Vulnerable buildings could someday be shielded from damaging earthquakes by surrounding them with “seismic invisibility cloaks”. An early prototype of such a cloak has been tested experimentally by researchers in France. In principle, the concept could be extended to create protective barriers that divert earthquake energy away from sensitive facilities such as nuclear power plants.
Traditional earthquake engineering is based on damping and dissipating the energy absorbed by a building when it is hit by shock waves. Now, computational physicist Sebastien Guenneau and colleagues at the Institut Fresnel and the geoengineering company Ménard have taken a different approach that involves modifying the ground around a building to divert seismic waves, effectively cloaking the structure from an earthquake’s destructive energy. They have also conducted preliminary field tests on the efficacy of the design for earthquake protection.
Their design is the latest extension of the concept of metamaterials, which were first suggested for electromagnetic waves in 1968 by Soviet physicist Victor Veselago. The first metamaterials were built in 2006 by a team that included John Pendry at Imperial College London and David Smith at Duke University in North Carolina. Pendry, Smith and colleagues created artificial materials with negative indices of refraction – properties that do not normally occur in natural materials. Pendry and others also developed a mathematical tool called transformation optics to describe how a metamaterial should be structured to have the desired properties.
Transformation seismology
The most famous application of transformation optics is the invisibility cloak, which guides electromagnetic waves around an object and so makes it appear to be invisible. In previous work, Guenneau and colleagues had shown that transformation optics transfers easily to seismology. While electromagnetic waves pass energy back and forth between the electric and magnetic fields, seismic waves pass energy between the potential energy stored in the deformation of the Earth’s crust and the kinetic energy contained in its movement. If the electric permittivity term is replaced by the soil density and the magnetic permeability by its elastic modulus, transformation optics becomes transformation seismology.
That is the theory, but actually making a practical cloaking structure that works for all destructive seismic waves is fiendishly difficult. This is because both the soil density and elastic modulus of the surroundings have to be controlled simultaneously. Furthermore, the elastic modulus needs to be different for deformations in different directions. Similar challenges faced the builders of the first electromagnetic cloaks, who worked around the problem by creating cloaks that worked in 2D rather than 3D. It turns out that this simplification also works for earthquake protection as most damage is caused by waves propagating directly across the Earth’s surface.
As a simple test of the hypothesis, Stéphane Brûlé and colleagues at Ménard buried a source vibrating at 50 Hz – the upper end of the frequency of earthquake surface waves – just below the surface of a sedimentary basin. Several metres away, sensors mapping the speed at which the earth vibrated recorded strong oscillations. The researchers then bored strategically placed, 5-m-deep holes to modify the density and elastic modulus of the soil, as they calculated would be necessary. With the holes in place, some regions on the other side of the boreholes recorded less than 20% of the oscillation amplitude, showing that the modified soil had indeed stopped much of the wave energy.
‘A completely new approach’
Pendry describes the research as “a completely new approach”, although he points out that the cloak would require similar space to the region being cloaked. “It’s not something you’re going to do in Manhattan,” he says. “On the other hand, it might be something you’d want to do if you had a very high-value strategic object such as a nuclear reactor.” Cloaking expert Andrea Alú also says the work is “neat”, but he sees a potential pitfall with the simple shield demonstrated here. “Whatever you don’t transmit, you reflect,” he says, “If you are in a crowded environment, you may cause problems for other buildings.”
The researchers openly acknowledge this limitation. They are currently building the full cloak, which should control the flow of waves, ensuring they do not damage neighbouring structures. Gunneau concludes the involvement of the Ménard civil-engineering company shows the research “is likely to have real applications”. “It’s not just for fun,” he says.
The research will be published in a forthcoming issue of Physical Review Letters.
