Seismic Invisibility Cloak Will Protect Buildings From Earthquakes

By Sean Kane email: , Feb 13, 2013 12:51 AM EST

French researchers have developed a method to make buildings "invisible" to earthquakes.

The team, made up of researchers from France's Institut Fresnel and the French division of Menard, studied invisibility cloaks that shield an object from light waves, rendering it invisible. The cloaks were made of metamaterials smaller than the wavelength of light.

Metamaterials are artificial materials that have been designed with properties unlike materials appearing in nature. This is achieved not through the actual composition of the material, but rather through its structure, often highly precise and organized. This unnatural form can cause unconventional interactions with energy waves. In some cases, metamaterials can divert a wave around an object, rendering it "invisible" to that energy source.

Light waves are tiny and are able to travel incredibly far distances, making it nearly impossible to create a suitable metamaterial. However, seismic waves are larger and travel shorter distances, making a seismic metamaterial much more possible.

The French were not the first to propose this idea. Physicists at the Australian National University in Canberra and Mokpo National Maritime University in South Korea created a model for a seismic invisibility cloak last year. But the French succeeded in actually building and testing the first seismic cloaking device, reported Phys Org.

A team used a vibraphone to send 50 hertz acoustic waves across a silty basin near Grenoble, France. They created a metamaterial by drilling holes into the basin. There were three rows of 10 holes, each 320 millimeter hole separated by 1.73 meters.

The system worked remarkably well. Energy levels near the source of the waves doubled, indicating that the holes (the metamaterial) were reflecting the waves back to their source.

Much more research is needed before a seismic cloaking device can actually be used to protect a building from an earthquake. Seismic waves caused by an earthquake are highly unpredictable, so the distance needed between the holes in the array would be equally difficult to determine. Equally challenging is deflecting the seismic waves in a way that wouldn't damage nearby buildings. A potential solution would be to develop a metamaterial that absorbed, rather than reflected, the seismic waves.

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