Can a ‘seismic cloak’ protect us from earthquakes?

According to a new study from French scientists, in the future, we may be able to protect our cities from earthquake damage by surrounding them with a 'seismic invisibility cloak'.

Scientists studying invisibility cloaks have been using metamaterials — materials engineered to have repeating micro-scale or nano-scale structures. By making the scales of these structures so small, they can influence waves that are much larger, like light and sound waves, redirecting them around objects to effectively make the objects invisible to those waves. However, when you're dealing with much larger waves, you can make the structures much larger as well; you just need to keep roughly the same difference in scales between the structure and the waves.

That's what researchers at the Institut Fresnel and the French geotechnical company Ménard have done with one of their latest project. Since they're dealing with seismic waves from earthquakes, which can have very large wavelengths, they were able to scale up the structure of their 'metamaterial' design to that of between 10s of centimetres to metres. To simulate a 'seismic cloak' for a large building, they dug a grid of 5-metre-deep shafts in the ground, each about 32 centimetres across. On one side of the grid they placed an array of sensors, with more sensors in between the bore holes and surrounding the area, and on the other they put a source of vibrations under the ground to generate a set of fake seismic waves.

The plots of their results show how the vibrations (in shades of red) spread out from the source, but are the vibration waves are reflected by the grid of bore holes so that they can only spread so far through it.

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This is just an early stage of the research into this, so we won't be seeing anything soon from it. There are still plenty of factors to look into, such as the exact arrangement of the bore holes to give the best results. Also, there's the problem of where the vibration energy goes when it's reflected, as it could amplify the seismic waves in other areas, causing even more damage there.

"Whatever you don't transmit, you reflect," said Andrea Alú, an expert on cloaking technology at the University of Texas, according to Physics World. "If you are in a crowded environment, you may cause problems for other buildings."

The researchers are aware of the problem and are apparently working on ways of using the idea to control where the redirected energy goes, ensuring that it doesn't affect the buildings around the one being protected.

(Images courtesy: Institut Fresnel/Ménard/Physical Review Letters)

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