IT’S the summer of 2012, and in a field outside the French Alpine city of Grenoble, a group of civil engineers are doing what they do best: boring. Each of the 30 or so holes they are drilling into the soft sediment would support a telegraph pole. But Sébastien Guenneau, the researcher directing the operation, has more ambitious plans in mind. He’s come here today to start an earthquake.
Guenneau, a physicist at the Fresnel Institute in Marseille, France, is one of a band of researchers aiming to shake up the way we deal with quakes. Rather than designing cities and their buildings to withstand them – with all the cost and potential for failure that implies – they believe we can engineer the ground so that destructive seismic waves never reach cities at all. From underground musical pipes to swaying metal rods to strategically planted trees and, yes, even the right sort of holes bored in the right places, they think some simple tools could help us conquer quakes once and for all. Now they’re setting out to test the idea on a grand scale.
Seismic waves come in two types: body waves, which travel underground directly from the source, and surface waves, which ripple along Earth’s surface. These are the most damaging to buildings, turning the ground beneath into a bucking bronco.
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For the last few thousand years, earthquake protection has involved finding ways to ride out the tremors. From layers of sheepskin in the foundations of temples – as described by the Roman author Pliny the Elder – to more modern solutions involving springs, ball bearings and special pads, these methods all aim to isolate the foundations of a building so it can remain stable while the ground shakes.
But in 2001, French seismologist Philippe Guéguen realised that getting buildings to sway could have unexpected benefits. He was studying the effect on Mexico City of a magnitude 7.4 quake that hit it in September 1995. While various parts of the city should have responded in the same way, some neighbourhoods shook for nearly twice as long as others. Softer sediments wobble more than solid rock, but underlying geology wasn’t enough to account for the discrepancy. Instead, it turned out that tall buildings were creating secondary waves in the ground around them, prolonging the shaking but muting the original surface waves. This suggested that the swaying high-rise towers were redistributing the earthquake energy, and providing some protection.
The idea that the waves themselves could be modified, rather than simply withstood, turned earthquake engineering on its head, says Guéguen, who is based at the Institute of Earth Sciences in Grenoble. The key is controlling the way in which the ground responds to incoming waves.
Guenneau had a surprising idea for how this could be achieved. While simply drilling holes in the soil may not seem like it would have much of an impact, he knew that the principle on which it was based had already achieved the impossible.
That was with very different waves in a very different setting. A decade ago, Guenneau was working with John Pendry, a physicist at Imperial College London, when he showed that it was theoretically possible to guide light waves around an object, effectively rendering it invisible. Pendry and a team from Duke University in North Carolina soon went on to demonstrate the concept in the lab.
The invisibility cloaks they made were designed to bend the path of microwaves so that they flowed smoothly around the object, like water flowing around a boulder in a stream. Making this happen involved building a metamaterial – a substance with a physical structure designed to give it unique properties not usually found in nature. Their material was a lattice of thin metallic wires, separated by gaps smaller than the wavelength the device was built to deflect.
That set Guenneau wondering if the invisibility cloak principle could be scaled up to work on seismic waves. In 2009 he floated the notion in a paper. “When I heard about this I thought it was a daring idea. Seismic waves obey rules similar to electromagnetic wave equations, so in theory there is no reason why it shouldn’t work,” says Pendry. The work also caught the attention of Stéphane Brûlé, a civil engineer with the French firm Menard, who had a long-standing interest in the way earthquakes interact with subsurface structures. “I realised that what they were talking about was close to my research and could be tested,” says Brûlé.
“Simply drilling a few holes into the soil could achieve the impossible“
Working together, Guenneau and Brûlé began to think about ways to mimic the super-thin wires Pendry used to disrupt electromagnetic waves, but tuned to wavelengths billions of times longer. “Our first attempt was simply to drill rows of holes in the soil,” says Guenneau, a project they undertook over three days in August 2012. They used an industrial crane to plant a large vibrating probe underground, where it emitted a deep bass rumble at the upper frequency limit of seismic waves.
Without any holes, the team found the shaking could be detected up to 50 metres away. With the holes in place, however, the shaking died out by the second row, just over 3 metres from the source. The scientists had stopped their mini-earthquake in its tracks.
“Cities could convert an earthquake’s energy into less harmful forms“
But there was a problem. Sensors in front of the row of holes recorded vibrations twice as intense as those produced by the probe. “In this case we reflected the seismic waves instead of deflecting them around the area. We created a seismic mirror,” says Guenneau. If this technique were used to protect a hospital or a vital power plant, the surrounding houses would end up being shaken twice as hard.
Guenneau’s more recent results suggest that this problem can be solved by narrowing the spacing between holes in subsequent rows. This way, he says, “the path of the seismic wave is gradually bent, rather than reflected”. Later this year Brûlé and Guenneau intend to put this idea to the test, drilling a new network of holes and dropping a 35-tonne weight from a 40-metre-high crane to simulate a magnitude 3.5 quake.
This all sounds promising, but truly devastating earthquakes are a different kettle of fish entirely. A magnitude 6 quake can release as much energy as the 1945 Hiroshima atomic bomb. The earthquake that hit Mexico City in 1995 was over a hundred times more powerful than that (see “Bad vibrations”). “Seismic waves that are dangerous for buildings are generally more than 100 metres long. This means that the seismic barrier would need to be several hundred metres wide, or extremely heavy, to be effective,” says Chiara Daraio, who designs metamaterials at the Swiss Federal Institute of Technology in Zurich. “It isn’t practical for protecting buildings in a dense urban area.”
Daraio proposes a more sophisticated way to modify earthquake waves. Together with her team, she has designed a cylindrical steel resonator, containing a vertical steel rod that sways back and forth when a seismic wave arrives. These resonators increase the stiffness of the soil, says Daraio, meaning a comparatively small barrier can have an impact on waves much larger than itself. When faced with an array of differently sized resonators, each tuned to respond to a different frequency, the surface waves will lose energy and be deflected downwards where they pose less of a threat.
In 2015 Daraio and her team tested their idea by inserting cigar tube sized resonators into a tray of sand and inducing mini earthquakes with an electromagnetic shaker. They then modelled how the system might perform for a real earthquake, using the magnitude 6.7 Northridge quake in California in 1994 as an example. “We showed that a barrier consisting of 35 resonators would have functioned as an efficient shielding mechanism,” says Daraio.
Now the team is scaling up the experiments and using more realistic materials. But when it comes to really large earthquakes, Daraio doubts her system could hold up. “The structural integrity of the resonators would be hard to maintain,” says Daraio. It is a criticism that she also levels at Guenneau’s system of holes, insisting that they too would collapse when shaken by a massive quake.
Daraio envisages her earthquake cloak being used to shield areas from small and medium-sized earthquakes, or from earthquakes induced by human activities such as fracking. “Fracking can induce seismicity in regions that don’t have natural seismicity, and so buildings are vulnerable because they were not designed to cope,” she says.
But the dream of rerouting the largest of earthquakes remains. One way of doing this might be to convert the earthquake’s movement energy into a less harmful form. With this in mind, Sang-Hoon Kim from the Mokpo National Maritime University in South Korea and Mukunda Das from the Australian National University in Canberra have designed a system of subterranean perforated concrete cylinders that would resonate when struck by surface waves. “It would be like thousands of underground concrete saxophones being played with seismic energy,” explains Kim. For a magnitude 8 earthquake, Kim and Das calculate that their underground saxophones would produce a 160 decibel roar – equivalent to the noise level of a shotgun firing. However, the structures would have to be impractically large – perhaps 100 metres long. Instead, Kim and Das envisage creating smaller structures, with roughly the same stopping power as Guenneau’s network of holes.
Unfortunately, their concrete saxophones have not yet been put to the test. “I live in Korea – an almost earthquake-free country – and nobody wants to spend money on this experiment,” says Kim.
Although Guenneau stands with Daraio in conceding the challenges associated with deflecting the largest earthquakes, he argues that seismic cloaks don’t need to be perfect to be effective. “We don’t need to completely stop the seismic wave – reducing shaking by 10 to 15 per cent would be enough to preserve buildings,” he says. And with this in mind he has recently teamed up with Guéguen and Philippe Roux, a geophysicist at the Institute of Earth Sciences in Grenoble, to try yet another seemingly outlandish idea.
The trio believe that just as the shaking of tall buildings in Mexico City redistributed the energy of the 1995 quake, tall trees might also be able to modify surface waves.
To test this idea, the three got together last summer and recorded ground tremors passing through a pine forest on the outskirts of Grenoble. At certain frequencies, rumbles recorded inside the forest were found to be six times smaller than those recorded outside. This summer, the trio intends to scale their experiment up, deploying a larger network of sensors and investigating how tree size and spacing affects seismic wave absorption. With the right configuration of trees, says Guéguen, “we can start to make self-resistant cities”.
But even the tallest of pine trees resonate at frequencies too high to disrupt the most dangerous seismic waves. A forest of record-breaking giant redwoods would be needed to do the job, although a ring of uninhabited buildings in a city’s suburbs might be more practical. With experiments under way to test the same principles on tsunamis, Guenneau’s days of shaking up the field are far from over.
Taming tsunamis
Building an invisibility cloak to shield cities from seismic waves is a challenge in itself, but Sébastien Guenneau from the Fresnel Institute in Marseille, France, doesn’t want to stop there. Mirroring the success of his land-based experiments (see main story), he envisages forests of artificial trees planted on the ocean floor, rising above the surface ready to calm mighty tsunamis. “My wife was born in Sri Lanka, which was heavily affected by the 2004 Indian Ocean tsunami, and after this event I started to think about ways to protect coastal towns,” he says.
Back in 2008 Guenneau and colleagues tested their first prototype tsunami cloak, measuring just 10 centimetres across. It comprised an arrangement of metal rods in seven concentric rings, with a greater spacing between rods in the outer rings. They exposed it to a mock tsunami wave made of a slippery organic compound even less viscous than water, seeing as the ordinary wet stuff would have been too sticky on such tiny scales.
The approaching wave flowed through the gaps between the rods, but then swirled around them to produce forces that pulled the water along the concentric “corridors”. “We are encouraging the waves to travel in curved trajectories rather than straight lines,” says Guenneau. The increasing density of rods made the turning forces progressively stronger towards the centre, ensuring that the water was eventually diverted around the ring of rods, keeping the centre dry.
In 2014 the team revisited the idea on a scale 10 times larger. Using the wave tank facility at near Marseille, they subjected their “invisibility carpet” to water waves and showed that it successfully – a step on the road to redirecting them completely.
This summer, Guenneau and his colleagues are scaling up their tsunami cloak yet again, testing a 2-metre-wide arrangement of rods at the same facility. But not everyone is convinced that tsunami waves can be tamed. “Tsunamis carry a vast amount of energy and the structures capable of controlling that energy would have to be very substantial – kilometres across. Engineering such a thing seems implausible,” says John Pendry of Imperial College London, who designed the world’s first invisibility cloak for electromagnetic radiation. Guenneau agrees that for now the construction challenges associated with protecting an island or stretch of coastline are prohibitively expensive, but he believes that smaller structures, such as oil platforms, are a more realistic proposition.
This article appeared in print under the headline “Movers and shakers”
