James Stewart

Data from a series of earthquakes that took place in close proximity at the South Sandwich Islands and also, crucially, a seismic station that had reliably monitored them for 32 years at College, Alaska.

By comparing the readings, they found that from 1967 to 1995, these similar waves were taking a faster and faster path through the Earth's core.

They had their answer.

Earth's core must have rotated at a different speed to the rest of the planet.

It was the first observational evidence of inner core rotation, and they calculated that it was spinning faster than its surroundings.

And now, with the technique proven, well, the floodgates opened.

In the last couple of decades, some studies have been published that agree the core rotates faster, while others have concluded that it spins more slowly.

Further studies have suggested that it fluctuates, sometimes spinning faster and other times slower, maybe as frequently as every couple of years.

So what's actually going on?

Who's right?

In 2023, scientists at Peking University in Beijing published a new research paper, arguing that inner core rotation has nearly paused and, relative to the mantle, has actually been rotating slightly backwards over the past decade.

They stated that such a change of rotation also occurred in the early 70s,

ultimately concluding that the Earth's inner core oscillates with a period of about 70 years.

The following year, another study was produced, this time by the University of Southern California, which approached the same problem from a subtly different angle.

Rather than looking for differences in waves from the same earthquakes, it looked for matching waveforms from different times, reasoning that if previous studies were correct and the inner core did go through a cycle of faster and slower rotation, then there would be times where, relative to the mantle, the inner core was back in the same position.

I think of this like a car and a bus driving in the same direction side by side.

If the car alternates between travelling slower and faster than the bus, then there'll be times when it's ahead of the bus, there'll be times when it's behind, and times when it's back aligned with the bus again.

So the study examined P waves produced from 121 earthquakes that took place between 1991 and 2023 in the South Sandwich Islands.

The waves passed through the Earth's inner core and were detected by seismometers in Alaska and Canada.

Using this methodology, they confirmed the findings of the previous year, that the changes in the inner core's rotation speed follow a 70-year cycle and, more than that, according to their calculations, it's just about due to speed up again.