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

且隨著都市的發展而越來越致命

with collapsing buildings posing one of the largest risks.

建築物的倒塌是最大的危機

Why do buildings collapse in an earthquake,

為什麼建築物在經歷地震時會倒塌？

and how can it be prevented?

我們又該如何預防？

If you've watched a lot of disaster films,

如果你曾看過災難片

you might have the idea

也許會認為

that building collapse is caused directly by the ground beneath them

建築物倒塌是因為直接接受地表

shaking violently, or even splitting apart.

劇烈的晃動，甚至是地裂

But that's not really how it works.

但實際上並不是那樣的

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

首先，大部分的建築物都沒有蓋在斷層上

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

況且，板塊運動是在比建築物地基還要深很多的地底發生的

So what's actually going on?

所以到底發生了什麼事呢？

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

事實上，地震對於建築物的影響

is a bit more complicated.

要比想像中複雜的多

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

為了搞懂這些，建築師和工程師利用模型

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

像是二維的線狀陣列來表示柱子和橫樑

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

或是一根根棒棒糖狀的東西來表示建築的質量

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

雖然是非常簡化的模型，但還是很有幫助的

as predicting a building's response to an earthquake

因為建築物對於地震的反應

is primarily a matter of physics.

可以用物理來解釋

Most collapses that occur during earthquakes

大部分在地震中倒塌的建築

aren't actually caused by the earthquake itself.

並不是地震本身造成的

Instead, when the ground moves beneath a building,

而是當建築物下方的土地搖晃時

it displaces the foundation and lower levels,

讓地基和低樓層的部分跟著移動

sending shock waves through the rest of the structure

接著往上傳導震盪波

and causing it to vibrate back and forth.

讓建築物開始前後搖晃

The strength of this oscillation depends on two main factors:

這種震盪的大小跟兩種主因有關：

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

第一個是建築物的質量，主要集中在建築物下方

and its stiffness,

第二個是剛性

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

也就是需要多少力量才能造成相當的位移

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

伴隨著使用的建材和柱子的形狀

stiffness is largely a matter of height.

剛性跟建築物的高度也有很大的關聯

Shorter buildings tend to be stiffer and shift less,

較矮的建築通常剛性較大且不容易位移

while taller buildings are more flexible.

較高的建築物則比較容易搖晃

You might think that the solution is to build shorter buildlings

你也許會認為解決辦法就是盡量蓋比較矮的建築

so that they shift as little as possible.

讓它們盡可能不要位移

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

但1985年的墨西哥城大地震已經證明了那是錯的

During the quake,

在搖晃期間

many buildings between six and fifteen stories tall collapsed.

很多介於六到十五層樓間的大樓倒塌了

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

奇怪的是，除了那些比它們矮的房子沒有倒之外

buildings taller than fifteen stories were also less damaged,

那些超過十五層樓高的大樓也沒有倒

and the midsized buildings that collapsed

事後發現那些倒塌的中等高度建築物

were observed shaking far more violently than the earthquake itself.

在地震時晃得比地震本身還大

How is that possible?

怎麼會這樣呢？

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

答案跟自然頻率有很大的關聯

In an oscillating system,

在一場震盪中

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

頻率代表著一秒鐘前後移動循環的次數

This is the inverse of the period,

跟週期正好相反

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

週期是一次循環需要花幾秒的時間

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

而一棟建築物的自然頻率（由質量與剛性決定）

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

就是會讓震動累積的頻率

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

增加一棟建築的質量會讓自然震動的速率變慢

while increasing stiffness makes it vibrate faster.

如果增加剛性，則會讓震動變快

So in the equation representing their relationship,

所以在表示它們之間關聯性的等式中可以看到

stiffness and natural frequency are proportional to one another,

剛性和自然頻率成正比

while mass and natural frequency are inversely proportional.

質量和自然頻率則是成反比

What happened in Mexico City was an effect called resonance,

在那次墨西哥城大地震中，發生了所謂的共振

where the frequency of the earthquake's seismic waves

也就是地震的震波頻率

happen to match the natural frequency of the midsized buildings.

剛好與中等建築物的自然頻率吻合

Like a well-timed push on a swingset,

就像是盪鞦韆時有人在最佳的時機推你一把讓你盪更高

each additional seismic wave amplified the building's vibration

每一次的震波都會讓建築物在當時搖晃的方向下

in its current direction,

晃得更厲害

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

讓它擺動越來越劇烈

eventually reaching a far greater extent than the initial displacement.

最後終於支撐不住而倒塌

Today, engineers work with geologists and seismologists

現今，工程師跟地質學家與地震學家合作

to predict the frequency of earthquake motions at building sites

想要預測地震在建築工地上搖晃的頻率

in order to prevent resonance-induced collapses,

以預防共振所造成的倒塌

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

他們運用土質、斷層種類，

as well as data from previous quakes.

還有過去發生的地震資料來計算

Low frequencies of motion will cause more damage to taller

低運動頻率會對較高和剛性低的建築

and more flexible buildings,

造成較大的損害

while high frequencies of motion pose more threat

相反的，高運動頻率則會對較矮且剛性高的建築

to structures that are shorter and stiffer.

造成較大的威脅

Engineers have also devised ways to abosrb shocks

工程師們也運用最新科技發展出吸收震盪

and limit deformation using innovative systems.

和減少變形的方法

Base isolation uses flexible layers

「基礎隔震」利用有彈性的樓層

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

來阻隔地基的位移造成整動建築物的搖晃

while tuned mass damper systems cancel out resonance

「調諧質塊阻尼器」則是減緩建築本身的擺動

by oscillating out of phase with the natural frequency

以避免共振的發生

to reduce vibrations.

進而減少震動

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

結論是，最堅固的建築並不會撐最久

but the smartest ones.

智慧建築才是我們需要的