Earthquakes and nuclear tests produce sound waves that travel even further than the coronavirus: through the atmosphere, ocean and earth. What do they say about their sources?
What sound reveals about the movement of the earth
After four years of research, the coronavirus forced PhD candidate Gil Averbuch of the Department of Geoscience & Engineering (Faculty of Civil Engineering and Geosciences) to stay even longer in Delft. Less than a week before his flight to Dallas for his postdoctoral study in seismo-acoustics, the USA closed the borders due to the corona crisis. “It was very bad timing for me,” says Averbuch, “because all my things were packed and my rental contract had ended. Luckily, a friend let me stay in his apartment, as he is not here at the moment.”
Averbuch, originally from Tel Aviv, met Prof. Läslo Evers (Faculty CEG) at a congress in Vienna. They both had a shared interest in the effect of underground sources on sound propagation in the atmosphere. Their curiosity was peaked by the North Korean underground nuclear tests. When North Korea secretly performed underground nuclear tests in 2013, infrasound signals – very low frequency acoustic waves – were detected in Russia and Japan. When the leader of North Korea, Kim Jong-un, launched another test of the same strength at the same place in 2016, almost no infrasound was measured in Japan and only a very weak signal in Russia. How come? Infrasound propagates through the atmosphere at frequencies of between 0.01 and 20 Hz. ‘Infra’ literary means ‘very low’, and sound is an acoustic wave. Together, infrasound is a very low frequency acoustic wave.
Seismic source and infrasound. (Illustration: Gil Averbuch)
Infrasound is by no means new to science. Observations of earthquake induced infrasound signal recordings hundreds of kilometres away date back to the 1950s. However, there were not as many measuring stations back then. Since the Comprehensive Nuclear-Test-Ban Treaty in 1996, the International Monitoring System (IMS) started continuously monitoring seismo-acoustic events to prevent countries developing nuclear weapons. This led to a huge inflow of data open to scientists from around the world. Initially, researchers tried to understand how sound propagates through the atmosphere. These were only speculations of how the waves are coupled to the atmosphere from any underground source. Nobody took it as the focus of his or her research, says Averbuch.
Source depth
“First we wanted to know if the condition of the atmosphere had an effect on the different detections of the North Korean underground nuclear tests in 2013 and 2016,” says Averbuch. Wind and temperature in the atmosphere affect wave propagation. Although the atmospheric conditions were different in 2013 and 2016, they could still not account for everything. “Then we realised that the most logical parameter left was the source depth of the nuclear tests. The variations in the source depth led to different amounts of energy being transmitted to the atmosphere.”
Averbuch made a model that simulates seismic and acoustic wave propagation in an earth-ocean-atmosphere system. Variations in source depth and source strength result in different amounts of energy coupled to the atmosphere and in different wave propagation paths.
The propagation of infrasound from the 2010 earthquake in Haiti to the IMS infrasound stations. Transmission loss is shown as a function of distance and altitude at 1.0 Hz. (Figure: Gil Averbuch)
How the earth moved
After having a basic understanding of how the coupling mechanism works, it was time to study more complex systems: earthquakes. In 2010 for example, an earthquake in Haiti generated infrasound that was detected on Bermuda Island, 1,700 kilometres away. Joining forces and scientific tools with fellow-researcher Dr Shani-Kadmiel, they used a ‘back protection’ method and projected the infrasound signals back to the possible location of the source. The back projection showed the exact spot of the epicentre of the earthquake in Haiti.
From left to right: Map of Haiti and Bermuda; earthquake epicentre; earthquake source mechanism polarity and intensity.(Figure: Gil Averbuch)
But infrasound recordings can reveal even more information. The polarity of the signals can also be used. The signals from the Haiti earthquake show a very interesting pattern that reveals the earthquake’s source mechanism. It basically tells you which parts of the earth dropped down and which parts went up. Averbuch: “We were all surprised to see this pattern, and we wanted to know if it was just a coincidence.”
Predicted earthquake source mechanism: red parts went up, blue parts down. (Figure: Gil Averbuch)
So the team simulated wave propagation in the atmosphere for a batch of sources, simulating the same source mechanism. There they saw the exact same pattern. “Infrasound really lets us extract information on how the earth moves during an earthquake 1,700 kilometres away.”
“In the next step,” Averbuch continues, “we will see the extent to which we can use infrasound recordings to get supplementary information about underground sources. We will therefore not separate seismic waves and acoustic waves. We have to look at them as a whole.”
At the moment, Averbuch has regular contact with both the research group of the Southern Methodist University of Dallas and TU Delft. “I can’t say that I am working as usual, but I do try to make the best of all the ‘forced’ free time I have. I read, learn new things, work on new ideas, and am finishing up unfinished projects.”
- Dr Averbuch, Gil, Conversations between the Earth and Atmosphere: A study on the seismo-acoustic wavefield, PhD supervisors: Prof. Läslo Evers. Defence, 9 March 2020.
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