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Earthquakes have always been 
a terrifying phenomenon,

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and they've become more deadly
as our cities have grown,

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with collapsing buildings posing
one of the largest risks.

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Why do buildings collapse 
in an earthquake,

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and how can it be prevented?

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If you've watched a lot of disaster films,

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you might have the idea

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that building collapse is caused directly
by the ground beneath them

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shaking violently,
or even splitting apart.

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But that's not really how it works.

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For one thing, most buildings
are not located right on a fault line,

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and the shifting tectonic plates 
go much deeper than building foundations.

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So what's actually going on?

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In fact, the reality of earthquakes
and their effect on buildings

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is a bit more complicated.

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To make sense of it,
architects and engineers use models,

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like a two-dimensional array of lines
representing columns and beams,

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or a single line lollipop with circles 
representing the building's mass.

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Even when simplified to this degree,
these models can be quite useful,

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as predicting a building's response
to an earthquake

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is primarily a matter of physics.

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Most collapses that occur 
during earthquakes

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aren't actually caused 
by the earthquake itself.

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Instead, when the ground moves
beneath a building,

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it displaces the foundation 
and lower levels,

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sending shock waves through 
the rest of the structure

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and causing it to vibrate back and forth.

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The strength of this oscillation
depends on two main factors:

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the building's mass, 
which is concentrated at the bottom,

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and its stiffness,

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which is the force required 
to cause a certain amount of displacement.

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Along with the building's material type
and the shape of its columns,

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stiffness is largely a matter of height.

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Shorter buildings tend to be stiffer
and shift less,

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while taller buildings are more flexible.

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You might think that the solution
is to build shorter buildlings

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so that they shift as little as possible.


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But the 1985 Mexico City earthquake is
a good example of why that's not the case.

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Durng the quake,

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many buildings between six 
and fifteen stories tall collapsed.

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What's strange is that while shorter
buildings nearby did keep standing,

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buildings taller than fifteen stories
were also less damaged,

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and the midsized buildings that collapsed

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were observed shaking far more violently
than the earthquake itself.

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How is that possible?

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The answer has to do with something
known as natural frequency.

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In an oscillating system,

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the frequency is how many back and forth
movement cycles occur within a second.

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This is the inverse of the period,

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which is how many seconds it takes
to complete one cycle.

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And a building's natural frequency,
determined by its mass and stiffness,

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is the frequency that its vibrations 
will tend to cluster around.

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Increasing a building's mass slows down
the rate at which it naturally vibrates,

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while increasing stiffness
makes it vibrate faster.

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So in the equation representing
their relationship,

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stiffness and natural frequency
are proportional to one another,

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while mass and natural frequency
are inversely proportional.

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What happened in Mexico City
was an effect called resonance,

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where the frequency 
of the earthquake's seismic waves

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happen to match the natural frequency
of the midsized buildings.

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Like a well-timed push on a swingset,

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each additional seismic wave
amplified the building's vibration

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in its current direction,

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causing it to swing even further back,
and so on,

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eventually reaching a far greater extent
than the initial displacement.

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Today, engineers work 
with geologists and seismologists

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to predict the frequency 
of earthquake motions at building sites

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in order to prevent 
resonance-induced collapses,

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taking into account factors
such as soil type and fault type,

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as well as data from previous quakes.

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Low frequencies of motion 
will cause more damage to taller

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and more flexible buildings,

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while high frequencies of motion
pose more threat

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to structures that 
are shorter and stiffer.

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Engineers have also devised ways
to abosrb shocks

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and limit deformation
using innovative systems.

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Base isolation uses flexible layers

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to isolate the foundation's displacement
from the rest of the building,

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while tuned mass damper systems
cancel out resonance

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by oscillating out of phase
with the natural frequency

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to reduce vibrations.

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In the end, it's not the sturdiest 
buildings that will remain standing

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but the smartest ones.