Earthquake is a problem of millions of people. According to the weather data and world statistics, this natural phenomenon with magnitude 8 and higher takes place at least once a year. More than 100 million people affected by such a disaster since 1983 until nowadays. Most of them became the victims because of the destroying buildings during earthquakes.
However, French scientists found out how in the nearest future the buildings could be isolated from earthquakes by being placed behind rows of trees. They have shown that certain seismic waves, known as Love waves, could be diverted away from the Earth’s surface as they pass through a forest containing trees of a certain height. The forest acts like a metamaterial – an artificial structure usually used to steer electromagnetic radiation around objects.
The interaction of the wave disturbance with the material depends on many parameters. In metamaterials it can be defined by element’s location, but not by the compound. For example, there are some metamaterials developed for the electromagnetic radiation. This invention allows distorting the trajectories of the light rays. In the nearest future, such a thing will disguise the objects from the detection. The physicists can develop the similar things for the mechanical waves in particular for the seismic waves. The idea here is to use arrays of suitably-sized objects either below or above ground – holes or posts of some kind – to divert seismic waves around vulnerable buildings.
How does it work?
Sébastien Guenneau, CNRS Research Director at Fresnel Institute in Marseille, believes whereas passive isolation typically targets a building’s resonant frequency, seismic cloaks could, in principle, be broadband. He says: «Such a thing would allow extensions to be added to buildings and could be used to protect historical monuments that cannot be altered. »
Sébastien Guenneau was part of a research team that demonstrated the basic principle of such “seismic cloaks” in 2012 by drilling a 2D grid of 5 m-deep boreholes into topsoil and measuring the grid’s effect on acoustic waves generated close by.
The scientists found out that just a couple of rows of boreholes could reflect around half of the wave energy back towards the source. A few years later, however, another group, which included Guenneau and Phillippe Roux from the University of Grenoble, showed that nature could do a similar job. They showed that a small pine forest in Grenoble could reflect most of the energy within certain frequency bands of Rayleigh waves, which travel just under the surface and are generated by the wind and vibrations from nearby road works.
In the new theoretical research, the physicists conceptualized that with help of the trees people can protect themselves from the seismic Love waves.
Like Rayleigh waves, these waves travel just below ground and are generated when seismic waves travelling away from an earthquake’s epicentre reach the Earth’s surface. But, whereas Rayleigh waves have both a horizontal and vertical motion, Love waves – which can severely damage a building’s foundations – cause a side-to-side, purely horizontal shaking. Like Rayleigh waves, Love waves should set up vibrations in tree trunks. The researchers identified a new kind of wave that they dub a “spoof Love wave” generated when a seismic wave propagates along the wooded ground, whose topsoil yields lower shear velocities than does the bulk. This wave is mathematically analogous to an electromagnetic wave known as a “spoof plasmon”, which can propagate along a metal surface studded with metallic pillars – the ground playing the role of the air above the surface while the trees stand in for the pillars.
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