Earthquake-proofing cities with engineered materials
Engineers in future could provide a solution to protect cities from devastating effects of earthquakes by building structures into the earth that dissipate powerful shock waves.
The approach has been tested in theory and experiments. It relies on the so-called metamaterials – specifically designed materials with properties not experienced in nature. Metamaterials are better known from photonics, where they can divert electromagnetic waves around objects to make them appear invisible.
Researchers have already tested that a similar approach could work with seismic waves, such as those that virtually flattened entire towns in the Italian region of Umbria this week, killing hundreds of people.
In an article in the New Journal of Physics, a team of European researchers described computer modelling experiments that prove that metamaterials could protect buildings and their inhabitants from catastrophic earth tremors.
The researchers propose an array of ground-based cavities filled with specially-designed metamaterials that could, in theory, be built around buildings or entire settlements. Unlike existing building techniques that help significant objects such as bridges withstand earthquakes, the metamaterial shield could be implemented retrospectively. That means it could be used to protect historical buildings or entire areas without any modifications to the buildings themselves.
The most efficient approach tested by the team from Italian universities of Torino and Trento and London’s Queen Mary University, requires two to three rows of equally spaced cross-shaped cavities to be dug into the ground. The cavities would subsequently be filled with the earthquake-attenuating metamaterials. The seismic waves, once they reach the shield, would be modified and emerge on the other side in a less dangerous form.
"The exact dimensions will depend on the soil type and the frequency range of the shield," explained Marco Miniaci of the Universities of Torino and Le Havre. "For sandy conditions and low-frequency seismic excitations, the width, spacing and depth of the cavities, which should be lined with concrete to prevent the surrounding soil from collapsing, could reach 10m."
In the simulations, the researchers further determined that adding further cylindrical cavities about 2m in diametre would further enhance protection.
"The next steps should involve experimental tests using scaled models in specialised geotechnical seismic and vibration labs," said Miniaci. "This would provide further validation of the proposed structures and help to build on earlier work in the field."
The idea to use metamaterials to virtually dissolve seismic waves is being studied by multiple research teams. In an article published in Scientific Reports in March this year, scientists from Imperial College London, Joseph Fourier University in Grenoble and University of Liverpool, described an experiment which proved that it is possible to use a large-scale metamaterial wedge, similar to a refraction prism, to change the so-called Rayleigh waves into harmless bulk shear waves.
Rayleigh waves are a specific type of waves generated in earthquakes that travel over the surface and spread the tremors away from the epicentre. These waves are similar to ocean surface waves and tend to cause the most damage.
The metamaterial wedge turns the devastating surface waves into what the researchers described as a ‘seismic rainbow’ of less severe tremors.
The concept of a metamaterial-based ‘seismic cloak’ protecting buildings from earthquakes has previously been explored by a team from the Institut Fresnel in France.