In this film we’re going to examine the anatomy of an earthquake

What happens to turn a seismic hazard, into a seismic disaster?

To do that we’re going to use this virtual cube of the Earth’s crust, to explore what

happens when an earthquake at depth meets a vulnerable population above

Here’s the unsuspecting target - a typical hill-top town

with a historic old part here and modern suburbs here and here

Home to a few tens of thousands of people with a constant stream of tourists adding

to the numbers. Life’s pretty normal really, there’s been

a few minor tremors in the preceding months but nothing out of the ordinary

Few if any of the inhabitants or the visitors have noticed this, though

a steep slope cutting right through the heart of the town

That is an earthquake fault line. If we zoom out we see that the fault descends

deep beneath the town, down to about ten kilometres, and down here the stress is at breaking point

The fault itself is locked. The blocks of rocks on each side are snagged

on rough patches that prevent the sides from sliding past each other

But the thing is those minor earth tremors are picking away at those strong points, breaking

them down, and eventually the points of resistance give way

With nothing holding them back, that section of the fault slips

The sudden rupture of the fault releases seismic energy

In this case the equivalent of a thermonuclear blast

- a magnitude 6.5 earthquake. Seismic waves radiate out from the point of

rupture, heading outwards. But to really understand what’s going on

we need to slow those waves down and zoom in

The seismic ruptures unleash different kinds of vibrations

One set are pressure waves, P-waves, that push and pull the rock and rapidly send pulses

of vibration ahead. Another set vibrate the rock from side to side

perpendicular to the motion of travel

Most rocks are weaker in this shearing motion, so the S-waves travel slower

If we speed things up again we see that those fast P-waves travel that ten kilometres in

just a matter of seconds but the sharp judders they produce go largely unnoticed

That all changes a few seconds later when the first sheer waves arrive

because their vertical and sideways lurches make people realise that they’re experiencing an earthquake

Almost immediately the full force of the earthquake is unleashed

and that’s because a third set of seismic waves has joined the action

Surface waves - slower but larger rolling and twisting motions

One after another the surface waves crash into the town

They wrench from side to side and up and down

Something like 95 per cent of the seismic energy gets released in the first ten seconds or so

But the shaking continues for 30 seconds, 40 seconds, a minute

People get thrown off their feet

and it’s those motions that buildings are most vulnerable to

Their fabrics strain as they try to bend and flex to the convulsing ground

Generally, it’s not earthquakes that kill people, it’s buildings

To the town’s inhabitants it would have seemed much longer, but it’s taken barely

60 seconds for the tremors to finally cease. But in that short period of time, much of

the historic centre has been levelled - the ancient masonry offering up little or no resistance

Nowadays, at relatively low cost, buildings

can be designed to be so-called life safe, maintaining their integrity without collapsing,

- kind of like a car bonnet designed to crumple on impact

These reinforced concrete buildings were put up decades ago, before the stringent building codes

They just weren't strong enough or flexible enough to withstand the intense shaking

The layout of our cities too has a big effect on the number of casualties

Many large new hospitals get built on the edge of the town where the land’s cheaper

But if key lifelines like roads and bridges fail, then the route to the hospital can be severed

cutting it off from many of the injured. The most tragic loss of life, though, has

occurred here close to the river In this area the softer sediments just make

the shaking far more intense and it lasts longer

The results are lethal A modern school, which should have been built

to tough earthquake standards, has crumpled. As the dust clears it’s obvious that many

of the fatalities lie among the ruined masonry of the historic centre

But it’s the poorly constructed modern buildings that claim most of the lives

Buildings that could have stayed up, should have stayed up, but didn’t

In countries around the world, public buildings like schools and hospitals and municipal housing

schemes tend to be built fast and cheap. Compromises are made on location and materials

Corruption of local officials allow the approval of designs that don’t even meet the most

basic seismic building codes. In the end, what makes populations fatally

vulnerable to earthquakes, is poverty, and poor governance

It’s a scenario played out to different degrees in countless cities across the earthquake-prone

parts of our planet. Here’s an image showing the earthquakes

that have happened across the globe. And here are our biggest cities

Today, more than half the world’s population live in towns and cities

And some of the biggest lie in earthquake zones

Over the next century, earthquakes will shake urban sprawls housing upwards of 12 million people

A direct seismic strike on such a megacity

has the potential to cause a million deaths. It’s a truly terrifying prospect, and one

that we’ve not seen the like of in human history

So the decisions that we make now about how to build and manage our cities

will go a long way to determining the fate of those who live in earthquake country