How are those toothpick skyscrapers in Manhattan earthquake proof?
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Hooray for tuned mass dampers! I can never hope to explain it as well as [Practical Engineering ](http://youtube.com/watch?v=f1U4SAgy60c&ab) did, but here goes anyway.
The tuning part is easy. Every pendulum has a frequency that it likes to oscillate at given by a fairly simple function using only length and gravity as variables. Gravity tends not to change noticably, so changing the length changes the frequency. A building is also a pendulum, just inverted. It’s well-anchored at the bottom, but wind (or earthquakes) can cause the top to move because steel and concrete are not perfectly rigid materials. All materials have some amount of flex. So you tune the pendulum to the same frequency as the building. This makes it fairly easy for the building to give some of its energy to the pendulum through a mechanism called resonance. The same reason armies must break step when marching over a bridge. And the same reason pushing a kid on a swing should happen at the same frequency as the swing happens.
The damping part is still not the worst, but it’s more complex than the resonance part. Once we’ve transfered energy to the pendulum, we need to bleed it off slowly so as not to cause too much deceleration (just as destructive as too much acceleration. As they say, it’s not the fall that kills you, but the sudden stop at the end). So using any number of methods from friction brakes to hydraulic dampers, they add resistance to motion which turns the energy into heat. Under damped systems can’t get rid of energy as quickly as the earthquake can add it, meaning the oscillations get worse until it bends beyond the yield point. Overdamped systems brake too hard, basically causing whiplash for the building which is also bad. So right in the middle is a sweet spot called critically damped. And that’s what they’re going for.
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