USC scientists have found evidence that the Earth's inner core is not rotating as fast as previously thought.
According to the analysis of seismic data, the inner core changed direction over the course of six years. The scientists theorize that the variation in the length of day is due to a model of inner core movement.
John E. Vidale is the Dean's Professor of Earth Sciences at USC and the co-author of the study. The inner core spun slower from 1969 to 1971 and then moved the other way from 1971 to 1974. The length of day was predicted to grow and shrink.
The likely interpretation is that the two observations are related.
The rotation rate is determined by the analysis of atomic tests.
In the past 30 years, our understanding of the inner core has grown. Over the years, the inner core has been shown to move and change. It's not possible to observe directly, meaning researchers have a hard time explaining the pattern, speed and cause of the movements.
The inner core rotates at a faster rate than the rest of the planet, according to research published in 1996. The idea that the inner core rotates at a slower rate was reinforced.
The inner core rotation was found to be 0.1 degrees slower than previously thought. The study used a novel beamforming technique to analyze the waves generated from the nuclear bomb tests.
Wang and Vidale applied the same methodology to a pair of earlier atomic tests beneath Amchitka Island at the tip of the Alaskan archipelago. They found that the inner core had reversed direction and was rotating at least a tenth of a degree per year.
It was the first time that the six-year oscillation had been indicated through seismological observation.
The community has been split on the viability of the inner core model. We expected to see the same rotation direction and rate in the earlier atomic tests, but we didn't see it. It was moving in the other direction.
Future research to find out why the core formed.
Future research depends on finding precise observations to compare against the results. Wang says that they have been able to locate the exact location and time of the earthquake by using seismological data from previous studies. The era of U.S. underground atomic testing ended in 1978 and the Montana LASA closed, meaning that the researchers would need to rely on relatively accurate earthquake data.
The theory that the inner core changes based on the length of day is supported by the study. The findings provide a compelling theory for a lot of questions.
The inner core is moving under our feet, and it seems to go back and forth a few times a year. Is the inner core locked compared to everything else in the long term or is it moving in a gradual way? It's important that we understand how the inner core formed and how it moves over time.
More information: Wei Wang et al, Seismological observation of Earth's oscillating inner core, Science Advances (2022). DOI: 10.1126/sciadv.abm9916. www.science.org/doi/10.1126/sciadv.abm9916 Journal information: Science Advances
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