Want to protect buildings from earthquakes? Turn the surrounding ground into Swiss cheese. Scientists have for the first time shown that a grid of holes in the ground can act as a kind of seismic wall, a development that could lead to technologies that protect buildings from the dangerous tremors of earthquakes.
"It's very cool stuff," says Ulf Leonhardt, a theoretical physicist at the University of St. Andrews in the United Kingdom who was not involved with the study. "It's a step toward manipulating seismic waves and done in a genius way."
For more than a decade, scientists have been manipulating electromagnetic waves with metamaterials—assemblages of conductors and insulators patterned at length scales shorter than the waves themselves. Metamaterials can change the speed and direction of the waves in bizarre ways, and researchers have used them to funnel light around objects in the first generation of invisibility cloaks. The successes of those experiments raise another question: Can researchers also manipulate the nonelectromagnetic seismic waves set in motion by an earthquake? Computer models imagining a larger metamaterial seemed to suggest they could. But the new work, by a team of engineers from the French ground improvement company Ménard and physicists from Aix Marseille Université in France, is the first to put a seismic wave cloak to the test.
The scientists created their jumbo-sized metamaterial in August 2012 by drilling holes in a thick bed of silt and clay near the city of Grenoble in the French Alps. The cylindrical holes stretched down about 5 meters into the earth, but were also skinny, only 32 centimeters wide. They were arranged in a rectangular grid of three rows of 10 holes each. The holes changed the density and stiffness of the earth and, thus, the speed and direction of vibrations rippling through the ground, forming a seismic metamaterial. The scientists then shook the earth on one side of the grid using a vibrating soil-compacting machine that they had placed underground. That machine created 50 seismic surface waves per second with a wavelength of 1.56 meters—about the same as the distance between the holes, though shorter than typical wavelengths from earthquakes.
Sensors placed throughout the site showed that the waves couldn't get past the grid of holes, bouncing off of it instead, the researchers report in a paper posted on the arXiv online preprint server. The waves just barely got by the second row of holes and couldn't even touch the third row, leaving the ground on the other side unshaken.
The large scale of the experiment really stands out, says Steven Cummer, an engineer at Duke University in Durham, North Carolina. "What this group is reporting, I think that is a pretty important step."
However, the work is not yet the earth-shaking advance that will render earthquakes harmless, says Nicholas Fang, a mechanical engineer at the Massachusetts Institute of Technology in Cambridge. Fang says the new experiment is "exciting" but notes that it does not address the complexities of the interactions between temblors and buildings. For example, at the experiment site the waves had to navigate only fine silty clay, whereas a real earthquake's seismic waves would run through a broad variety of rock, influencing their strength and direction. "I think there's great potential, but we don't have a complete answer for [protecting buildings] yet."