New research reveals that Earth's inner core, a 1,500-mile-wide sphere of iron and nickel, may not be entirely solid and its outer boundary has changed shape over recent decades. The study, published in Nature Geoscience on February 10, 2025, by researchers at the University of Southern California, analyzed seismic waves from earthquake pairs in the South Sandwich Islands, recorded at seismometer arrays in Alaska and Canada from 1991 to 2004.
The analysis of these earthquake pairs showed that while seismic signals at the Fairbanks, Alaska array remained consistent, those at the Yellowknife, Canada array differed. This discrepancy suggests changes near the inner core's outer boundary, as the Yellowknife array received seismic waves that traveled less deeply into the core.
John Vidale, a professor of earth sciences at USC, suggests that turbulent flow in the outer core or gravitational pull from the mantle could be deforming the inner core boundary. "The most likely thing is the outer core is kind of tugging on the inner core and making it move a little bit," said Vidale.
The findings contribute to ongoing debates about the inner core's rotation rate and shape. Hrvoje Tkalcic, a geophysics professor at the Australian National University, notes that the study reconciles previous arguments by proposing a combination of both rotation rate and shape changes. However, Lianxing Wen, a geosciences professor at Stony Brook University, remains unconvinced about differing rotation rates.
Further research, including enhanced seismological infrastructure in remote areas and the ocean floor, is needed to fully understand these phenomena. Xiaodong Song, a professor at Peking University, emphasizes that this study should spur further exploration into the Earth's core. Vidale estimates a 90% confidence level in the findings, acknowledging the need for more conclusive evidence.