Breaking news in southern Taiwan.
Rescuers are frantically trying to find an uncounted number of victims
Who may be trapped beneath the rubble of several collapsed buildings.
Japan was struck this afternoon by a major earthquake.
This one is reported to be a magnitude 7.1.
Residents here are bracing for more aftershocks.
The death toll in the Italian earthquake is rising,
At least 250 now dead, nearly 400 others injured.
That earthquake in Ecuador, country reeling from a powerful shock, as the death toll rises above 400
[Ross] So far this year, earthquakes have killed one thousand, two hundred, and eight people.
In the last thirty years, the death toll stands at over a million.
The most catastrophic events are those which result in tsunamis or landslides.
But, more frequently, it’s not the earthquake which kills someone.
It’s the building which falls on top of them.
But to understand why buildings collapse, first we have to understand the anatomy of an earthquake
[Anna] The Earth, when it formed, was very very hot.
And it condensed down and formed different layers as any kind of material condensing would.
Think of your glass of orange juice sat on your breakfast table, if you leave it long enough
you’ll end up with a thick layer at the bottom and little bits floating on the top and it’ll look a bit watery.
Earth’s done the same. There’s a big iron core in the middle,
and then there’s a liquid layer, and then there’s something called the ‘mantle’.
[Melody] The mantle is solid, but if you look at it over a really long period of time
– like a million years – it behaves like a liquid.
[Anna] And then there’s a cold bit on the outside, that’s this shell protecting everything underneath it.
And what the Earth is trying to do is – we’ve dunked it in space, and space is cold,
so it’s just loosing heat, because that’s all it can do.
So all that heat comes from the centre out through the Earth.
And in doing so, the inside of the Earth, the bits that can move try to move.
And so we see bits that rise up and bits that fall down.
And on top of that, the crust that we all walk about on is trying to stay still,
but everything underneath it is moving, so eventually cracks form and it splits and it moves.
We call it ‘plate tectonics’, but it’s just the Earth trying to lose its heat and
the knock-on effect is creating earthquakes.
An earthquake is a release of energy, and it’s caused by the movement of things.
Now that movement causes a shockwave, and just like you can hear me talking because
there’s a wave travelling through the air; that’s the sound wave.
Basically, an earthquake is made of sound-waves in the earth.
Transmitted by just shaking the ground around it.
So it propagates through as a wave, just like ripples in a pond, watching them spread out.
[Ross] These ripples through the earth are what we experience when an earthquake hits.
But it’s the frequency of these waves that has the greatest impact on our buildings.
To find out more, we visited the University of Bristol’s earthquake shaking table.
[Joel] Most of the things that we put on the earthquake table will break because we’re
trying to understand the failure mode.
So that maybe we can prevent it or improve its response later on,
so we’re generally aiming to break things.
And the table itself can accelerate at up to 6G – so six-times the force of gravity.
So a tubby guy like myself, who might weight 100 kilos, if I stood on the table and it
accelerated upwards, for a fraction of a second.
I would my 100-kilos going down that I’m experiencing normally, and 600 from the table pushing upwards.
So, for that time of acceleration, I would feel as if I weighed 700-kilos
and my knees would be much closer to my face than I’d probably like.
Buildings all have frequencies that they like to vibrate at, and we have here three models
of buildings and they all have different natural frequencies.
So, if I pluck this smaller building a little bit like a guitar string, you’ll see that
it vibrates reasonably quickly.
If I pluck the medium one, we’ll see that this one has a frequency that it also
likes to vibrate at naturally,
and it’s slightly slower than this one.
And if I go all the way to the taller one, and pluck this one:
see it’s much, much slower indeed.
Natural frequencies is an important factor in earthquake engineering,
because if we have an earthquake
that is effectively tuned, or has frequencies that are close to one particular type of building,
then that building will effectively be amplified in its response.
So, if we say this is two-storey, this is three-storey and that’s a four-storey building.
If I apply an earthquake that’s got a similar natural frequency to the three-storey building;
see that that one is moving an awful lot, but the other two are staying reasonably static.
And the impact of this in an earthquake zone would be that all of the buildings that happened
to be tuned to that frequency
will suffer considerably more damage, and collapse.
[Ross] So we know that earthquakes are most likely to occur along the fault lines of tectonic plates.
and we understand how the frequency of these waves travelling through the earth can impact buildings.
But is there a one-size fits all solution to the problem of counteracting these forces?
[Joel] Earthquakes vary a lot, so it’s not as simple as finding a one-size fits all solution
that will always be OK in every single earthquake.
Obviously we could build our buildings incredibly strong, to avoid any sort of earthquake.
But the reality of doing something like that is that it would be far too expensive for
anybody to afford, it would be hugely unsustainable.
So it’s about the trade off between the earthquake engineering side, and also the
financial side, and the usability.
So generally people want to be able see out of their structures, so we put windows in them etc.
So there are things that we need to include in buildings that might not help its strength,
but they’re a requirement of the building itself, whether we’re trying to avoid earthquakes or not.
[Ross] There are over 13,000 earthquakes every year.
And each one varies in its intensity, magnitude and frequency.
And we can’t accurately predict exactly when one is going to happen.
So is it even possible to engineer an earthquake-resistant building?
[Matt] The main means by which we going about designing anything,
but particularly in earthquakes, is to use structural codes and standards
which kind-of define the process that we need to go through to design something.
You have what’s viewed as your less sophisticated techniques such as brace frames, or rigid frames.
But you have a lot more complicated means of achieving the same result;
one of those is tuned mass dampers,
where you place basically a really large pendulum at the top of the building
that moves in a way which can counteract the effects of earthquakes.
Or things like base isolation.
[Alice] When you have base isolation, you have springs that are between the foundation and the ground floor.
So therefore it stops the earthquake forces from going up through the top building.
Which means that the foundation takes all of the strain of the earthquake.
[Ross] These are just two of the tools that engineers have at their disposal.
But it got me thinking, for countries that have a low risk of experiencing earthquakes, like the UK,
is it worth us constructing buildings which are earthquake resistant?
[Alice] It would be looking at the severity of the earthquake consequence.
So, how many people would die?
What would the cost of the damage be?
Versus the likelihood of an earthquake hitting the area.
The only one building for the UK would be a nuclear power plant.
[Matt] That’s the only real situation where the damage that could occur, even though still
relatively small, has potentially very large consequences.
So we don’t like to leave it to chance.
[Ross] And thankfully, we don’t have to leave it to chance.
Designing an earthquake-resistant buildings isn’t impossible, but it is an extremely difficult challenge.
The odds might seem stacked against us,
but with the novel solutions and innovation of engineers around the globe
we can continue to save lives through engineering.
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