The Earth’s inner core, a solid mass of mostly iron at the center of the planet, has long been known to change over time. However, the exact nature and rate of this change has been a subject of scientific debate. A new study by researchers at the Chinese Academy of Sciences and the University of Southern California has provided compelling evidence that the inner core gradually rotated eastward relative to the Earth’s surface from 2003 to 2008, then reversed direction and rotated back westward from 2008 to 2023 at a slower rate. This discovery was made possible by analyzing subtle changes in the waveforms of seismic waves that travel through the inner core.
Background on the Inner Core
The inner core is a region of immense heat and pressure, with temperatures estimated to be as high as 9,000 to 13,000 degrees Fahrenheit. Despite these extreme conditions, the inner core is solid due to the enormous pressure exerted by the overlying material. The inner core is primarily composed of iron, with smaller amounts of nickel and other elements. It has a radius of about 800 miles and is located about 3,200 miles beneath the Earth’s surface.
The inner core plays a crucial role in the Earth’s magnetic field. As the molten outer core moves and convects, it generates electric currents that produce the magnetic field. The solid inner core interacts with this field and may influence the long-term evolution of the field. Additionally, the inner core’s structure and dynamics can provide clues about the early history and formation of the Earth.
Seismic Waves and the Inner Core
Seismic waves are vibrations that travel through the Earth, typically generated by earthquakes or explosions. There are several types of seismic waves, each with distinct properties and paths through the Earth’s interior. One type, called PKIKP waves, travels through the inner core and can be detected at seismic stations on the opposite side of the Earth from the earthquake source.
As PKIKP waves pass through the inner core, they are affected by the material properties and structure of the inner core. Changes in the inner core, such as rotation or variations in crystal alignment, can alter the speed and waveform of the PKIKP waves. By comparing PKIKP waveforms from earthquakes that occur at the same location but at different times, scientists can infer changes in the inner core.
The New Study
The researchers in this study compiled a dataset of 121 earthquakes in the South Sandwich Islands region from 1991 to 2023. These earthquakes were recorded at two seismic arrays in northern North America: the Eielson array in Alaska and the Yellowknife array in Canada. The South Sandwich Islands earthquakes and the North American arrays are nearly antipodal, meaning they are on opposite sides of the Earth. This geometry is ideal for studying PKIKP waves that travel through the inner core.
From the 121 earthquakes, the researchers identified 143 pairs of repeating earthquakes. Repeating earthquakes are events that occur at the same location and have very similar seismic waveforms, indicating they likely have the same source mechanism. By comparing the PKIKP waveforms for each pair of repeating earthquakes, the researchers could isolate changes in the inner core from other factors that might affect the waveforms.
The key finding was that for some repeating earthquake multiplets, the PKIKP waveform changed between the first and second event, then changed back to match the first event for the third earthquake in the multiplet. This pattern of changing and then reverting waveforms suggests that the inner core rotated to a new position between the first and second earthquakes, then rotated back to its original position for the third event.
Evidence for Inner Core Reversal
By examining the time intervals between repeating earthquake pairs with similar PKIKP waveforms, the researchers constructed a model of the inner core’s rotation. They found that pairs with long time intervals between events tended to have their midpoints around 2008, with the first event occurring earlier for pairs that extended back in time and the second event occurring later for pairs that extended forward in time.
This pattern is consistent with an inner core that gradually rotated eastward from 2003 to 2008, reached a maximum rotation around 2008, then reversed direction and rotated back westward from 2008 to 2023. Importantly, the westward rotation occurred at a slower rate, about 2.5 times slower, than the earlier eastward rotation. The researchers estimated that the reversal in rotation direction occurred around 2008.5, give or take about 0.2 years.
Implications and Future Work
This study provides the clearest evidence yet for oscillatory motion of the Earth’s inner core. Previous work had suggested the inner core rotates steadily eastward, but this new finding indicates the rotation is more complex, with reversals in direction and changes in rate. The different rates of eastward and westward rotation are a key result that will need to be explained by future models of inner core dynamics.
The reversal in inner core rotation may be related to changes in the magnetic field generated in the outer core or gravitational interactions between the inner core, outer core, and mantle. However, more work is needed to test these hypotheses and develop a comprehensive model of inner core rotation and its coupling with other parts of the Earth system.
The methods developed in this study, particularly the use of repeating earthquake pairs to isolate inner core signals, provide a new tool for monitoring inner core rotation. As more repeating earthquakes are detected and analyzed, it may be possible to construct a continuous record of inner core motion over the past few decades. This could provide insights into the forces that drive inner core rotation and its variability over time.
Beyond the inner core, this study highlights the power of seismology to probe the deep interior of the Earth. Seismic waves are one of the few tools available to directly sample the material properties and dynamics of the Earth’s core and mantle. As seismic instrumentation and analysis techniques continue to improve, we can expect new discoveries about the structure and evolution of our planet’s interior.
Summary
The Earth’s inner core, though remote and inaccessible, holds clues to the history and dynamics of our planet. This new study has revealed that the inner core undergoes oscillatory rotation, with reversals in direction and changes in rate over decades. This finding challenges previous models of steady inner core rotation and opens new questions about the forces that drive this motion.
As we continue to probe the deep Earth with seismic waves, we can expect more surprises and insights into the complex processes that shape our planet’s interior. The inner core, with its extreme conditions and dynamic behavior, will undoubtedly remain a frontier of Earth science research for years to come.
