Image: Amy Qualls/Flickr
The United States is currently gripped in a bout of earthquake mania, following a series of significant tremors in the West. And any time Yellowstone, LA, or San Francisco shakes, people start to wonder if it's a sign of The Big One™ to come. Yet even after decades of research, earthquake prediction remains notoriously hard, and not every building in quake-prone areas has an earthquake-resistant design. What if, instead of quaking in our boots, we could stop quakes in their tracks?
Theoretically, it's not a crazy idea. Earthquakes propagate in waves, and if noise-canceling headphones have taught us anything, it's that waves can be absorbed, reflected, or canceled out. Today, a paper published in Physical Review Letters suggests how that might be done. It's the result of French research into the use of metamaterials—broadly, materials with properties not found in nature—to modify seismic waves, like a seismic cloaking device.
The team, which included a duo of civil engineers from Ménard, a French industrial firm, and a pair of researchers from Aix-Marseille University, notes that "in theory, it seems realistic to influence seismic waves passing through an artificial anisotropic medium," but real-world soil doesn't necessarily have the perfect properties for quake-stopping. So to see what they were working with, the team found a test site with homogenous clay soil and set it vibrating with a vibrocompaction probe.
The goal was to simulate Rayleigh waves, which are the type of wave caused when energy released from tectonic plates deep in the Earth reaches the surface and refracts along it. As Ping Sheng notes in an accompanying APS article, Rayleigh waves "have a much slower speed, hence a smaller wavelength, than underground waves and are usually the most destructive seismic waves."
The research team attempted to counteract those waves by drilling bore holes (sized 0.32 meters in diameter, 5 meters deep) in a grid pattern spaced 1.73 meters apart, which was designed to approximate the wavelength the 50 Hz waves produced by the vibroprobe. The goal was to see if by breaking up the uniformity of the clay soil, waves could be reflected.
The figures below show how seismic waves propagate in an alluvium basin like the test site, and how the grid was drilled:
Image: Brûlé et. al
In the field, the experiment looked like this (photo is the reverse of the image above), with the borehole mesh stuck in between the vibroprobe and a series of sensors:
And the result? The image below shows the relative change in wave strength before and after the grid was drilled. As you can see, in the region where bores were drilled, wave strength dropped immensely. Near the source, the strength increased, as waves were reflected backwards.
In short, the test worked. "The signal hardly exceeds the second row of boreholes in [the figure above], showing the efficiency of this device for this geometry and a 50 Hz source in soft soils," the authors write.
Note that they're very specific: While this test provided good evidence that seismic waves can be managed, it's still a very particularized case. "This numerical investigation suggests that one should be extra cautious with anti-earthquake designs before some definite conclusions are drawn," the authors write.
"The concept of a seismic cloak to protect a building from earthquakes would at least require in-depth analysis to achieve some judicious—but not necessarily periodic—arrangement of boreholes," they continue.
Now, taking a design that's successful under set, experimental conditions and making it work under actual environmental conditions, where earthquake epicenter and magnitude is unpredictable, is a huge challenge. There's another potential problem, as Sheng explains: "It is worth noting that the scheme blocks and cloaks seismic surface waves by redirecting the wave energy elsewhere, which could worsen the earthquake’s effects in the area surrounding the protected region."
But even if the results are largely limited to those specific conditions, it's still a sign that seismic cloaking is possible. Elastic cloaking has been a popular topic in physics research in the last few years, and that's translating into the use of metamaterials to influence seismic waves.
In 2012, the duo of Sang-Hoon Kim and Mukunda P. Das published research demonstrating a device that "reduces the amplitude of the wave exponentially." In another 2013 paper, they found that by "designing huge empty boxes with a few side-holes corresponding to the resonance frequencies of seismic waves and burying them around the buildings that we want to protect, the velocity of the seismic wave becomes imaginary."
Again, the study isn't saying that all we need to do is drill some holes around fault lines to create earthquake mirrors. However, the mere fact that experiments are showing positive results is heartening, especially with further development of designs aimed at dampening, and not just reflecting, seismic waves. While still a long ways off, it's nevertheless fascinating to think that one day we may just be able to build a seismic cloak.