28 April 2014

Earthquake cloaking could protect cities from temblors

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French researchers say they could soon offer quake-proofing with seismic ‘invisibility cloaks’ that cancel out shock waves. Philip Dooley reports. 
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A collapsed building in Christchurch, New Zealand, after the 7.1 magnitude earthquake in 2010. A new means of dispersing seismic shockwaves could save cities from similar damage.
CREDIT: JOSEPH JOHNSON/GETTY IMAGES

Nuclear power plants, and perhaps even whole cities, could be protected from earthquakes using “seismic invisibility cloaks” if researchers’ theories are proven right. Scientists from the Institut Fresnel in Marseilles have teamed up with geo-engineering company Ménard to show that by drilling boreholes in a precise pattern into the ground, they could divert seismic waves that travel through the earth’s surface during earthquakes.

“You can build on this knowledge to create an invisibility cloak which will actually protect a specific site from seismic waves,” says the leader of the team, physicist Sebastien Guenneau, who published the work in Physics Review Letters in late March. The team is now planning to test whether they can defend an area about the size of a football field from a simulated earthquake.

How can a set of boreholes deflect an earthquake? It turns out they create an invisibility cloak for seismic waves in the same way that other materials can create an invisibility cloak for light waves – the cloaked car in the Bond movie “Die another day” was not total fantasy.

In the early 2000s John Pendry from Imperial College London, shook the world when he proposed an invisibility cloak based on a material that deflected light. In 2006 he actually built one. The so-called “metamaterial” guided microwaves around a copper cylinder. Anyone looking at it through a microwave detector saw empty space. Researchers are now developing metamaterial cloaks that work with visible light.

To produce invisibility, metamaterials rely on geometric structures smaller than the wavelength of the light they are deflecting. In the case of earthquakes, the waves are seismic, and the structures Guenneau’s team used were boreholes in the soil. The team calculated that for earthquake wavelengths of around 1.5 metres, the boreholes would need to be 30 cm wide and spaced 1.73 metres apart. In theory each hole would scatter the incoming seismic wave, and the precise placement of the boreholes would guide the scattered waves so they cancelled each other out.

A similar approach had been successfully used to block soundwaves from travelling through metamaterials. But, says Guenneau, “Soil is a different story. Its properties are difficult to characterise, and depend on different things, such as the weather! It makes the mathematical models much more difficult.”

Some scientists initially ridiculed the idea. But Guenneau fortuitously met Stephane Brûlé, an open-minded geo-engineer who was also influential enough to persuade his company, Ménard, to collaborate.

‘If your aim is to stop an earthquake you don’t get to choose the frequency.’

So it was that the team of twenty studied the weather forecasts carefully and chose three days of consistent sunny weather in August 2012, to do the experiment at a site near Grenoble, at the foot of the French Alps.

Using a “vibroprobe” that vibrates the ground at 50 times a second they first measured the propagation of the waves in the undisturbed soil. Then, after carefully drilling three rows of five metre deep boreholes, they repeated the experiment. Just as the model predicted, most of the wave energy was reflected by the bore pattern: only 20% reached the detector.

“It’s interesting because these are the first experimental results on this topic,” says physicist Boris Kuhlmey who studies electromagnetic metamaterials at the University of Sydney. But he points out the bore pattern the team used would only reflect seismic waves of a specific frequency. “If your aim is to stop an earthquake you don’t get to choose the frequency,” he says.

However, Guenneau’s mathematical models also suggest a pattern that might provide a “zero stop band”, which can stop a wide range of earthquake waves. “That would be really key to get it to work well. Maybe it’s possible, on the scale of a city, to diminish the impact of an earthquake considerably,” says Kuhlmey.

Guenneau’s next experiment will certainly push the boundaries. The team will inflict magnitude 6.0 earthquakes, with frequencies of between two and 12 vibrations per second, on their test site, which they plan to protect by a ring of boreholes 200 metres in diameter.

“It would be a dream for me to see this done for real one day, not just tests,” muses Guenneau. In the meantime he is already turning his skills to other problems, such as tsunami control. “Imagine some columns of wood, 200 m from the sea shore, arranged in a similar fashion to the bore holes in the seismic experiments. The effect will be that you deflect the tsunami to a non-sensitive coastal area.”

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