USC scientists have found evidence that the Earth's inner core oscillates,
contradicting previously accepted models that suggested it consistently
rotates at a faster rate than the planet's surface.
Their study, published today in Science Advances, shows that the inner core
changed direction in the six-year period from 1969-74, according to the
analysis of seismic data. The scientists say their model of inner core
movement also explains the variation in the length of day, which has been
shown to oscillate persistently for the past several decades.
"From our findings, we can see the Earth's surface shifts compared to its
inner core, as people have asserted for 20 years," said John E. Vidale,
co-author of the study and Dean's Professor of Earth Sciences at USC
Dornsife College of Letters, Arts and Sciences. "However, our latest
observations show that 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."
Analysis of atomic tests pinpoints rotation rate and direction
Our understanding of the inner core has expanded dramatically in the past 30
years. The inner core—a hot, dense ball of solid iron the size of Pluto—has
been shown to move and/or change over decades. It's also impossible to
observe directly, meaning researchers struggle through indirect measurements
to explain the pattern, speed and cause of the movement and changes.
Research published in 1996 was the first to propose the inner core rotates
faster than the rest of the planet—also known as super-rotation—at roughly 1
degree per year. Subsequent findings from Vidale reinforced the idea that
the inner core super-rotates, albeit at a slower rate.
Utilizing data from the Large Aperture Seismic Array (LASA), a U.S. Air
Force facility in Montana, researcher Wei Wang and Vidale found the inner
core rotated slower than previously predicted, approximately 0.1 degrees per
year. The study analyzed waves generated from Soviet underground nuclear
bomb tests from 1971-74 in the Arctic archipelago Novaya Zemlya using a
novel beamforming technique developed by Vidale.
The new findings emerged when Wang and Vidale 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.
This latest study marked the first time the well-known six-year oscillation
had been indicated through direct seismological observation.
"The idea the inner core oscillates was a model that was out there, but the
community has been split on whether it was viable," Vidale says. "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."
Future research to dig deeper into why inner core formed
Vidale and Wang both noted future research would depend on finding
sufficiently precise observations to compare against these results. By using
seismological data from atomic tests in previous studies, they have been
able to pinpoint the exact location and time of the very simple seismic
event, says Wang. However, the Montana LASA closed in 1978 and the era of
U.S. underground atomic testing is over, meaning that the researchers would
need to rely on comparatively imprecise earthquake data, even with recent
advances in instrumentation.
The study does support the speculation that 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. Vidale says the findings provide a compelling theory
for many questions posed by the research community.
"The inner core is not fixed—it's moving under our feet, and it seems to
going back and forth a couple of kilometers every six years," Vidale 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? We're trying to understand how the inner core formed and how it
moves over time—this is an important step in better understanding this
process."
Reference:
Wei Wang et al, Seismological observation of Earth's oscillating inner core,
Science Advances (2022).
DOI: 10.1126/sciadv.abm9916