One of the mysteries of the Earth's core is the seismic variations that have occurred in recent decades. Scientists believe they have proven changes in the direction of rotation of the inner core, but are still arguing about their periodicity.
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Due to the liquid outer core that surrounds Earth's deepest layer, it can rotate independently of the mantle and crust for about 35 years; the last reversal may take place in the mid-2040s.
Scientists examined seismic vibration data from the 1960s to the 1990s and found that waves (usually caused by earthquakes or nuclear explosions) recorded in the last decade showed relatively little vibrations, which prompted them to think about the nucleus's stopping motion.
Similar findings were discovered in the early 1970s, and the findings were consistent with changes in day length.
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John Vidale, a seismologist at the University of Southern California, is debating whether the inner core changes every 6 years due to the data on nuclear explosions from the late 1960s to the early 1970s. Other theories claim that the inner core shifted between 2001 and 2013, then stopped.
Hrvoe Tkalcic, an Australian National University geophysicist, believes the core changes direction every 20-30 years. He also questions the validity of all of the proposed theories, because seismic data provides limited information about what is happening inside the Earth.
The remaining theories suggest that the inner core might have another core inside.