The wind may be gusty, but it doesn’t push in a regular rhythm that could match the building’s natural frequency, so there’s a limit to how bad the swaying can get. But the jolt of an earthquake sends out ripples in the ground, huge waves traveling out from the epicenter, slowly tipping the earth from side to side. What happens when a tall building meets an earthquake?
On the morning of September 19, 1985, Mexico City started to move. Tectonic plates underneath the edge of the Pacific Ocean, 217 miles away, ground over each other to generate an earthquake of magnitude 8.0 on the Richter scale. In Mexico City, the shaking lasted for three to four minutes, and it tore the city to pieces. It’s estimated that ten thousand people lost their lives, and massive damage was inflicted on the city’s infrastructure. Recovery took years. The US National Bureau of Standards and the US Geological Survey dispatched a team of four engineers and one seismologist to assess the damage. Their detailed report showed that a horrid coincidence of frequencies was responsible for a lot of the worst damage.
First of all, Mexico City sits on top of lake-bed sediments that fill a hard rock basin. The earthquake-monitoring devices showed beautiful regular waves with a single frequency, even though normally earthquake signals are much more complex than that. It turns out that the geology of the lake sediments gave them a natural frequency of oscillation, and so they had amplified any waves that lasted about two seconds. The whole basin had temporarily become a tabletop shaking at almost exactly one single frequency.
The amplification was bad enough. But when they looked at specific damage, the engineers discovered that most of the buildings that had collapsed or were badly damaged were between five and twenty stories high. Taller or shorter buildings (and there were plenty of both) had survived almost untouched. They worked out that the natural frequency of the shaking closely matched the natural frequency of the midsized buildings. With a long-lasting regular push at exactly the right frequency, these buildings had been twanged like tuning forks, and they didn’t stand a chance.
These days, controlling the natural frequency of buildings is taken very seriously by architects. Management of shaking is even sometimes celebrated. In the Taipei 101, a 1670-foot monster in Taiwan that from 2004 to 2010 was the largest building in the world, the place to visit is the viewing galleries on the 87th–92nd floors. This section of the building is hollow, and suspended inside it is a 660-ton spherical pendulum, painted gold. It’s beautiful and weird—and practical. It’s there not just as an aesthetic quirk, but to make the building more earthquake-resistant. The technical name for it is a tuned mass damper, and the idea is that when there’s an earthquake (a common occurrence in Taiwan), the building and the sphere swing independently. When an earthquake starts, the building sways one way and pulls the spherical pendulum sideways too. But by the time the sphere has moved in that direction, the building has swayed back the other way, and is now tugging the sphere back. So the sphere is always pulling in the opposite direction to the movement of the building, reducing its sway. The sphere can move 5 feet in any direction and it reduces the overall oscillation of the whole building by about 40 percent.# The humans inside would be far more comfortable if the building never moved. But earthquakes shove the building out of equilibrium so that it has to move. The architects can’t stop that happening, but they can tweak what happens on the return journey. The occupants of the building have no choice but to sit tight as the huge tower sways past the equilibrium position and back again, until the energy is lost and serene stasis is restored.
THE PHYSICAL WORLD is always ticking along toward equilibrium. This is a fundamental physical law, known as the Second Law of Thermodynamics. But there’s nothing in the rules to say how quickly it has to get there. Every injection of energy kicks things away from equilibrium, moves the goalposts, and the winding down has to start all over again. Life itself can exist because it exploits this system, using it to shunt energy around by controlling the speed of flow toward equilibrium.
Plants still sneak into my life, even though I live in a big city. From my office, I can see bright sunlight falling on the lettuce seedlings, strawberry plants, and herbs on the balcony. The light falling on the wooden decking is absorbed by the wood, which heats up, and that heat is eventually dispersed through the air and the building. Equilibrium is reached quite quickly, but nothing very exciting happens along the way. But the sunlight that falls on those coriander leaves is entering a factory. Instead of being converted straight into heat, it’s diverted to serve the needs of photosynthesis. The plant uses the light to boot molecules out of equilibrium, and so keeps the energy for itself. By controlling the easiest path back toward equilibrium, the machinery of the plant uses that energy in stages, to make molecules that act as chemical batteries, and then uses those to convert carbon dioxide and water into sugars. It’s like a fantastically complex system of canals carrying energy, complete with lock gates, bypass sections, waterfalls, and waterwheels, and the flow of energy is controlled by changing the speed at which it passes through each section. Instead of streaming straight to the bottom, the energy is forced to build complex molecules on the way. These aren’t in equilibrium, but the plant can store them until it needs their energy, and then it places them somewhere where they can take the next step down toward equilibrium, and then the next step after that. As long as light is falling on to that coriander plant, it’s supplying the energy to keep the factory on the hop, continually chasing after equilibrium as the injection of energy moves the goalposts. Eventually, I’ll eat the coriander, and that will provide an injection of energy to my system. I’ll use that energy to keep my own body from equilibrium, and as long as I keep eating, the system won’t be able to keep up. Equilibrium won’t be reached. But I choose when to eat, and my body chooses when to use that energy, all by controlling the floodgates.