3D Imaging Technique Allows Exploration of Earth’s Interior Using Microseisms

There is more to our planet Earth than we can see on its surface, and its interior still holds some mysteries for humans. While we have explored other worlds and have deployed satellites into orbit, the inner portion of our planet remains off-limits from us.

Our modern understanding of the Earth's interior is based on data gathered from seismic monitoring. This involves the measurement of seismic waves generated by earthquakes and examining how the different layers of the Earth cause them to slow down. In a new study by Japanese researchers, a 3D imaging technique was developed to explore the Earth's mantle using microseisms.

What are Microseisms?

In seismology, microseisms refer to a faint tremor in the Earth caused by natural phenomena. Also known as the "hum," these are dominant background seismic and electromagnetic noise signals on Earth.

Microseisms are small vibrations generated by the constant activities in the ocean. These bodies of water are constantly roiling and swelling, producing body-wave microseisms. Human activities such as vehicular traffic can also create the nearly imperceptible vibrations.

Unlike seismic waves, which are only generated by earthquake shocks, microseisms always happen. Like large earthquake waves, microseisms can travel along the Earth as surface waves or through its interior as body waves. Although microseisms are less powerful than earthquakes, they can offer valuable information about deep-Earth structures.

Extraction of Mantle Discontinuities

S. Kato and K. Nishida from the Earthquake Research Institute of the University of Tokyo investigated how body wave microseisms emerge from the ambient noise of the ocean and travel through the mantle. They looked at data from 690 seismic stations that recorded 5,780 microseisms occurring in the North Atlantic and the North and South Pacific oceans.

Instead of studying the microseisms using seismic interferometry, the method used in analyzing seismic noise under the assumption that these small vibrations are generated everywhere, Kato and Nishida assessed the tiny tremors more like they would larger earthquakes. By analyzing the data obtained from seismic stations, the experts created a 3D image of the structure of the Earth's mantle.

In this research, microseismic body waves are treated as having isolated source locations; a new receiver function method is used to analyze the constant background hum generated by the oceans. This approach can lead to a better understanding of the inner structure of our planet that is not available from the analysis of larger seismic events and provide insights for future exploration.

The imaging results confirm the depths of mantle discontinuities. These rock density and composition changes mark the transition zone between the upper and lower mantle at 410 and 660 kilometers below the surface.

The researchers extracted the P-S waves found at mantle discontinuities using the ambient noise excited by the ocean swells. They developed the source deconvolution technique in generalizing a receiver function method to P-wave microseisms. The migration result of P-S waves shows consistency with previous studies, which demonstrated the potential of P-S microseisms to seismic structures.