'Invisible' Building Design Could Reduce Earthquake Damage

Physicists propose shielding buildings to re-direct vibrations around them.

July 19, 2009— -- WASHINGTON (ISNS) -- Engineers have been developing earthquake-resistant buildings for years, but a group of physicists now believe it's possible to make an entire building effectively disappear from an earthquake's destructive path, avoiding serious damage.

Inspired by the recent development of novel materials that precisely control the flow of light waves around objects, they've shown that the same ideas can work whether the waves make up light, sound or earthquakes.

Earthquakes are some of the most destructive forces in nature. The waves they produce ripple across the earth's surface, much as water waves travel across the ocean. The waves from earthquakes crumple buildings, bridges, and other structures, causing millions of dollars in damage and often death.

Despite efforts to understand earthquakes and reinforce buildings against them, damage from the shaking ground is nearly impossible to avoid. But that may not be the case for long, say a team of physicists in France and the United Kingdom.

Inspired by Invisibility Cloaks

Recently, physicists have been developing better and better invisibility cloaks, which hide an object from sight by causing incoming light waves to bend around the object, and come together behind the object.

Physicists Mohamed Farhat and Stefan Enoch of the Fresnel Institute in Marseille, France, and Sebastien Guenneau of LiverpoolUniversity in England wondered if they could use the same principles to hide an object from the destructive waves produced during an earthquake.

In a paper to be published this week in the journal Physical Review Letters, the three physicists show that the answer may be "yes."

Guenneau said that it's possible to shield an object, even a building, so that an incoming earthquake wave behaves as if the object weren't there. The building in the path of the wave is like a rock in afast-flowing river, he said.

"It's the same picture, the wave pattern, as for a water wave that is propagating in a river, and it's bent smoothly around the rock and will be reconstructed around the rock." The object, or building, is "invisible" to the mechanical waves.

Shield Could Re-Direct Vibrations Around Buildings

A series of concrete rings would surround a building or other structure, forming the shield. The shield would redirect the vibration around the object inside. "Each ring is going to wobble in such a way that the wave will bend around (the object)," Guenneau said.

Earthquake waves come in varying lengths, with many peaks and troughs in a given distance, or just a few. To effectively shield a building from short and long waves that earthquakes generate, several rings could be built around a structure, each "tuned" to a different wavelength.

A 1,000 square foot house, for example, would need a circular shield with a 33-foot radius, which could be built with commercially available concrete. Guenneau suggested that the method might be used to protect a large building like a stadium, where people could seek shelter after an earthquake and be protected by the rings from possible aftershocks.

'Interesting' But Not There Quite Yet

Guenneau warned that there are some limits to what the cloak can accomplish. He and his colleagues could not find a way to shield a structure from the types of earthquake waves that travel below the earth's surface. He noted that surface waves are typically the most destructive in an earthquake.

Jim Beck, an earthquake engineering expert at the California Institute of Technology in Pasadena, wondered if the ring would be worth building if it couldn't protect a structure from different types of shaking, not just different wavelengths. The cloak might only work perfectly in special circumstances, he said.

"It sounds like an interesting idea, but I think there's a long way to go before they get to what they would like to see done," he said.

Guenneau said he hopes that others will take the idea and explore its promising applications. Last year, Guenneau and his colleagues made headlines by building a prototype tsunami invisibility cloak that uses ring-shaped channels to redirect water waves around an object. Now they're probing that idea further in a large-scale experiment.

Not Science Fiction

The reality of making buildings seem invisible to the destructive forces of nature, be they the waves from earthquakes or tsunamis, "seems a bit crazy, but it's not science fiction," Guenneau said. "We gave the people the concept, now people can try to improve it to make it more tractable."