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Earth’s Inner Core Off-Centre By Tens of Kilometres, Say Geophysicists

Seismic waves seem to travel faster through the eastern half of the Earth’s inner core than the western half. Now two geophysicists think they know why

The Earth’s inner core is a solid ball some 2400 kilometres across made almost entirely of iron. It is surrounded by a liquid outer core made largely of iron and nickel.

In recent years, geophysicists studying the way that seismic waves bounce off and pass through the inner core have noticed something odd. Seismic waves seem to travel more rapidly through the eastern hemisphere than through the western.

That’s hard to explain. Geophysicists have had a crack at an answer by assuming that the eastern and western halves must have a subtly different composition. They think that perhaps the rate of growth of iron crystals is different in each hemisphere, leading to a difference in density and therefore in wave propagation speed.

That might be possible if the east and western hemispheres were somehow at different temperatures. But that raises another problem in explaining how such a temperature difference could be maintained.

So they have another epicycle in the theory. Perhaps the inner core rotates at a different speed to the outer core and mantle, allowing a temperature differential to be maintained.

There is even some evidence that the inner core super-rotates, although exactly how fast is much disputed. Whether this process could maintain the required temperature difference is a moot point.

Today, Calin Vamos at the “T. Popoviciu” Institute of Numerical Analysis in Romania and Nicolae Suciu at the Friedrich-Alexander University of Erlangen-Nuremberg in Germany put forward another idea.

Their hypothesis is that there is no difference in seismic wave speed in the eastern and western hemispheres. Instead, they say the measurements can be explained if the Earth’s inner core is merely off-centre by a few tens of kilometres, making the eastern boundary closer to the surface than the western boundary.

So instead of travelling faster through the eastern hemisphere, seismic waves simply have less distance to go. Vamos and Suciu have carried out a numerical analysis of the timings from a de-centred core to show that they fit the data.

That’s a much simpler explanation than the existing ones so it deserves consideration.

However, an off-centre core is likely to have other consequences for the mechanical, thermal and magnetic properties of the core. These will need to be properly characterised and compared against known measurements. Until then, this remains an exciting hypothesis but little more.

Ref: arxiv.org/abs/1111.1121: Seismic Hemispheric Asymmetry Induced By Earth’s Inner Core Decentering

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