New information about the deep core of Earth has been uncovered thanks to the resonances of earthquakes, as demonstrated by research released by Nature Communications this week. There has been accumulating evidence over the last few decades that the planet’s solid inner core is composed of multiple layers2,3, but the characteristics of these layers have remained unknown. Scientists have employed an array of seismometers to study the way seismic waves are bent as they travel through the solid ball at Earth’s core, yielding important insights on the structure of the inner core. The Earth, following a major earthquake, will tremble like a bell for many days.
The waveforms were captured by scientists near to the epicenter of the earthquake and in the antipode, the polar opposite, in order to pick up on these tremors. This allowed them to examine the several routes that get to the center of the planet. Seismometers detected as many as five echoes from a single tremor, with each reverberation taking about twenty minutes to travel from one half of the world to the other.
Each of the first earthquakes was of magnitude six or higher, but the waves gradually diminished in strength as they traveled deeper into the Earth. The researchers used a stacking approach, whereby they stacked the waveforms from a single event to construct a more comprehensive image of the distortion from the deepest core.
They discovered that the waves behaved differently inside the inner core, which they estimated to be around 650 kilometers deep, compared to the outer core. A wave traveling through the deepest section of the core would slow down in one direction, while a wave traveling through the outer layer would slow down in the other way. What this suggests is that the inner core’s major element, iron, is likely arranged differently in its crystals than its outer shell.
University of Connecticut in Storrs geophysicist Vernon Cormier says the work is significant because it provides a measurement of Earth’s deepest portion that has previously been impossible to get. In order to estimate the wave speed in the extremely deep core of the Earth, it is necessary to locate seismic waves caught at very long distances and that are very weak in amplitude, and then enhance the amplitude. Minerals exploration uses this method often, while geophysicists seldom use it.
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