In recent years, cloaking technology has taken the world of physics and engineering by storm. The possibility that any object can be hidden from incident waves has numerous applications, both practical and fantastical.
One of the more interesting is the possibility of protecting buildings from seismic waves. The idea here is to surround a building, or at least its foundations, with a metamaterial that steers seismic waves around the structure. Various groups have explored ways of doing this.
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Today, however, Sang-Hoon Kim at the Mokpo National Maritime University in South Korea and Mukunda Das at The Australian National University in Canberra, suggest another idea. They point out that while seismic cloaks can protect buildings, they steer waves towards other buildings. “The cloaked seismic waves are still destructive to the buildings behind the cloaked region,” they say.
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Instead, they suggest that metamaterials could instead dissipate the energy in seismic waves by converting them into evanescent waves, which die down exponentially as they travel.
They calculate the properties of such a metatmaterial and how it might be constructed with a basic repeating unit in the form of a concrete cylinder some 18 metres in diameter, with four perpendicular holes in its sides (see picture).
These cylinders, perhaps varying in size to absorb a range of seismic wavelengths, would need to surround the foundations of a building in cylindrical shells some 60 metres across.
That needn’t be prohibitively expensive but it would be a big structure that could only be constructed around isolated buildings (thereby somewhat negating the supposed benefit that other buildings in the earthquake ‘shadow’ might also be protected).
That would be suitable, they say, for structures deemed vital for society, such as power plants, dams, airports, nuclear reactors, oil refining complexes and such like.
But the work leaves one important question unanswered: what happens to the seismic energy that is dissipated?
Kim and Das suggest that the absorbed energy will turn into sound and heat. However, they offer no evidence to suggest that their concrete cloaks could cope with the energy releases involved in big earthquakes. How much sound and heat is this equivalent too?
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That’s something they’ll need to work on. A convincing argument that the metamaterials involved can cope with this kind of dissipation will be crucial if anybody is to run with this idea.
