Earth’s core softer than thought

New modelling offers insight into the planet’s deepest reaches. Nick Carne reports.

The Earth, with the inner core at its heart.

The Earth, with the inner core at its heart.

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Earth’s inner core is probably softer than previously thought.

Seismologists from the Australian National University (ANU) have adapted a method used successfully to study the Earth’s crust and upper mantle to provide new evidence of what lies inside.

The idea of there being an inner core – a ball inside a molten outer layer – was proposed more than 80 years ago, when a Danish seismologist analysed anomalous results from seismograms following earthquakes in New Zealand.

The inner core was assumed to be solid, but that has been hard to establish, in large part because the shear waves, known in the jargon as PKJKP, that would provide direct evidence have proven very difficult to detect, even after large earthquakes.

Instead, low-frequency normal mode oscillations of the Earth have been deployed to propose a measure of the core’s solidity. These are incorporated into the Preliminary Reference Earth Model (PREM), a one dimensional model developed in the 1980s and widely used in seismology.

In their recent work, the ANU’s Hrvoje Tkalčić and Thanh-Son Pham cross-correlated recordings of a range of large earthquakes on a planetary scale and identified previously unobserved seismic phases in Earth’s correlation wavefield.

In their paper published in the journal Science, the pair say they were able to detect shear waves in the inner core, but in so doing developed new estimates for properties such as shear-wave speed, resistance to shear and attenuation.

They report shear-wave speeds and shear moduli that are 25% lower than in PREM, suggesting that the inner core is “solid, but relatively soft”.

“This inference represents an advance in our understanding of structure and dynamics of the inner core – Earth’s deepest time capsule that has been probed by the global correlation wavefield,” they write.

In a related commentary, published in the same journal, Jessica Irving from Princeton University in the US says a better understanding of the inner core’s shear wave properties can help ascertain dynamical processes including what, if any, kind of convection is taking place in it and whether it is rotating relative to the mantle.

“The inner core provides an important part of the energy budget for the geodynamo – the mechanism that generates Earth’s magnetic field – as latent heat is re leased and light elements are preferentially segregated into the fluid outer core,” she writes.

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