Long theorized to have a solid center, researchers have documented proof that the Earth’s inner core is indeed solid. The evidence lies in the detection of shear, or “J” waves, which are only transmitted through solid The inner core is solid but somewhat squishy. Source: Shad.off/Depositphotosobjects.

Direct detection of these waves has been impossible because of their small amplitudes, prompting Australian National University researchers to apply a correlation wavefield method, previously used to analyze earthquake signals and to measure Antarctic ice thickness, to the problem. The approach zones in on similarities between signals at two receivers after a major earthquake.

A cross-correlation of similarity is generated by analyzing data from receiver pairs to extract the similarity between seismograms, resulting in a global correlogram. The data demonstrated the occurrence of J waves and enable inference of shear wave speed in the inner core.

The surprising finding: the inner core is solid, but relatively soft. Shear-wave speeds and shear moduli of 3.42 ± 0.02 km per second and 149.0 ± 1.6 gigapascals (GPa) near the inner core boundary and 3.58 ± 0.02 km per second and 167.4 ± 1.6 GPa in Earth’s center were observed.