Earthquakes are one of the planet’s most lethal natural forces
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