Repeated seismic signals reveal a more dynamic outer core

Scientists have gained a new way to investigate Earth’s inaccessible outer core, finding evidence that its liquid metal is less uniform and more changeable than previously understood. The analysis also identified a previously unrecognised seismic wave, named PKrKP, that reflects within the middle of the outer core rather than travelling through or bouncing off the inner core.

The study examined signals generated by underground nuclear tests conducted at Mururoa in French Polynesia between 1977 and 1995 and recorded at a seismic station in Kazakhstan. Because the explosions occurred at closely spaced locations, their waves followed nearly identical paths through Earth. Comparing 112 pairs therefore provided unusually controlled snapshots of the planet’s interior over time.

Waves crossing the outer core arrived between roughly 0.1 and 0.2 seconds earlier or later in different years. Though small, such shifts are meaningful across journeys of thousands of kilometres and indicate that the waves encountered material with changing properties.

One possible explanation is a large region of solid material suspended and moving within the liquid outer core. The researcher estimated that the anomaly could span more than 700 kilometres and be about 100 kilometres thick beneath the low-latitude South Pacific. Its composition and precise form remain uncertain, but the observations support a picture of vigorous mixing deep inside Earth.