Love waves from the bottom of the ocean

Vibrations travel across our planet in waves, like chords resonating from a strummed guitar. Earthquakes, volcanoes and the hustle and bustle of human activity excite some of these seismic waves. Many others reverberate from wind-driven ocean storms.

As storms shake the world’s seas, wind-whipped waves at the surface interact in a unique way that produces piston-like pressure hits on the sea floor, generating a stream of faint tremors that ripple across the Earth to every corner of the globe .

“There is an imprint of those three earth systems in these environmental seismic data: atmosphere, rocky outer layers of the Earth and ocean,” said Lucia Gualtieri, a geophysicist at Stanford University, lead author of a paper Proceedings of the National Academy of Sciences which helps solve a decades-old puzzle about the physics of seismic waves related to ocean storms.

Known as secondary microseisms, the small seismic waves excited by the noisy oceans are so ubiquitous and chaotic that seismologists have long put aside the data. “When you record these waves, the seismic record looks like random noise because there are so many sources, close to each other across the extended area of ​​a storm. They all act at the same time and the resulting wave fields interfere with the one another, “said Gualtieri. “You just want to discard it.”

Yet over the past 15 years, researchers have found a way to extract meaning from this noisy data. By analyzing the speed at which wave pairs travel from one seismic station to another, they began to gather information about the materials they move through. “We use seismic waves like X-rays in medical imaging to scan the Earth,” said Gualtieri, assistant professor of geophysics at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth).

Unlike a single ocean wave rolling on the surface, which dies before it reaches the deep sea, the chaotic interactions of waves traveling in opposite directions during a storm can create an oscillating up and down motion on the surface that pulsates all the way. to the solid Earth below. The vibrations known as Rayleigh waves then travel outward from the impulse, moving the ground up and down as they go.

For decades, scientists have understood the vertical component of ocean storm microsisms, where Rayleigh waves dominate. But there is another set of vibrations recorded during ocean storms that are inexplicable in accepted theories of how stormy seas generate motion in solid Earth. These vibrations, called Love Waves in honor of their 20th century discoverer, push the underground rock particles from side to side, perpendicular to their forward path, like a crawling snake. “These waves shouldn’t be there at all,” Gualtieri said. “We didn’t know where they came from.”

Scientists have presented two plausible explanations. One idea is that when the vertical force pumping down from colliding ocean waves meets a slope on the sea floor, it splits and forms the two different types of surface waves: Rayleigh and Love. “In that case, the source of the love waves would be very close to the source of the Rayleigh waves, if not in the same location,” said Gualtieri.

But Gualtieri’s research, in collaboration with geoscientists at Princeton University, finds that the slopes and slopes of the seabed are not steep enough to generate the strong horizontal force necessary to produce the waves of Love detected by seismic recorders. Their findings, published on November 9, support an alternative theory, in which the waves of love originate within the Earth itself. It turns out that when windswept seas reduce the pressure to the bottom of the sea, the mosaic structure of the solid Earth below responds with a quiver of its own.

“We understand how earthquakes create waves of love, but we never understand exactly how ocean waves create them,” said environmental seismic noise expert Keith Koper, professor of geology and geophysics and director of seismographic stations. from the University of Utah, who was not involved in the study. “This is a bit embarrassing because the waves of love generated by the ocean have been observed for over 50 years.” The paper led by Gualtieri, he said, “provides conclusive evidence” of how ocean waves generate this particular type of vibration on Earth.

Using the Oak Ridge National Laboratory Summit supercomputer, the researchers simulated the complex interactions that occur between storms, ocean waves, and solid Earth over three-hour periods. Accurate to four seconds, each simulation included 230,400 pressure sources scattered around the globe. “We are using the computer as a laboratory, to allow seismic waves to propagate from realistic sources in all oceans of the world based on the known physics of how and where seismic waves are generated by ocean storms, as well as how they move through the ocean. Earth, “Gualtieri said.

One version of the Earth model represented the planet as a simplistic layered world, in which properties vary only with depth, like a layer cake. The other, more realistic, model captured more of the three-dimensional variation in its underground terrain, like a chocolate chip cookie. For each release, the researchers turned underwater depth data on and off to test whether seafloor features like canyons, ravines and mountains – as opposed to the deeper structure – could produce waves of love.

The results show that love waves are sparsely generated in the one-dimensional Earth like a layer cake. Given about 30 minutes and a rumbling ocean, however, the waves of Love emanated from beneath the sea floor in the three-dimensional model. When Rayleigh waves and other seismic waves generated by ocean storms encounter warmer or colder areas and different materials on their lateral journey across Earth, the study suggests that their energy is dispersed and refocused. In the process, part of the wave field converts into love waves. “If you apply those sources of pressure from the interfering ocean waves and wait, the Earth will give you the entire wave field,” Gualtieri said. “It is the Earth itself that will generate the waves of Love.”

According to Gualtieri, a better understanding of how these vibrations arise and propagate across the Earth could help fill the gaps in knowledge not only of our planet’s interior but also of its climate change. Analog seismic records date back to before the satellite era, and high-quality digital data has been recorded for several decades.

“This database contains information on environmental processes, and it is virtually unused,” he said.

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