Imagine we could scale up noise cancellation technology in earphones and apply it to earthquake prone zone, what challenges beside engineering and cost would make such a machine impossible to realise? If we could somehow mitigate or minimise the shaking so that the buildings can never hit resonance freq and thus would not collapse, is such technology too far fetch?
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2$\begingroup$ I understood your question as a noise cancelllation technology at the base of each buildings to protect them but L.Dutch and Arcanist Lupus understood the question differently. Could you give a bit more precision on if you're asking about a local cancellation or a broad one ? $\endgroup$– EchoxDec 16, 2019 at 13:23
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$\begingroup$ @Echox: since every object have unique resonance frequency thus if we can avoid it somehow we could eliminate the aftermath. $\endgroup$– user6760Dec 16, 2019 at 13:36
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1$\begingroup$ Earthquakes aren't just a think that shakes the ground. They move the ground. Remember how fast tectonic plates move? That's an average; what actually happens is that parts of the plates get stuck, bend and compress... until the pressure gets too great, and the whole stuck bit jumps ahead violently. So if you have a plate that moves at 10cm/year, which (in some place) gets stuck for a hundred years, when it finally gets "unstuck", it can move ten meters in one leap (in practice, it's usually less - not all of the bending gets "restored", so some of the length of the plate is lost). $\endgroup$– LuaanDec 18, 2019 at 8:17
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$\begingroup$ "Never hit resonant frequency" doesn't guarantee no collapse. $\endgroup$– Geoffrey BrentDec 18, 2019 at 9:59
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5 Answers
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what challenges beside engineering and cost would make such a machine impossible to realise?
The same reason for which you have noise cancelling earphones but not noise cancelling speakers.
Earthquakes produce different types of seismic waves, like P-wave, S-wave and many more, each traveling with a different velocity.
If you want to nullify the disruptive effects of the earthquake in a certain location, you will have to produce waves of equal amplitude and opposite phase in that location. But then those waves would travel across the crust, finding nothing to nullify them. You will end up nullifying the damage in a limited area but doing more damages all around it.
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1$\begingroup$ Active acoustic cancellation is employed in industrial environments where large areas experience sufficiently predictable periodic background sound. Headphones work quickly in a localized area because one sample point is sufficient - large areas have to be sampled and averaged over a longer term, AND account for the reductions the make in ambient sound. Your final point is genuinely correct, but your opening premise is not. $\endgroup$– user8827Dec 16, 2019 at 15:08
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9$\begingroup$ You've inspired me to go look up the difference between noise cancelling and white noise, because I thought they were basically the same idea, and there are white noise machines. I found out that not only was my own definition of white noise wrong (getting it confused with sound masking), but that it's different from noise cancelling because it makes no effort to combat specific sounds and instead just kinda covers all frequencies. I've had an interesting morning, thank you. $\endgroup$– Davy MDec 16, 2019 at 16:21
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$\begingroup$ you also have things like shear wave roll which can't be canceled by sound because they are not compression waves . $\endgroup$– JohnDec 17, 2019 at 3:53
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1$\begingroup$ The reason there are headphones but not speakers is that active cancellation works by overlaying waves. These waves originate from a different point, so the cancellation effect can be observed only in certain spots, and the headphone is built so one of these spots is in the ear. At other points, the amplitude will be doubled. $\endgroup$ Dec 17, 2019 at 10:35
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This technology, called "Tuned Mass Dampers", is in practical use for decades, see the corresponding Wikipedia article. In skyscrapers, they do exactly what is asked for: changing the resonant frequency of a building (by means of a pendulum, for example) in a way that it never resonates to the frequency typical for an earthquake.
In regard to earthquakes, the article states:
The seismic waves caused by an earthquake will make buildings sway and oscillate in various ways depending on the frequency and direction of ground motion, and the height and construction of the building. Seismic activity can cause excessive oscillations of the building which may lead to structural failure. To enhance the building's seismic performance, a proper building design is performed engaging various seismic vibration control technologies. As mentioned above, damping devices had been used in the aeronautics and automobile industries long before they were standard in mitigating seismic damage to buildings. In fact, the first specialized damping devices for earthquakes were not developed until late in 1950.
A small-scale experimental setup is described (including video) by the Practical Engineer.
EDIT: If you need/want an active setup (in the sense that some sort of controller predicts the next movement and actively tries to cancel the shaking using some sort of actuator, like in noise cancelling), I imagine that you could just extend the pendulum variant of a Tuned Mass Damper with really strong motors that react to an array of seismic sensors around the building (assuming P-waves in granite at 5000m/s, they need to be in about 5 km distance if the sensors, communication line and the motors together have a reaction time of 1 second). This would minimise the shaking further, but a misinterpretation of the sensors then would induce active shaking.
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1$\begingroup$ This answer is worthy of a bounty. $\endgroup$ Dec 16, 2019 at 14:35
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4$\begingroup$ +1 Not exactly the solution the OP had in mind. This is comparable to ear plugs vs. active noise cancelling. This dampens the resonance (like ear plugs) and shifts the resonant frequency of the building. Nevertheless, this is the only realistic solution. $\endgroup$ Dec 16, 2019 at 19:25
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1$\begingroup$ The technology described in this answer is a completely different, nearly unrelated technology from the technology asked about in the question. $\endgroup$ Dec 17, 2019 at 2:43
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5$\begingroup$ @aidan.plenert.macdonald Judging from OP's sentence "If we could somehow mitigate or minimise the shaking so that the buildings can never hit resonance freq", I surmise it is exactly what is asked for. This technology minimizes that part of the shaking that is induced by resonance. It is not an "active" technology (in the sense that it does not have a controller that predicts the next movement and actively tries to cancel it out using some sort of actuator, like in noise cancelling), but the point is: it does not need to be an active technology to "mitigate or minimize" the shaking. $\endgroup$– orithenaDec 17, 2019 at 10:16
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1$\begingroup$ @aidan.plenert.macdonald Anyhow, I extended the solution to active dampening. $\endgroup$– orithenaDec 17, 2019 at 10:48
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You can't cancel the stress
L.Dutch provides a good answer as to why your specific technology doesn't work. But more fundamentally, there's a bigger problem with any sort of reactionary earthquake prevention tech.
The shifting of Earth's crust on the mantle causes buildup of enormous stresses. An earthquake is what you get when a bunch of those stresses release all at once. If you could push a button and "cancel" that earthquake, then those stresses would still be there, and soon you'll have another earthquake on your hands, probably even bigger than the one you prevented. Or, as L.Dutch described, you might prevent the effects of the quake in one area and get increased damage in other areas.
In theory, a technology could be created that would bleed off the stresses gradually so as to prevent them from snapping all at once - effectively replacing single large earthquakes with thousands of micro quakes. But it would be a very different technology than the one you describe.
Incidentally, there is a related concept in forest management, where small natural fires are allowed to burn freely to prevent the proliferation of undergrowth that could fuel unstoppable firestorms.
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1$\begingroup$ Putting your forest analogy into effect on the earth's tectonic plates would mean massive, massive cuts into the crust. Which would render the earth uninhabitable. It's the equivalent of flattening a house with a nuke. It's flat, but now you have bigger problems. $\endgroup$– MastDec 16, 2019 at 14:03
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4$\begingroup$ @Mast None of that is necessarily true. Earthquakes generally occur because plates that are trying to slide against each other get stuck. The longer they're stuck, the more stress builds up. The 'forest' analogy in this case would just involve identifying the places where things are stuck and breaking that rock up, deep underground, effectively replacing really big, infrequent earthquakes with much smaller, more frequent ones that allow the plates to move more smoothly. Nukes deep underground would work just fine, and wouldn't make the surface even slightly less habitable. $\endgroup$ Dec 16, 2019 at 14:43
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$\begingroup$ @MorrisTheCat Fair point, although I imagine it would be quite a feat to pull that off. $\endgroup$– MastDec 16, 2019 at 14:48
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$\begingroup$ @Mast as opposed to massive cuts in the crust? Even "noise cancelling" a quake requires generating energy on the scale of the quake. All of these are extraordinary. $\endgroup$ Dec 16, 2019 at 15:27
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2$\begingroup$ I'd also like to point out that this applies to avalanches as well. Small detonations with dynamite tossed from helicopters prevent massive buildups that might lead to devastating avalanches on the towns below. $\endgroup$– coblrDec 16, 2019 at 23:51
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Even if it works, consider that if this were to work in a certain area where the local waves are perfectly phased to cancel out, other nearby areas could experience constructive interference. Other neighborhoods might not appreciate that ;)
Constructive vs destructive interference in 2D space
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2$\begingroup$ Something about the premise of the question was really bothering me, but I couldn't quite put my finger on what. Thanks for this post! $\endgroup$ Dec 17, 2019 at 20:24
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To prevent buildings from experiencing shaking due to an earthquake, Its not necessary to cancel the waves (active isolation). A much easier (totally passive) approach is to put the buildings on a giant shock isolator which has a lot of attenuation at the typical frequencies produced by earthquakes. Effectively you would be shifting the resonance frequency of the system to avoid damage.
This basically amounts to putting the buildings on some sort of stiff spring, and using the combination of the spring and the buildings mass to attenuate the incoming shaking.
