause.

Utilizing data from the LASA (Large Aperture Seismic Array), a US Air Force facility in Montana, Vidale found the inner core rotated slower than previously predicted - approximately 0.1 degrees per year.

Lab staff developed a novel beam-forming technique to analyze waves generated from Soviet underground nuclear bomb tests from 1971 to 1974 in the Arctic archipelago Novaya Zemlya.

The latest results emerged when they applied the same methodology to a pair of earlier atomic tests beneath Amchitka Island at the tip of the Alaskan archipelago - Milrow in 1969 and Cannikin in 1971.

Measuring the compressional waves resulting from the nuclear explosions, they discovered the inner core had reversed direction, sub-rotating at least a tenth of a degree per year.

It marked the first time the well-known six-year oscillation had been indicated through direct seismological observation.

Vidale said: "The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable.

"We went into this expecting to see the same rotation direction and rate in the earlier pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving in the other direction."

By using seismological data from atomic tests in previous studies, the researchers have been able to pinpoint the exact location and time of the very simple seismic event.

LASA closed in 1978 and the era of US underground atomic testing is over, meaning they would need to rely on comparatively imprecise earthquake data, even with recent advances in instrumentation.

The study supports speculation the inner core oscillates based on variations in the length of day - plus or minus 0.2 seconds over six years - and geomagnetic fields, both of which match the theory in both amplitude and phase.

It provides a compelling theory for many questions posed by the research community.

Added Vidale: "The inner core is not fixed - it is moving under our feet, and it seems to going back and forth a couple of kilometers (1.25 miles) every six years.

"One of the questions we tried to answer is, does the inner core progressively move or is it mostly locked compared to everything else in the long term?

"We are trying to understand how the inner core formed and how it moves over time — this is an important step in better understanding this process."

The study is in Science Advances.

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