Up-to-fivefold reverberating waves through the Earth's center and distinctly anisotropic innermost inner core

Probing the Earth's center is critical for understanding planetary formation and evolution, but seismological inferences have been limited by a lack of probes sensitive to the Earth's center. By stacking waveforms recorded by a growing number of global seismic stations, we observe up-to-fivefold reverberating waves from selected earthquakes along the Earth's diameter. Differential travel times of these previously unreported arrival pairs complement and improve existing constraints. The inferred transversely isotropic inner-core model includes an ~650-km-thick innermost ball with P-wave speeds ~4% slower at ~50° from the Earth's rotation axis. In contrast, the inner core's outer shell exhibits much weaker anisotropy with the slowest direction in the equatorial plane. These findings strengthen evidence for a distinctly anisotropic innermost inner core (IMIC) and a transition to a weakly anisotropic outer shell, potentially recording a significant global event in Earth's past.