By detecting special seismic tremors in the aftermath of a massive Indonesian earthquake, the seismologists from Northwestern University and the French Atomic Energy Commission have shown without doubt that at the heart of the Earth is a solid iron-nickel ball, 2400km in diameter.

Destructive earthquakes can yield scientific benefits

Emile Okal and his French colleague, Yves Cansi, used an eight-station French seismic network to study the earthquake occurring on the other side of the world.

"The 1996 Flores Sea earthquake, which was a big earthquake at about 600km depth, was perfect in geometry for recording in France," said Okal. It was a rare opportunity as massive, deep earthquakes are needed to probe the necessary 5000km below the surface of the Earth but only happen infrequently.

Breakthrough applauded

Experts hailed the research, announced at the American Geophysical Union meeting in San Francisco, as a breakthrough.

Professor Kathy Whaler, a geophysicist at Edinburgh University told BBC News Online, "No-one has unambiguously taken these waves from the inner core before. And there is a group in Utrecht who are getting the same result but using a different analysis of a different earthquake, 1994 in Bolivia - that gives us confidence."

As long ago as the 1930s, scientists predicted that despite a temperature of thousands of degrees, the crushing pressure at the Earth's centre would cause the iron-nickel alloy to freeze. But because the telltale signals are so weak they have taken 60 years to detect.

Capturing faint seismic tremors dispelled doubt

The key to the breakthrough is the behaviour of the two types of seismic waves. Pulse waves can travel through both liquids and solids as they move by compressing and then relaxing the material in the direction of travel.

Shear waves, in contrast, can only pass through solids. They vibrate at right angles to the direction of travel and as liquids have no material strength the signal rapidly dissipates in the fluid.

So detecting a shear wave coming all the way from the inner core would prove it was solid. But the scientists' task was made even more difficult by the fact that the liquid outer core surrounding the inner core blanks out all shear waves.

Wave energy converted

So they then had to exploit the different speeds at which the waves travel. Imagine a seismic wave rumbling down through the Earth. When it reaches the outer core, all shear waves are lost and only pulse waves continue. When the pulse waves reach the inner core the waves are partly refracted and reflected.

This allows part of the energy to convert into shear waves that then travel through the inner core. Shear waves travel more slowly than pulse waves so they reach the opposite side of the inner core later. Here they are partly converted back to pulse waves.

It was these delayed pulse waves that the scientists detected. The reason it has taken 60 years to do so is because the conversions from pulse to shear to pulse sap the energy of the waves so the signals are exceedingly weak.

Improvements in instrumentation over the last 15 years were crucial to the new finding, Okal said, as were computer capabilities, developed in France, to sort the signals from the noise.