The Earth’s core is solid, according to a pair of geophysicists who claim to have solved an 80-year-old conundrum concerning the planet's center.
The suggestion that our home world has a liquid outer core containing a smaller solid inner core was put forward by Danish seismologist Inge Lehmann in the 1930s. Scientists have been searching for direct evidence ever since.
Fast forward to today, and boffins say they think they've detected a smoking gun: shear waves, also known as J-waves, rippling through the planet after an earthquake. These waves indicate the presence of a solid core.
J-waves are tiny, and difficult to detect during seismic activity: they are drowned out by much more powerful movements and rumblings within the planet. Now, Hrvoje Tkalčić and Thanh-Son Phạm, an associate professor and a PhD student at the Australian National University, believe they have finally found these tiny waves by studying seismogram recordings of major earthquakes.
Crucially, they ditched the first few hours of measurements after each quake, and started looking when all the signals had died down enough for minuscule J-waves to become apparent. Very clever, huh?
“We're throwing away the first three hours of the seismogram and what we're looking at is between three and 10 hours after a large earthquake happens,” Tkalčic said. "We want to get rid of the big signals.
"Using a global network of stations, we take every single receiver pair and every single large earthquake – that's many combinations – and we measure the similarity between the seismograms. That's called cross correlation, or the measure of similarity. From those similarities we construct a global correlogram - a sort of fingerprint of the earth."
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Specific features in the global correlogram showed the presence of J-waves. The results published in Science on Friday show that the Earth’s inner core is solid, but softer than previously thought.
There are still many puzzles that can be solved by studying seismic waves, Tkalčić said. “For instance we don't know yet what the exact temperature of the inner core is, what the age of the inner core is, or how quickly it solidifies, but with these new advances in global seismology, we are slowly getting there.
"The understanding of the Earth's inner core has direct consequences for the generation and maintenance of the geomagnetic field, and without that geomagnetic field there would be no life on the Earth's surface." ®