Posted: July 31, 2009

Earthquake protection through an invisibility cloak

(Nanowerk News) Schools and hospitals could one day be protected from feeling the effects of an earthquake. CNRS researchers at the Institut Fresnel in Marseille (1) have come up with a system that could protect buildings from the most destructive seismic waves. Based on mathematical models, such an 'invisibility cloak' could find applications ranging from the automobile and aviation industries to earthquake protection. The paper appears in the journal Physical Review Letters dated 10 July 2009 ("Ultrabroadband elastic cloaking in thin plates").
The 'invisibility cloak' is a thin plate with a structure that controls the propagation of certain waves so as to deflect them from an obstacle. Thus, such waves will not affect an object placed in the center of this cloak but will simply go round it. This protection takes the form of concentric rings made up of different materials. The whole thing makes up a metamaterial which has properties that are not found in natural materials.
However, the invisibility cloak cannot be used to deflect all types of seismic wave (2).
Surface waves, which generally have a greater amplitude than the other types of wave, produce the most destructive effects in earthquakes. The equations governing this type of wave are invariant under geometric transformation: this characteristic, revealed by the researchers at the Institut Fresnel, enabled them to design the invisibility cloak (3).
Putting it into practice against earthquakes will require working in collaboration with geologists in order to adapt it to specific ground conditions. On a smaller scale, the cloak could do away with unwanted vibrations in the automobile and aviation industries.
In 2008, the same team had already designed an invisibility cloak against waves, with the same effect but working according to a different physical principle. This wave "cloak" is undergoing large-scale trials, and applications under consideration include facilitating fish breeding in tropical areas, and protecting limestone coastlines such as the white cliffs of Dover.

(1) CNRS / UniversitÚ Paul CÚzanne / Ecole centrale Marseille / UniversitÚ de Provence

(2) Seismic waves fall into three categories: pressure waves, shear waves and surface waves.

(3) Surface waves are governed by biharmonic equations resulting from the theory of thin plates.

Pressure waves and shear waves are governed by Navier equations, which are not invariant under geometric transformation.

Source: CNRS
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