Earth’s Inner Core is Solid, but Softer than Previously Thought: Latest Study

A recent paper published in Science has reported to have detected seismic J waves or shear waves for the first time, since the prediction that the inner core of the Earth is solid was made 80 years ago. This prediction had made it apparent that the solid core should support the shear waves or seismic J waves. From then onwards, detecting these J waves had been an unaccomplished task and considered a “holy grail”.

Associate professor Hrvoje Tkalčić and PhD scholar Than-Son Pham of Australian National University (ANU) have devised ways of detecting these feeble waves for the first time. 

The detection of J waves in the inner core not only provides confirmatory evidence about the solid character of the Earth’s core, but also opens up new possibilities of unveiling the core’s greater details. Their data also have shown that although the inner core is solid, it is softer than was thought previously. They were able to measure the speed of the J waves. Based on the data, the researchers have proposed that the inner core of the Earth has some elastic properties like that of gold or platinum- which means that a solid base with flexibility is what our planet’s core is made up of. 

The shear waves or J waves are kind of waves that can only propagate through solid objects. If Earth’s inner core is solid, then these kinds of waves are also present in the core and should be detectable. But detecting these waves in the Earth’s inner core had been of extreme difficulty as they are very tiny and feeble, and can’t be observed directly. This had been a great challenge to the field of seismology, until now. 

Correlation Wave Field Method: A Breakthrough

The ANU researchers came out with an innovative approach the break the nut. They used the technique called Correlation Wave Field Method. In this method, they study the similarities between the signals at two receivers after a major earthquake has occurred. The ice thickness in Antarctica was also measured by the same team using a similar technique. 

They took the data from the seismogram (a graphical output by the seismograph. It is the record of the ground motion) after a large earthquake, and analysed the data between 3 to 19 hours after the quake. The first three hours data from the seismogram were avoided by the researchers to avoid the big signals. 

The seismogram data had been taken from a huge global network of stations. They analysed every single receiver pair and every large earthquake, thus creating a huge matrix of seismogram data for analysis. They examined the similarities between the seismograms in a method called cross correlation, or the measure of similarity. From the similarity data, they constructed a global corelogram. This, they claim is a sort of fingerprint of the earth. 

Understanding of the Earth’s inner core is an important aspect in the field of seismology—especially to understand how the geomagnetic field of the earth is generated and maintained, which is essential for the sustenance of life on Earth.  

Detailed information of the inner core of Earth is also important to know how the Earth evolved.  Questions like the exact temperature of the inner core, the age of the inner core, or how quickly it solidifies etc. will become easier with the new advances in global seismology and this new finding is a step forward in this direction.