Earthquake whispers reveal Earth’s secrets

By listening to earthquake ‘whispers’ scientists at the University of Calgary believe they have gained insight into what’s really going on in the Earth’s core.

Professor David Eaton and PhD student Catrina Alexandrakis analysed faint signals – seismic wave speed – produced by 44 earthquakes on the other side of the world to measure the sound speed at the top of the Earth’s core.

“Observation of distant earthquakes is one of the few tools that scientists have to investigate deep parts of the Earth,” said Alexandrakis, “This isn’t the first time earthquake data has been used, but our research method is the most definitive.”

The duo developed a technique called Empirical Transfer Function analysis to try and determine the materials that make up the core, 2,891km below the surface. Professor Eaton told Laboratory News: “It represents application of a technique borrowed from exploration seismology, called Wiener Deconvolution, to a problem in global seismology. The method allows us to remove most of the earthquake signature (which varies from event to event) from the recording. In so doing, we can average (or stack) many observations to reduce noise.”

He said this method provides a limit on some of the assumptions that need to be made when evaluating their data - different assumed compositions can be used to predict seismic waves speed, and having this new constraint allows these compositions to be evaluated, and where necessary changed.

 “Scientists have proposed a region of sediment accumulation at the top of the core, or even distinct liquid layers,” said Professor Eaton, “The study shows the core is, in fact, well mixed. This inaccessible region is composed of molten iron, nickel and other as-yet unknown lighter elements such as silicon, sulphur, carbon or oxygen.”

Understanding the composition and state of the Earth’s core is key to unravelling the source of the Earth’s magnetic field and the formation of our planet. The results will help to guide research at laboratories where core composition is studies by simulating conditions that exist in the Earth’s core – like extreme pressure and temperature.

“This work informs our understanding of planetary formation processes (e.g., differentiation of the core from the rest of the Earth) as well as models for the geodynamo, which sustains the Earth's magnetic field,” Professor Eaton concluded.