On January 14th, 2018 we learned about

Stands of trees can function as shields against some seismic vibrations

A well-placed tree can do wonders for your home. It can provide shade that lowers your cooling bill, increase property values, and lower stress levels, absorb carbon dioxide, prevent erosion, etc. Researchers are now realizing that a properly-arranged group of trees may even be able to help your home survive an earthquake, based on the same principles that are being used to develop invisibility cloaks.

Rerouting seismic waves

That may sound like some sort of science fiction word salad, but the ideas behind it have been tested in controlled conditions. Lots of work is being done with metamaterials, which can control how light, as electromagnetic energy, interacts with their surface. Instead of reflecting off an object, light can be bent and routed around an object, like water flowing around a rock in a stream. This allows an object to effectively become invisible, as a viewer ends up seeing whatever is behind the cloaked object as if it weren’t there.

When it comes to trees and earthquakes, the trees act like a metamaterial, and the vibrations in the ground act like light, moving around a space without hitting it directly. Researchers first tested this idea by making a grid of posts in the ground, then pumping sound waves through the ground, since an actual earthquake isn’t terribly practical to plan around. As the vibrations moved through the dirt, they would interact with the posts in one of two ways: Smaller posts would shake in response to the incoming wave, dissipating much of its energy straight down. Large posts could vibrate in a way that actually reflected the wave back in the opposite direction. The combination of different sizes of posts, or better yet, trees, could then stop a seismic vibration from reaching a nearby structure at full strength.

Practical earthquake protection?

So how practical is “arboreal shielding” at this point? No tests have been conducted with actual trees and buildings yet, mainly because of the logistics involved. More simulations are being done to better understand optimal spacing and sizing for the trees involved, as the heights of the trees is an important factor in just how well they’ll reroute incoming seismic vibrations. Early simulations have used trees arranged in a grid, but ideally models will be developed that can account for more naturalistic distribution of tree growth, in case this concept is beneficial for more than the most carefully controlled green spaces. Additionally, researchers are finding that it’s easier to block horizontal pulses of energy, such as those found in Love waves, than tremors that also move vertically, as in Rayleigh waves.

Nonetheless, the potential benefits would be very significant. For a 10-story building, a 30- to 50-foot tree could make a huge difference. These trees could be planted around buildings that can’t use traditional seismic reinforcements, such as historical structures and monuments. Even if the trees couldn’t completely protect a structure, they could reduce the amount of protection that would be needed in the building itself, potentially cutting engineering costs, and adding all that shade, erosion control, green space, etc.

My third-grader asked: Wouldn’t wooden buildings work just as well?

Aside from the structural limitations wood creates when compared to concrete and steel, the wood itself isn’t the key ingredient in this seismic shielding. An early test actually replaced the trees or posts with holes in the ground, as the key was their size and placement in the ground. As long as those were properly calibrated to divert vibrations that might match the resonant frequency of the building, no lumber was necessary.

Source: How forests could limit earthquake damage to buildings by Edwin Cartlidge, Physics World

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