Earthquakes: from Fukushima to Haiti, they leave behind nothing but death and devastation. A tool for predicting earthquakes could save lives, but infrastructure would still be at the mercy of plate tectonics. To make us even more helpless, the forces driving tectonic plates are enormous—it seems unlikely that prevention is ever going to be realistic. So we can't predict them, and we can't stop them. But what if we could hide from earthquakes?

On the face of it, the idea sounds ridiculous. The shaking of an earthquake is due to the propagation of pressure waves along the surface of the Earth. Surely, the only way to hide from an earthquake would be to leave the Earth, right? Not exactly. In optics, we know how to hide an object from light. These invisibility cloaks have been demonstrated, and they even sort of work. And a wave is a wave, so maybe, a group of researchers thought, this might be a fruitful approach for engineering.

Going meta

The key here is a concept called a metamaterial. To understand metamaterials, let's jump back into the world of optics. The way that light travels through a material is determined by nature—light may bend, slow down, or speed up as it enters different materials, depending on their properties. Materials do not have infinite variation, so it's not possible to use material properties to get light to flow around an object to make it disappear from view, for instance.

This is where metamaterials come in. Material properties are determined by the material's atoms and molecules. But the wavelength of light is much longer than a single atom or molecule, so if we structure a material on the scale of the wavelength, we can make light behave as if it were moving through an impossible material. Butterfly wings and beetle shells take advantage of this principle to produce iridescent colors.



To get an idea of how this works, imagine that we take two materials—say glass and air—and stack thousands of layers on top of each other. Then we attempt to shine light through the stack. At each interface, some light is reflected. But there are thousands of such interfaces, so all the light bounces back and forth between all these different interfaces—there are so many interfaces that it's highly unlikely that a single photon will make it through without being reflected.

We have light "trapped" inside the stack. All of the light waves mix together and interfere, and this interference produces a peculiar effect. Imagine that the glass layer and the air layers are exactly one-quarter of a light wavelength thick. In this case, the reflections in one direction (back out of the stack) add up in phase, and the light is all reflected—we have created the perfect mirror for just one wavelength. In contrast, if the layers are a half-wavelength in thickness, the reflections for light traveling through the stack add up in phase, giving us perfect transmission—again, for a single wavelength. This is the principle for anti-reflection coatings on camera lenses and glasses.

That's a one-dimensional example, but similar behavior can be obtained in two dimensions by drilling holes in a sheet of glass, and in three dimensions by stacking glass rods.

The simple case of creating perfect reflectors and perfect transmission was just the beginning of the story. To create an invisibility coat, the internal placement of the materials is designed such that all light that would enter the hidden area experiences destructive interference, while light that passes around the hidden area experiences constructive interference, meaning that all the light flows around the hidden area. The design of such structures was very difficult, but by borrowing tricks from general relativity, optics researchers can now work backward from a desired behavior to the required metamaterial properties.

Back to the shaking earth

The cool thing about physics is that ideas from one branch can be transferred to another with relative ease. Waves are waves, regardless of whether they are light waves in glass, electrons in a crystal, or pressure waves generated by an earthquake. That means that metamaterials should work for surface waves as well.

The researchers set out to test this theory. They drilled an array of 30 cm diameter holes into the ground to a depth of 5 m. On one side of this array, they placed a thumper operating at 50Hz—well, it was probably a little more sophisticated than a thumper, since they needed something that emits 50Hz waves as purely as possible. To detect the propagation and strength of the surface waves, an array of velocimeters was placed around the source. Because experimental physics is a bit noisy, the researchers examined the difference between the surface wave strength before and after the bore holes were drilled.



Further Reading Bending space to create a super-antenna

The results show that it worked reasonably well, with the bore holes attenuating the surface wave by a factor of five or so toward the end of the array. Correspondingly, the strength of the surface waves in the vicinity of the source increased by a factor of five.

On the one hand, it's unsurprising that this experiment worked—waves are waves, and the physics remains the same. But in practice, it's incredible that it worked. Unlike optical experiments where we have beautiful materials that have consistent properties, the ground is very messy. There are variations in density and material types. Inclusions of rocks in dirt, for example, mean that the surface wave is already traveling through a metamaterial—a random metamaterial, generated by the geology of the region. So taking some basic measurements, constructing a regular array of holes, and observing that it works was quite astounding.

Of course, this success is partially due to a good choice of location; making it work in areas where the structure of the ground is more variable will be much more difficult. Not only that, but as the authors point out, the vibrations were amplified in the vicinity of the source—actually using a cloak on a building could be highly detrimental to surrounding buildings. It's pretty clear that such an approach would require citywide (or even region-wide) planning, with a detailed picture of the ground's structure across the entire area.

Physical Review Letters, 2014, DOI: 10.1103/PhysRevLett.112.133901