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Seismic Cloak Successfully Deflects Earthquake Waves

Engineers are experimenting with a variety of ways to cloak vital infrastructure—even entire cities—from earthquakes.

ByPhil McKennaNOVA NextNOVA Next
Seismic Cloak Successfully Deflects Earthquake Waves

MIMIZAN, FRANCE—For an experiment with such ambition, the setting couldn’t have been more humble. Yet scientists from across Europe converged here—a dirt lane between a farmer’s field and a small wood in southern France—in the fall of 2016 to test a provocative idea: could they make the seismic waves of an earthquake disappear?

Straddling rows of canola and maritime pine, they assembled an array of more than 1,000 seismometers, devices so sensitive they could detect a beating heart, treetops swaying in the breeze, or ocean waves crashing on distant shores. Then, with a robotic-looking device they’d nicknamed R2-D2, they planned to shake the ground beneath their feet.

The experiment was the largest field test thus far of ongoing efforts in Europe and the United States to create a seismic barrier that would shield or perhaps even cloak key infrastructure from an earthquake’s deadly force.

The technique is still in its infancy, yet government agencies are beginning to take notice. Seismic cloaks could someday protect nuclear reactors, crude oil pipelines, or even entire cities.

“The payoff for this could be huge,” said Eric Dunn of the U.S. Army Corps of Engineers’ Research and Development Center in Hanover, New Hampshire. “If this works, it’s fantastic.”

Before there could be any chance at a payoff, however, the geophysicists gathered in France needed help from their farmer host. The generator they’d brought with them from Grenoble refused to start. Without it, they couldn’t power R2-D2, and without the ground-shaking device, there would be no experiment.

Protective Trees

Several years ago, researchers at the National Center for Scientific Research in Grenoble noticed that if they connected long aluminum rods to a thin metal plate and then vibrated the plate, each rod would absorb and dissipate a small amount of the vibration wave’s energy. If they connected enough rods to the plate, the forest of resonating rods would cancel out the wave entirely.

“One tree doesn’t have that much effect, but if you have many trees combined together you can kill the seismic wave.”

Would the same phenomenon occur at a larger scale, with an actual forest and a simulated seismic wave? Initial results from the forest experiment in southern France, published earlier this year in the journal Seismological Research Letters , suggest the answer is “yes.” The experiment “confirms the strong influence that a dense collection of trees can have on the propagation of seismic waves,” the study concludes .

Velocimeters placed on the trunks of several trees showed that as the wave passed underground, each tree absorbed some of the wave’s energy, causing the trees to vibrate ever so slightly. Vibrations from the trees then caused new waves to spread from the trees in all directions, dissipating the energy of the seismic wave. The waves emanating from the trees were also out of sync with the original seismic wave and thus had a canceling effect on the original wave operating in much the same way that noise-cancelling headphones eliminate background noise. The collective vibration of hundreds of trees, each weighing roughly 500 pounds, was enough to nullify the simulated wave.

“One tree doesn’t have that much effect, but if you have many trees combined together you can kill the seismic wave,” said Philippe Roux, director of research for the National Center for Scientific Research in Grenoble and the study’s lead author.

While maritime pines worked well in mitigating a small, simulated seismic wave, much taller trees would be needed to protect against a real earthquake.

R2-D2, the shaker used to simulate seismic waves, generated waves at frequencies ranging from 10 to 100 Hz. Seismic waves from an actual earthquake have longer wavelengths, i.e. lower frequencies, ranging from 0.1 to 10 Hz. The maritime pine trees tested in the experiment were all approximately 45 feet tall and resonated the most when exposed to significantly shorter wavelengths around 50 Hz.

To absorb the longer waves of an earthquake, the trees would have to be between 260 and 300 feet tall, roughly the height of the tallest redwoods, and would need to be of varying heights to dissipate waves across a broader range of frequencies. Planting redwoods around critical infrastructure and then waiting decades or perhaps even centuries for the trees to mature, isn’t viable. But redesigning cities in such a way that buildings act as artificial resonators to collectively reduce or redirect a seismic wave may be.

Peacekeeping - MINUSTAH
Specially designed cities could help buildings resist damage caused by earthquakes.

Many of the same researchers working on the forest experiment are now considering how groupings of buildings could act as metamaterials—artificial materials with properties not found in nature—to mitigate against seismic waves.

In a study published in June in the journal Innovative Infrastructure Solutions they proposed “a district of buildings could be considered as a set of above-ground resonators” and expressed the hope that “our proposal will contribute to all theoretical and experimental efforts in [the] design of cities of the future, from a metamaterial standpoint.”

In short, they suggest that, like trees, buildings could be used to dissipate the energy of a seismic wave. Alternatively, the buildings could bend the wave around part of the city that they want to protect, like an invisibility cloak for seismic waves. The upshot is that much of a seismic wave’s destructive force could be rerouted around the most sensitive areas of a city, including power plants, hospitals, and schools, the authors wrote.

As fanciful as an invisibility cloak for earthquakes may seem, the idea has some credence. In 2006, physicists at Imperial College in London and Duke University showed it was theoretically possible to bend light around an object, effectively making it invisible. Later the same year, the group used metamaterials several nanometers tall to make a partial cloak for microwaves, whose frequencies are slightly larger than those of visible light.

In the experiment, the metamaterials didn’t absorb the waves like resonators but bent the waves, like water in a stream flowing around a stone. In theory, you could scale up the metamaterials used to bend longer waves, such as seismic waves.

Stéphane Brûlé, the lead author of the cities study and a geotechnical engineer with French civil engineering firm Menard, has done some preliminary studies suggesting boreholes or concrete columns in the ground could work as well as buildings sticking above the ground to deflect seismic waves .

If holes or columns in the ground could reduce even small vibrations, Brûlé and his colleagues wouldn’t have to look far for potential applications.

Perfume manufacturer Chanel threatened to close one of its facilities in the French city of Grasse in 2016 over plans for a high-speed rail line. The company claimed the project threatened the flowers it used to make its best-known fragrance, Chanel No 5.

“The construction of a viaduct and the regular passage of high-speed trains over these fields of flowers would force Chanel to cease supporting its artisanal activities in the region,” the company said in media reports . Chanel did not reply to a request for comment, however ground vibrations from high speed rail have been well documented elsewhere.

Researchers in the U.S. suggest an approach similar to Brûlé’s could reduce much more powerful seismic waves.

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Subterranean Defenses

“If we had a magnitude 7 earthquake, the structure that we designed would bring it down to a magnitude 5.5 which is significantly less in damage scale,” said Robert Haupt, a staff scientist at the Massachusetts Institute of Technology’s Lincoln Laboratory.

Haupt and colleagues at MIT presented their approach at the Institute of Electrical and Electronics Engineers symposium on Homeland Security technologies last April.

The group proposes a “V” shaped array of boreholes or trenches approximately 500 feet deep filled with either air or a fluid surrounding an area needing protection. The technique could reduce the magnitude of naturally occurring earthquakes as well as man-made earthquakes caused by oil and gas operations. In Oklahoma, for example, nearly 900 earthquakes shook the earth in 2015, many of which were likely caused by large volumes of drilling wastewater injected deep underground. The continual low-magnitude quakes, nearly three per day, pose growing economic and environmental risks for oil pipelines and storage tanks in the state.

“We’ve been speaking with the states of Oklahoma, California, and then government agencies as well, because there are real problems with earthquakes, and people don’t know what to do about it,” Haupt said.

The growing threat also poses a potential opportunity for researchers to test a seismic barrier against an actual earthquake rather than a simulation like the one in France.

“We are talking with the state of Oklahoma, since they have three a day, maybe we can take advantage of those naturally occurring earthquakes—the ones that have been induced by man—and use that as a source to see how this works on a small scale,” Haupt said.

If real world testing shows that the boreholes or trenches that Haupt and colleagues are proposing can significantly reduce the magnitude of a seismic wave, the results would be game changing.

Protecting Vital Infrastructure

“It would be huge to be able to do that, especially when you consider something like nuclear facilities, power facilities, and the ability to get a community back on its feet after it’s been destroyed by an earthquake,” Dunn of the Army Corps said.

A magnitude 9.0 earthquake in the Pacific Northwest, a “megaquake” that most experts say isn’t a matter of “if” but “when,” would immediately kill 8,000 people in Washington State alone according to a recent estimate by Federal Emergency Management Agency (FEMA) . Reducing the magnitude of such a powerful quake in urban areas could save countless lives.

Meet the earthquake that drowned the Pacific Northwest and transformed a once lush landscape into a ghost forest.

Dunn says protecting an entire city with such seismic barriers would be cost prohibitive, but securing key infrastructure would be of tremendous benefit.

“If you were able to retain power, water, and protect some dams and have hospitals ready to receive people, it’s going to be much easier to recover.”

Still, Dunn is cautious, noting that, at least for the MIT research, they have yet to test the concept in the real world.

“The fact that the modeling shows it works, is much better than the modeling showing that it wouldn’t work, but models depend a lot on assumptions,” he said. “We’ll have to see how this goes.”

“It’s Happening Now”

Researchers still have a long way to go to develop an invisibility cloak or other barrier for seismic waves, but as the first comprehensive field test in France came to a close, the excitement was hard to contain.

“It’s like a kid’s dream,” said Sébastien Guenneau, the theoretical physicist who first came up with the idea of an invisibility cloak for seismic waves. “You feel almost like a wizard,” he said, but “it’s real science, it’s happening now.”

As the researchers packed up their equipment, a thick fog enveloped their test site. After wandering a short distance into the canola field, I stopped and turned back around. Looking up, the physicists and the forest, cloaked in fog, had completely disappeared.

Travel for this story was supported by a science journalism fellowship from the European Geosciences Union.

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