In August 2012, civil engineers from the French firm Menard drilled three rows of shallow holes into the soft clay soil at the base of the Alps near Grenoble. Each hole was roughly the depth and diameter of what a utility company might dig to secure the base of a telephone pole.
Then, on one side of the boreholes, they lowered a vibrating probe several feet into the soil. On the surface, they placed a network of sensors to record vibrations. By the time the waves reached the second row of boreholes, a distance of 11 feet, their signal, which would typically travel through 160 feet of soil, hardly registered. The holes had stopped the vibrations in their tracks.
For more than 2,000 years people living in earthquake zones have tried to keep the shaking beneath their feet from destroying their buildings. In his history of ancient Greece, Pliny the Elder describes how a temple’s foundation included a layer of sheepskin to allow the ground to slip and slide without damaging the building above. The state of the art for earthquake protection today, including bearings, springs, moveable pads, and other “base isolation” systems, works in much the same way as those employed by the ancient Greeks.
Now, simple rows of boreholes like those drilled in the field could make all of that obsolete. They appear to work as a “seismic cloak” that could hide a building—or perhaps an entire city—from an earthquake’s deadly waves. “If we drill holes around the building, forget about all your earthquake protections inside the building. We don’t need them anymore,” says Sébastien Guenneau, the originator of the idea and a physicist at the Fresnel Institute in Marseille, France.
Guenneau compares the effect to that of what we see in a mirage when light, which typically travels in a straight line, is bent by low density, hot air immediately above the surface of the earth. “Instead of having a ray of light that goes straight to the Earth, now it is curved, which is why if you are looking at a mirage, you don’t see the soil but you see the sky,” Guenneau says. “The same thing happens to seismic waves when you change the density of the soil by drilling boreholes. Instead of bending a ray of light, you are bending a seismic wave.”
From Microscopic to Massive
Guenneau’s efforts at seismic cloaking stem from recent efforts to cloak or hide objects from light. In 2006, John Pendry, a physicist at Imperial College in London, along with researchers from Duke University, showed it was theoretically possible to bend light around an object, effectively making it invisible. Later the same year the group made a partial cloak for microwaves, whose frequencies are slightly larger than those of visible light.
But cloaking the foundation of an entire building from seismic waves would likely be a much greater challenge. “I was a bit wowed by the daring of this paper,” Pendry says. “But when you think about it, it’s a reasonable proposition. Earthquakes are waves—or those are the things which do the damage—and they propagate along the surface of the Earth. If you’re smart enough, presumably you can deflect them to go different places.”
Though Pendry doesn’t say it, he likely presumes Geunneau is smart enough. Before Pendry made headlines around the world for his invisibility cloak, Guenneau spent over a year working for him as a postdoc on a Defense Advanced Research Projects Agency (DARPA) project for the U.S. Department of Defense. Guenneau can’t say exactly what all the project entailed, in part because the work he did was basic research and he says he doesn’t know what DARPA did with it. He does add, however that “they are very interested in invisibility cloaks.”
For more than a decade since, Guenneau has continued to work on metamaterials, artificial materials that have properties not found in nature, such as the ability to bend waves in directions they wouldn’t normally go. The vast majority of people working in the field, Pendry included, work in optics, trying to bend light to create things like cloaks and “super lenses” that have incredibly high resolution. Largely as a way to distinguish himself in an increasingly crowded field, Gueunneau started thinking on a completely different scale, much to the amusement of many of his colleagues.
“They were just laughing, to be honest,” Guenneau says when he first told coworkers about his plans for a seismic cloak. They ticked off a laundry list of problems, including the fact that soil is viscoelastic—both viscous and elastic, complicating how it reacts to forces—and maddeningly heterogeneous in its composition. “There have been people working on soil structure for many years,” he says his colleagues told him. “And you think you will make a revolution?”
Guenneau hasn’t sparked a revolution just yet, but the results he and his colleagues have obtained are compelling. Their goal is a true cloak, a circle of boreholes that bends seismic waves around the holes, like water flowing around a boulder in a stream. What they demonstrated in Grenoble was a good first step. There, rows of boreholes caused seismic waves to reflect backwards, like light hitting a mirror. The reflections, however, created seismic waves in front of the boreholes that had twice the strength as those initially generated by the probe. Such an approach could prove catastrophic in real world applications.
“Behind the mirror, you may have protected your nuclear plant, but in front of your mirror, you have a hospital or a school that gets destroyed,” Guenneau says. The group now hopes to conduct a follow up study that demonstrates their ability to create a true cloak.
For all its daring, creating a seismic cloak may actually be easier than constructing an invisibility cloak for light waves, Pendry says. For metamaterials to work, they have to be smaller than the length of the wave they cloak. Visible light requires nanometer scale metamaterials, devices that are proving incredibly difficult to create. Seismic waves, by contrast, require meter-scale metamaterials like the boreholes used in Grenoble. Yet even if the group is able to hide a building from seismic waves, Pendry questions how widely applicable their approach will be.
“I don’t think this technology is anything that is going to work in a place like San Francisco because the cloaks tend to be the same size as object you are hiding,” he says. “In a dense city, you wouldn’t be able to excavate the area around buildings without knocking down their neighbors.”
Guenneau says they aim to protect high value—and potentially highly vulnerable—facilities like nuclear power plants with a sufficient amount of undeveloped land surrounding them where they could drill their holes.
Still, others say the recent study does little to show that the approach will work at all. “The mechanism of earthquake shaking is very different than a single discrete source that is very close to the boreholes,” says Youssef Hashash a civil engineering professor at the University of Illinois in Urbana-Champaign. “It comes from underneath you. Some from the surface, but a lot of it comes from below these boreholes, and the technique they have doesn’t affect anything underneath you.”
Jeroen Tromp, a geologist at Princeton University, disagrees, saying surface waves are typically more powerful and do more damage than waves coming up from below. “Surface waves spread in two dimensions as opposed to subsurface waves that spread in three dimensions,” Tromp says. “The energy is distributed over a surface rather than a volume, so they are much more destructive.”
Tromp cautions, however, that there is no one-size fits all approach to seismic protection. Smaller structures like single-family homes resonate, or shake, at higher frequencies, making them more prone to damage from the less powerful but higher frequency waves coming from underground.
Hashash adds that the vibrating probe used in the study only tested a single, 50 Hz frequency, while the most damaging surface waves typically occur across a wider and lower range of frequencies between 0.1–10 Hz. “You cannot produce the richness of waves that an earthquake produces with that device,” Hashash says.
Stéphane Brûlé, a seismologist and geotechnical engineer with Menard and lead author of the Grenoble study, says the initial experiment was a proof of concept to show that boreholes can alter the path of seismic waves similar to how prior studies altered microwaves. Brûlé and his colleagues completed a second experiment several months after the initial test that used significantly wider holes that were seven feet in diameter. This time they dropped a mass of 21 tons from a height of 164 feet to simulate an earthquake. Shaking in the immediate area registered between 2.0 and 3.0 on the Richter scale, and, perhaps more importantly, created waves in the 3 to 12 Hz range, similar to those seen in an actual earthquake.
At the same time, Guenneau is moving forward with cloaking techniques to protect against entirely different, yet equally destructive waves. In 2008, he and colleagues from Liverpool University in the U.K. and the Fresnel Institute in France showed how a series of rods arranged in concentric rings could redirect liquid waves, similar to what Pendry and others had shown for microwaves.
“Tsunami waves and surface waves for earthquakes are pretty much the same in terms of the mathematical equations,” Guenneau says. “You can have a huge storm, you can have waves of very high amplitude, but in center of the cloak you will have calm water.”
The initial experiment was conducted with an eight-inch diameter disk, a scale so small that the group had to use methoxynonafluorobutane, a compound similar to Freon once used in refrigerators, instead of water because of water’s high viscosity at such a small scale.
In February 2015, Guenneau and a team of French researchers published a follow up study for an “invisibility carpet,” a significantly larger rectangular pad with several rows of rods jutting out of the water. In wave tank testing, the carpet reflected the waves, which would otherwise have washed over a small seawall, back in the direction from which they came.
Guenneau says he is now working with a private company to develop a larger scale prototype. “The cost would remain quite affordable compared to the cost of a nuclear plant which is destroyed or a city which is devastated,” Guenneau says. “You prefer to have a carpet which costs perhaps 10 or 20 million euros than have 50,000 human beings wiped from the surface of the earth.”
Pendry, however, questions whether such a carpet could withstand a tsunami. “I’d be very, very skeptical just because of the enormous forces involved,” Pendry says. “It would involve a very large, very expensive structure, and maybe one that you couldn’t even find the materials to build in the first place.”
Whether or not columns sticking out of the sea could provide adequate protection against a tsunami, similar structures on land may already be absorbing earthquake waves.
In 2014, researchers in France found that a forest of long aluminum rods extending vertically from a horizontal plate canceled out vibration waves. The rods didn’t cause the waves to reflect as if bouncing off a mirror nor bend around a protected area like a cloak. Rather, each rod acted as a resonator absorbing a small fraction of the wave’s energy. Collectively, the rods canceled the wave entirely.
The finding made the researchers wonder if an actual forest would have a similar effect on seismic waves. In a yet-unpublished study, researchers in Grenoble recorded ambient vibrations in the ground using a pair of seismometers, one located within a small wooded area and one located just outside the wood’s western border. Both instruments detected vibrations from road construction roughly 100 yards away to the north. Vibrations inside the wooded area were six times weaker than those outside.
“If you have many trees close to each other you can completely cancel the incoming wave,” says Philippe Roux, director of research for the National Center for Scientific Research in Grenoble and co-author on both papers.
Trees, however, have their limitations. Like the metal rods, they only resonate, or absorb wave energy at certain frequencies based on their height. The 33–66 foot pine trees that damped the construction vibrations resonate at 30–45 Hz, a significantly higher frequency than the most dangerous seismic waves that are less than 10 Hz.
To absorb energy from the most destructive waves, Roux says structures 260 to 330 feet tall would be required. Instead of using trees, we might someday design cities so that a forest of skyscrapers or other tall structures neutralize seismic waves, Roux says.
Ping Sheng, a physicist at Hong Kong University of Science and Technology, says the energy absorbed from seismic waves could then be converted to electricity. In a recent paper Sheng demonstrated it is possible to convert energy from acoustic waves into electricity, suggesting the same could be done for seismic waves. “If you could have a large-scale dissipative mechanism, then you may be able to lower the effective Richter scale by one while simultaneously converting a lot of energy,” Sheng says. “That would be huge.”
Cloaking buildings from earthquakes and tsunamis or converting the energy of seismic waves into electricity may seem far-fetched, but probably no more so than it once did to isolate marble temples from the ground with a layer of sheepskins. Researchers will have to continue testing their cloaks and resonators in increasingly complex, real-word settings against a number of different seismic waves before we’ll know just how effective they will be. But in certain applications they might just work. “We are at the stage when people stop laughing,” Guenneau says.