23 July 2009

Building cloak protects against earthquakes

An invisibility cloak could protect buildings from earthquakes.

No, it is not about turning something invisible, rather using technology to divert incredibly damaging waves that occur during a quake.

Seismic waves produced by earthquakes include body waves, which travel through the earth, and surface waves, which travel across it. New technology can control the path of surface waves, which are the most damaging and responsible for much of the destruction which follows earthquakes, said researchers at the University of Liverpool in the UK.

The technology involves the use of concentric rings of plastic, which could fit to the Earth’s surface to divert surface waves. By controlling the stiffness and elasticity of the rings, waves travelling through the “cloak” pass smoothly into the material and then compresses into small fluctuations in pressure and density. The path of the surface waves can convert into an arc that directs the waves outside the protective cloak. The technique could work on buildings by installing the rings into foundations.

“We are able to ‘tune’ the cloak to the differing frequencies of incoming waves, which means we can divert waves of a variety of frequencies,” said Sebastien Guenneau, from the University’s Department of Mathematics, who developed the technology with Stefan Enoch and Mohamed Farhat from the Fresnel Institute (CNRS) in Marseilles, France. “For each small frequency range, there is a pair of rings which does most of the work, and these move about a lot—bending up and down—when they are hit by a wave at their frequency.

“The waves are then directed outside the cloak where they return to their previous size. The cloak does not reflect waves—they continue to travel behind it with the same intensity. At this stage, therefore, we can only transfer the risk from one area to another, rather than eliminate it completely.

“This work has enormous potential in offering protection for densely populated areas of the world at risk from earthquakes. The challenge now is to turn our theories into real applications that can save lives; small scale experiments are underway,” Guenneau said.

Seismic waves also include coupled pressure and shear body waves, which are less destructive than surface waves. Guenneau and Sasha Movchan at the University of Liverpool, together with Michele Brun at Cagliari University, designed an “elastic” cloak to protect against these particular seismic waves, and the team is currently seeking a suitable material to accommodate the elastic parameters of the cloak.

For related information, go to www.isa.org/networks.