Earthquakes 10 Things You Didn't Know About...

The world is full of stunning and dramatic landscapes,

all formed by the complex history of our planet.

A country like Greece may seem like the perfect holiday destination -

beautiful scenery, gorgeous beaches and a fascinating history -

but, for geologists like me, Greece has other attractions.

It's the earthquake capital of Europe.

Earthquakes are one of the most destructive natural forces on our planet

and, as a geologist, it's my job to try and understand them.

I'm here in Greece, following this dramatic earthquake fault line as it slices through the landscape.

On the way, I'm going to tell you ten of the most remarkable

earthquake stories from around the world.

There will be plenty of surprises along the way.

So, if you don't know how Cold War spying gave scientists a crucial clue to understanding earthquakes...

where quakes last 60 times longer than anywhere on the planet...

or which earthquake fault line causes hallucinations, then stay around, as I reveal:

DEEP RUMBLING

About an hour west of Athens, nestling in the hillside

and surrounded by olive trees,

lies the sleepy little village of Pisia.

It might not be on the tourist trail, but it's a fascinating place.

It sits right on top of an earthquake fault line.

The fault itself is best seen just outside the village.

This is a pretty special place for me...

first came here in 1986 when I was a student studying earthquakes and

this is the very earthquake fault I came to study.

This smooth, polished rock face is the actual fault surface

that gets violently pushed out the ground during an earthquake.

It's impossible to imagine how destructive those forces are.

And it was here that I first came face-to-face with

people who had actually experienced the devastation of an earthquake.

In February 1981, a major earthquake struck this region.

More than 15 people died and hundreds were seriously injured.

When I came to study this fault a few years later,

all the villagers wanted to know was - when will it happen again?

Could I calculate when the next earthquake would happen?

For them, you know, it was a case of life and death.

I hadn't thought about it until then, but then

you realise how important the work is that geologists do, how it really affects people's daily lives.

Earthquakes usually last for just a few seconds...

yet in those few seconds they can cause destruction on a massive scale.

Greece is the most active earthquake zone in Europe.

Pretty bad luck, then, that this was also the birthplace of western civilisation.

And that's the subject of my first story.

The effect of earthquakes on our ancient heritage.

Here in Greece, we're surrounded by the remnants of one of the greatest of all civilisations.

Everywhere you look you see the ruins of a world that's been lost to us.

Of course, many of these magnificent buildings were destroyed by war and pillage, but an astonishing amount

were destroyed by earthquakes - even whole cities have all but vanished.

The list is endless.

The beautiful city of Helike, on the coast, lost to an earthquake.

The wonderful Temple of Apollo at Corinth, destroyed by an earthquake.

What must the ancient Greeks have thought when they saw their

magnificent monuments shaken to destruction in a matter of seconds?

How did they explain the terror of earthquakes?

Well, not surprisingly, the Ancient Greeks thought it was all down to the anger of the gods.

They blamed Poseidon, Earth shaker and god of the sea, for all the carnage and ruin.

But in the fourth century BC, the Greek philosopher Aristotle

came up with one of the first rational explanations of how earthquakes happened.

He thought that there were deep, underground winds that caused the ground to fracture and shake.

He may have been wrong, but at a time when every aspect

of life was explained by the actions of the gods, this was at least a rational explanation.

Today, we can explain most earthquakes through the theory of plate tectonics.

The surface of our entire planet is made up of a number of moving plates.

These tectonic plates fit together like giant pieces of a jigsaw puzzle.

They sit on top of a hot, plasticky mantle of rock that's in constant motion.

The cold, rigid plates move slowly over this hot, soft interior just a couple of centimetres a year,

but as they move, they grate and tug and get caught up with each other at the plate boundaries.

And that's how earthquakes begin.

You get a real sense of how plates move when you come up to a fault line like this.

As the vast plates slide past each other, they get snagged along their edges.

When they get stuck, the pressure builds up along fault lines.

Eventually the strain gets so great that the fault ruptures, releasing

huge amounts of energy which shakes the ground for miles around.

We measure that shaking on a scale of magnitude, sometimes called the Richter scale.

Each number on the scale registers an earthquake 10 times larger than the last.

So a magnitude 6 earthquake is 10 times larger than a 5, and an 8 is an earthquake 1,000 times larger.

Greece lies close to the edge of one of these plates, in what's probably

one of the most geologically complex parts of our planet.

That's why it's so earthquake prone.

You can find dramatic earthquake fault lines like this all over the country.

It's no surprise, then, that so much of the ancient civilization that

flourished here 2,000 years ago has disappeared forever.

I wonder how different this country's architectural heritage

would have been if it hadn't been in an earthquake zone?

And what other unseen treasures from around the world have we lost to earthquakes?

Without earthquakes, we might even have some of the great wonders of the world still standing,

because we know that three of the seven Ancient Wonders of the World were devastated by earthquakes.

The Colossus of Rhodes.

In 226 BC, a powerful earthquake struck this region.

The poor Colossus snapped at the weakest point, his knees, and fell to the ground, lost forever.

And, of course, it's not just Greece.

Over in Egypt, another wonder, the Lighthouse of Alexandria.

Probably the tallest building in the world when it was built in 300BC.

But it disappeared, toppled by earthquakes.

And over in Turkey, there was the Mausoleum of Halicarnassus,

a tragic and romantic monument built in 351 BC.

The wife of King Mausolus was so distraught at his death that she built this beautiful tomb for him.

Built for love, it was destroyed by earthquakes.

You don't often think about it, but if it weren't for the awesome destructive power of earthquakes,

many of the greatest lost treasures from the Ancients would still be with us today.

Not just in Greece, but throughout the world.

Earthquakes have dramatically changed our legacy from the past.

My next story takes me back to the Great San Francisco earthquake of 1906.

This was one of the worst natural disasters in American history.

But it was also a turning point for geologists because it led to a real

breakthrough in our understanding how earthquakes happen.

1906.

San Francisco was a boom town, the largest city in the American West,

thanks to the Californian Gold Rush.

But while its 400,000 inhabitants were chasing the American Dream, they had no idea that the city was

perched on one of the most unstable fault lines in the world, the San Andreas Fault.

But on the 18th April 1906, they discovered the horrible truth.

An earthquake of magnitude 7.9 devastated San Francisco.

In the first few seconds of the quake, many people died in their beds

as brick chimneys crashed through walls and crushed them as they were waking.

The shaking set church bells ringing.

It must have seemed like the end of the world.

The quake lasted for just 45 seconds, but that's all it took to destroy the city.

Gas mains burst

and fires burned out of control for three days.

The earthquake, and its aftermath, killed up to 6,000 people and left more than 200,000 homeless.

This earthquake stunned everyone.

Not even the scientists understood what had happened.

In 1906, the study of earthquakes was still in its infancy.

Very little was understood about how and why earthquakes occurred.

But this was about to change.

The scale of the devastation in San Francisco was so shocking

that it stirred the scientific community into action.

Our understanding of earthquakes was about to take a giant leap forward.

Within a week of this event, a team of nine scientists met for the first time,

determined to revolutionise our understanding of earthquakes.

First, they studied the way the ground had shifted throughout the area.

And it was through this detailed analysis that they made

-the crucial link between the earthquake

and the fact that the San Andreas fault was somehow the cause.

The San Andreas Fault is one of California's

most distinctive natural landmarks, a gigantic gash across the state.

We now know that this fault is where two massive tectonic plates meet,

and they're slowly moving past each other all the time.

But back in 1906, no-one knew anything about these plates.

Even so, the scientists did give us a lasting legacy that came from this study.

One member of the team, Henry Fielding Reid, became fascinated by

the way features like farm fences had been shoved out of line by the earthquake.

The fences showed that the two sides of the fault line had shifted

violently past each other in a consistent pattern.

And, from this simple observation,

Reid came up with the first real theory about how earthquakes happened.

Reid was the first person to work out that, in some places,

the Earth's crust is being strained and stretched to breaking point

until finally, it snaps.

And that's what an earthquake is.

After this breakthrough, the whole study of earthquakes and geology took off

and eventually revealed the secrets of how our planet works.

So, out of the ashes of the terrible San Francisco earthquake came the first glimmerings

of scientific understanding about this terrifying force of nature.

My next story is an intriguing tale about

one of the eureka moments in earthquake science, though it reads rather like a Cold War spy plot.

The 1960s was a time when America and the Soviet Union were locked in a nuclear arms race.

And it was America's paranoia with secret Soviet nuclear testing

that helped to prove one of the biggest theories in geology.

So my next story's not an earthquake site, but a bomb site.

This is Novaya Zemlya Island, in Russia, one of the Soviet Union's secret nuclear test sites.

The Soviets began testing their weapons in this remote place in 1954 at the height of the Cold War.

Over a period of 35 years, hundreds of nuclear devices were detonated.

But, of course, the Americans were determined to find out exactly what the Russians were up to.

Rather than send in their spies, the Americans came up with a brilliant plan.

They decided do some long-distance spying using seismometers.

Seismometers are instruments that can register and measure shaking ground

from thousands of miles away, even from the most secret locations.

And, of course, nuclear explosions shake the ground,

so seismometers would be able

to record every explosion happening deep inside Russia.

So, in 1961, the Americans installed 120 seismometers

at sites all around the world and soon they were recording every Soviet nuclear test.

But all the time this network of seismometers was recording nuclear explosions,

it was also monitoring the natural phenomenon that shakes the ground all over the world - earthquakes.

And that earthquake data produced a major surprise for science back in the '60s.

Earthquakes weren't scattered chaotically across the planet.

They followed a narrow set of lines around the globe.

In their search for nuclear explosions,

they'd coincidentally created an accurate map of the world's earthquake zones.

And what was the significance of this map?

Well, the earthquake lines revealed the hidden edges of the Earth's tectonic plates.

In the 1960s, the theory of plate tectonics was a revolutionary new idea.

But it was still just a theory.

Now there was proof because the seismometers were able

to measure the direction the plates were moving in during an earthquake.

For the first time, geologists had solid evidence that showed

how all these plates were moving against each other at the fault lines and causing earthquakes.

Who could have imagined that spying for nuclear secrets would end up providing

key evidence for the most important geological theory ever developed,

the theory of plate tectonics.

This discovery was a major breakthrough for earthquake science,

and one of the few good things to come out of

the bitter Cold War rivalry between America and the Soviet Union.

My next story is about the deadliest earthquake in recorded history.

It happened in China in 1556.

In a matter of seconds, this one earthquake killed nearly a million people.

The reason it was so devastating is because, in this particular region,

people had always lived in man-made caves.

And this would prove lethal in the earthquake.

The world's deadliest earthquake happened

four centuries ago in central China, on a vast, fertile plateau rising above the Yellow River.

This is a region with freezing winters and baking hot summers,

and the Chinese have come up with an ingenious solution to cope with these extremes of weather.

Instead of building traditional houses, the locals have

burrowed into the hillsides, creating caves, or Yaodongs.

It's a tradition that goes back at least 2,000 years.

Tunnelling through the soft soil to depths of hundreds of metres,

these amazing structures are cool in the summer and warm in winter.

Probably the most famous caves are in Yan'an.

It's here that Mao Zedong and his troops holed up during the Communist Revolution.

What I find so surprising is that even today, up to 40 million Chinese still live in these kinds of caves.

That's two-thirds of the entire population of Britain.

The fine, silty soil here is easy to dig out to make caves.

But that means it's also highly unstable, and collapses easily.

And it's this characteristic that proved to be so fatal in 1556.

On the 23rd of January, a huge earthquake struck the region.

Modern estimates put the earthquake at a magnitude 8.

It shook a vast area of ground.

Everything within 1,300 square kilometres was destroyed.

But it's the loss of life that makes this the deadliest earthquake in history.

Nearly 1 million people perished in this disaster.

And it was the very structures they relied on for shelter that killed them.

Their Yaodongs collapsed like a pack of cards as the soft soil gave way

under the huge shockwaves, burying everyone inside them.

A Chinese account from the time gives us a vivid description of how

the force of the earthquake rearranged the landscape.

Mountains and rivers changed places, and roads were destroyed.

In some places, the ground suddenly rose up and formed new hills,

or it sank in abruptly and became new valleys.

The Shaanxi earthquake may not have been the biggest ever recorded, but it was certainly the deadliest.

And when you think that today 40 million people still live

in the same sort of caves as their ancestors, you realise that there could be much worse to come.

It's not earthquakes that kill people, buildings do.

And the ancient Greeks were the first to try and design their buildings against earthquakes.

Some of their temples had metal rods inserted in the columns.

In our next story, we'll see how innovative building design can save lives.

And it turns out, surprisingly, that one particular kind of skyscraper

might be the safest place to be when an earthquake strikes.

The teeming metropolis of Mexico City lies in one of the worst earthquake zones in the world.

What's more, large parts of the city are built on the site of an old lake bed,

drained by the Spanish after they conquered the Aztecs.

It's a highly unstable site.

The soil is a mixture of soft sands and clays, which are full of water.

Yet this is where developers decided to build the tallest skyscraper in Latin America.

It's called the Torre Mayor.

Completed in 2003, the building is 225 metres high.

And yet the people who come to work here are confident that it will keep them safe in an earthquake.

I would rather be in Torre Mayor than anywhere else in the city whenever there's an earthquake.

That's a pretty astonishing claim.

But the Torre Major is probably the strongest building in the world,

built to withstand an earthquake of a magnitude of 8.5.

That's one colossal earthquake.

So what makes this building so earthquake-proof?

And what on Earth made them reach for the sky in the most unstable part of the city?

The answers lie long before the Torre Mayor was built, in the events of 1985.

Early in the morning of September the 19th, Mexico City was struck by a massive earthquake.

At a magnitude of 8.1, it was the worst earthquake in Mexico's history.

At least 9,000 people died, with 30,000 injured.

Despite the scale of the tragedy, there were some amazing miracles.

Three days after the quake, 58 newborn babies were pulled alive

from the wreckage of a maternity ward.

Rescuers continued to find survivors up to a week later.

The greatest destruction happened in the area of the old lake bed.

That's because when the shockwaves hit the lake bed, something strange happened.

The soft sediments under the buildings

actually amplified the shockwaves, making them far more destructive.

These powerful waves cracked the foundations of buildings,

setting up vibrations that shook them into rubble.

And that's not all. Something else even stranger happened to the soil beneath these buildings.

It's called liquefaction.

Liquefaction is when apparently solid ground disintegrates

and water trapped in the soil starts leaking out.

When wet ground gets shaken, the land takes on the properties of a liquid.

Buildings simply sink into the ground, regardless of how solid their foundations,

and torrents of water get erupted out of the surface.

Liquefaction has only been caught on camera once, during an earthquake in Japan in 1964.

It's an amazing sight.

Buildings just slip into the ground.

It wasn't as severe as this in Mexico City,

but in 1985, liquefaction did add to the devastation in the drained lake area.

So it's astonishing that it was in this very area of maximum destruction

that developers decided to build the tallest building in Mexico.

It sounds crazy, but engineers were confident that in the Torre Mayor they could construct a building

in this vulnerable place to withstand almost anything that nature could throw at it.

Their first challenge was the foundations.

Without solid foundations, this building could become an enormous glass and steel coffin.

To get past the loose marshy sediments of the lake bed

and anchor the building in solid bedrock, engineers drilled down 60 metres.

That's three times as deep as the foundations of the Empire State Building.

Over 250 piles secured the main structure to solid ground.

But what really helps the tower ride out an earthquake

is an altogether different and unique piece of engineering.

Behind the oceans of glass lies the strongest possible skeleton -

a network of super diagonal diamond braces.

Where the super diagonals overlap, they form three smaller diamonds.

At each junction rests four huge shock absorbers.

And here's the magic of the design.

They look and operate just like a regular shock absorber in your car.

The only difference is that they're about the size of your car.

And this is how they work.

The moment a seismic wave strikes the tower,

giant pistons inside the shock absorbers are forced inward.

These dampen the severe vibrations from the shock waves,

and absorb their energy.

Instead of crashing to the ground, the tower flexes safely.

I think Torre Mayor is a benchmark.

Everybody would like to build buildings as important, as safe, as Torre Mayor is today.

In the past, Mexico's been struck by massive earthquakes,

five times more powerful than anything Torre Mayor has faced.

Yet the impressive team of engineers that built this skyscraper

are confident that it would survive an earthquake of that size.

They believe that in one of the planet's worst earthquake zones,

this could remain one of the world's safest buildings.

Earthquakes usually last for a few seconds.

Occasionally they go on for minutes, but there's one place where quakes can last for up to one hour.

But no matter how long they last or how violent they are, these quakes pose no threat at all.

The reason that you've never heard of them is that they're out of this world.

INDISTINCT

The engines are on.

My next earthquake story

is all about our close encounter with the moon.

On July 16th, 1969, when Apollo 11 blasted off towards the moon,

it wasn't just carrying a trio of astronauts.

Apart from its valuable human cargo,

Apollo 11 also carried a number of scientific instruments.

What's extraordinary to realise now is that in these early flights to

the moon, these science experiments were almost an afterthought.

NASA scientists were so intent on simply getting the craft to the moon

that they didn't give too much thought

to what Buzz Aldrin and Neil Armstrong would do once they got there.

So, in something of a rush, they put together instruments that

would measure the Moon's magnetic fields and investigate solar winds.

But the one that interests me is a specially designed seismometer,

placed carefully on the moon's surface by Buzz Aldrin.

These seismometers would one day reveal something totally unexpected about the moon.

Considering how rushed the NASA boffins had been,

the seismometer was a very smart bit of kit.

It was powered by solar panels, which would work for as long it was in sight of the sun.

Once the lunar night kicked in, after about two of our weeks, the seismometer would close down.

To operate properly, it had to be perfectly level.

The astronauts didn't have time to do this,

so the seismometer was fitted with motors, so that it could be levelled by remote control from Earth.

For two weeks, the seismometer made the first ever recordings of the moon's jolts and tremors.

It even picked up the vibrations made by Buzz Aldrin climbing up the ladder into the lunar module.

But that wasn't all.

As more seismometers were sent up, the data sent back showed,

to everyone's amazement, that the moon had thousands of quakes every year...

moonquakes.

This came as a surprise to scientists, because the moon has

none of the geological features that cause earthquakes here on Earth.

The moon is almost entirely solid rock.

It has no tectonic plates rubbing up against each other.

So what was the cause of these moonquakes?

Well, they finally worked it out.

And amazingly, many of these quakes, the really deep ones, are caused by the Earth.

In just the same way that the moon's gravity pulls at our oceans,

creating tides, Planet Earth exerts a gravitational pull on the moon.

Now, the moon doesn't have oceans or tides, but as the Earth pulls at its rocky structure,

there's a build up of strain as these rocks deform, and this strain is then released through moonquakes.

And what came as a real surprise is that, unlike the short, sharp events we have on Earth,

moonquakes can last for up to an hour or more.

That's 60 times longer than most quakes on Earth.

But it turns out there's a very simple reason for this.

When seismic waves travel through the Earth's crust, they die away pretty quickly.

Even the biggest earthquake shakes the ground for just a couple of minutes.

And that's because there's water on the Earth.

And rather than acting like solid rock, the crust acts

like a giant sponge, soaking up the energy of the seismic waves.

But the moon has no water on its surface,

and when a moonquake strikes it sets the whole sphere vibrating like a tuning fork.

So that's why quakes last so long up there.

There's no liquid to absorb the vibrations, so they just keep going and going.

Just imagine the destruction an hour-long earthquake would produce in the average city here on Earth.

Of course, if we ever went to live on the moon we'd have to make sure our houses were moonquake-proof.

It's one small step for man, one giant leap for mankind.

My next story couldn't be more different.

It's about an earthquake fault that's destroyed empires and deposed kings.

But not because of the violence of its shaking,

but rather because of its extraordinary powers of prediction.

This is Delphi, on the slopes of Mount Parnassus.

The centre of the universe for the Ancient Greeks.

Here stood one of the most famous of all Greek temples, dedicated to the sun god, Apollo.

Built in the 7th century BC, this temple has a surprising dependence on earthquakes.

For centuries, this was the site of the Delphi Oracle, THE place to come

if you wanted to ask Apollo what the future held in store.

His answers came through the medium of a priestess known as the Pythia.

She would enter into the temple's inner sanctum, a small, enclosed basement chamber,

sit on a three-legged stool, and begin to issue prophecies to her paying customers.

People came from all over the Mediterranean to ask if they'd be lucky in love or successful in war.

Her answers determined the fate of people and nations.

But things could go horribly wrong.

For instance, legend has it that in the 6th century BC,

the Pythia was consulted by Croesus, the last King of Lydia.

One of the ancient world's super rich,

he ruled over a huge empire and had his sights on the Persian one, too.

The Oracle at Delphi told Croesus that if he attacked the Persians,

he would destroy a great empire.

So he did attack the Persians, but to his great surprise, lost the war.

So the oracle came true, but not in the way he wanted.

An empire WAS destroyed -

unfortunately, it was his own.

By now you're probably wondering, what's the connection between earthquakes and prophecies?

Well, Delphi actually sits on top of two intersecting earthquake faults.

Geologists have recently found

that these faults pass right through the inner sanctum,

where the Pythia was thought to have sat.

Now, we know that some faults emit noxious, and even hallucinogenic gases.

And according to ancient writers, before going into a trance

the Pythia would drink from a spring running through the site.

She would inhale intoxicating vapours from deep within the Earth.

Vapours which would loosen her tongue.

Geologists have found that water from a spring near the Delphi Oracle contains a gas called ethylene.

This sweet-smelling gas is a narcotic, so when the Pythia went

to her enclosed, subterranean chamber to foresee the future,

she could have been exposed to concentrations of ethylene,

coming out of the fault line, strong enough to induce a trance.

In other words, she was as high as a kite.

It's thousands of years since people flocked to the Oracle at Delphi to have their futures foretold.

These days we don't have to travel as far.

Simply pick up a magazine and read your horoscope.

My next earthquake is the most surprising of all.

Because it happened in a place that should never have had an earthquake.

It's nowhere near an active earthquake zone, in fact, it's nowhere near a plate boundary.

To find out more about it, I need to pop over to the land of reindeer.

I'm here in Lapland, in Northern Sweden, miles from any earthquake zone.

So you'd think there would be no earthquakes here at all.

But you'd be completely wrong.

I'm standing on the Parve Fault.

Parve in the Lapp language means wave, and when you look at it

you can see it undulating away into the distance.

And what's really impressive is it's length.

It may only be a low cliff but it runs across the landscape

for 150 kilometres, a great tear in the Earth's crust.

There's only one thing that rips through the ground like this, and that's an earthquake.

DEEP RUMBLING

What gets me about this earthquake, what makes it so surprising,

is that it's so far from any active earthquake zone.

The nearest one is in Iceland, over 1,500 kilometres back there.

So, at first sight, it seems that this earthquake in Northern Sweden is inexplicable.

It shouldn't be here.

The height of this cliff, about 10 metres, also reveals the strength of the earthquake.

It's been calculated that the one that caused this uplift of the ground

must have had a magnitude of around 8.2.

That's an unbelievably large earthquake,

in an area that's nowhere near a tectonic plate boundary.

So what caused this mysterious earthquake?

When geologists began to study the Parve Fault,

they dated the earthquake event that thrust this cliff out of the ground at around 8,500 years ago.

But while they had a date, they couldn't figure out the cause.

And then it dawned on them.

Something else had happened during this geological period.

And that was the emergence of this entire region from the Ice Age.

During the last Ice Age, this area was covered in ice to a depth of three kilometres.

That's about 3,000 tons of ice pressing down on every square metre of land.

Or think of it this way.

The weight of two Eiffel Towers pressing down on an area the size of a snooker table.

This exceptional weight of ice literally pushed the land

down by many metres, for thousands and thousands of years.

And when all this ice melted, the Earth's crust reacted in an unusually violent way.

DEEP RUMBLING

With the enormous weight of ice now suddenly lifted off the land,

it began to spring up, back to its original height.

Normally the land rises slowly and gradually, at about a centimetre or so a year. But not always.

Sometimes, as the land is rebounding, stresses build up along a line of weakness in the rock

until they reach such a point that an earthquake rips along and relieves the pressure.

And that's exactly what happened here.

All across Scandinavia, from Lapland to Norway, you can see these earthquake scars.

They are the marks of the Earth's crust springing up after thousands of years of colossal pressure.

We now believe that what happened in Norway and Sweden 8,500 years ago could happen again.

And that's because of climate change.

Some scientists believe that global warming could melt the ice caps that weigh down Antarctica and Greenland.

And if that happens the land will rebound, producing big earthquakes in the most unexpected of places.

Geologists know lots about earthquakes now, but surprisingly,

even after decades of research, we still can't predict exactly when and where they'll happen.

That's because the build up of pressure deep inside the Earth

is so slow, and usually gives few warning signs that we can measure.

But for some geologists, prediction remains the Holy Grail.

And over in Turkey, there's one fault where we think we can make an earthquake forecast.

We think we know exactly where the next earthquake will strike and that's an exciting breakthrough.

The only trouble is, we don't know when.

Here in Turkey, there's a fault line that slices through the country for 1,500 kilometres.

It's one of the most treacherous faults on the planet.

Millions of people live along its path.

It's known as the North Anatolian Fault.

Unlike most other faults, the North Anatolian Fault runs in a relatively straight, simple line.

And that means it may be easier here for scientists to calculate where the next earthquake will happen.

That's because scientists now know that after an earthquake, the stress from that one earthquake

travels further down the same fault line and builds up in a new location along the fault.

So in theory, if you can pinpoint where the stress has gone to

after one earthquake, you know where the next earthquake will strike.

The key is to work out where the stress from the last earthquake has travelled to.

And one scientist, Geoffrey King, has been developing a computer model that can do just that.

King and his team of scientists plot where all the aftershocks happen after a major earthquake.

That tells you the broad areas where the stress has transferred to.

But to pinpoint more precisely where the next earthquake might strike,

this model analyses some key geological data about previous earthquakes.

For instance, how deep underground the rupture was,

the nature of the rocks in the area, and how much the fault had slipped.

The calculations of where the stress had gone show up in red.

To see if their model worked, in the 1990s, this team focused their attention on Turkey

and fed all the geological data from the North Anatolian Fault into their computer model.

Using this model, they then calculated exactly where

earthquakes should happen along this particular fault line,

according to how stress gets transferred.

They came up with seven locations where they believe that earthquakes would happen.

One after another, in a sequence running east to west.

Then they compared these predictions with what had happened in the past.

It was uncanny.

They discovered that those seven sites, running from east to west,

were almost exactly where earthquakes had actually occurred

from 1939 to 1967.

Their model worked.

It was a triumph.

The model confirmed that the stress generated in one earthquake

was being transferred west along the fault line.

The earthquakes seemed to be triggering each other like a set of falling dominoes.

So the question was - where did the model predict that the next quake would happen?

In 1998, the team fed all the necessary equations

into their computer and calculated where the stress would travel next along the North Anatolian Fault.

The area that showed up red

was the Bay of Izmit, home to 500,000 people.

The scientists couldn't say when, but they were sure that an earthquake would strike Izmit.

Newspapers and journals printed this remarkable news, but surprisingly,

the warning barely registered and life for the people of Izmit continued as normal.

And then, just one year after they'd made their prediction,

in August 1999, the scientists' forecast came true.

DEEP RUMBLING

The earthquake, measuring 7.4 in magnitude, lasted just 45 seconds.

But that was enough to destroy much of the city and claim the lives of 25,000 people.

But Izmit is not the end of the story.

Everyone knows there will be another earthquake along this fault.

Where next? Well, using the data from the Izmit quake,

scientists have now calculated where the stress has transferred to along the fault line.

The area they've pinpointed as the next earthquake site will be an area west of Izmit.

It includes one of the greatest cities in the world...

Istanbul.

Scientists are sure that the earthquake that hits Istanbul

will be as big or bigger than the one that hit Izmit.

They're certain it will come. They just can't say when.

More than 10 million people live in Istanbul.

It contains some of the world's most treasured buildings.

The scale of the catastrophe, when it happens, will be almost unimaginable.

Being able to accurately forecast where an earthquake will happen is a major breakthrough.

But even if we could take it a step further and predict the exact time

and place, how can you evacuate a city of 10 million people

in just a few days?

It would be practically impossible.

We've finally reached the end of our fault line here in Greece, where it plunges down into the sea.

These lines etched into the cliff face are the former levels of the sea

that is been left stranded as the land has risen in a series of earthquake jumps.

Now here, those earthquake jumps are a matter of tens of centimetres.

But in truly giant quakes, they are of the order of a few metres.

And that brings me to my final story.

The story of the most powerful and violent kind of earthquakes that can happen on Earth.

They are called mega-thrust earthquakes.

This particular story is about one coast where a mega-thrust earthquake

happened in the past - and where we know one will happen again.

The people of California live under a terrible shadow.

They know that one day the San Andreas Fault will rupture, unleashing an enormous earthquake.

But all this time, an even more powerful hazard lies just a little further north.

It's a completely different fault.

And it's going to unleash an earthquake up to 30 times more

devastating than anything the San Andreas Fault can produce.

The source of all this danger lies deep beneath the waters

of the Pacific north-west coast.

It's an huge gash in the Earth's crust

that's nearly 1000 kilometres long.

It runs from British Columbia in Canada

and ends in northern California.

It's called the Cascadia Fault.

The Cascadia Fault lies on what geologists call a subduction zone.

A subduction zone is where two giant plates meet head to head,

and one of them gets pushed right down under the other.

Subduction zones can produce the biggest earthquakes on the planet.

Generally speaking, if you have two great masses of rock,

and you're scraping them one underneath the other,

they won't move very easily,

we'll get a lot of friction there.

And I liken it to sort of two cheese graters pushing past one another.

Very difficult to get any smooth sort of movement there.

Subduction zones cause earthquakes when the plate that's being pushed down gets stuck.

As it pushes, the upper plate gets squeezed and distorted.

Eventually, the strain becomes too much.

The upper plate slips and that's what creates a rare event -

a mega-thrust earthquake.

The power of an earthquake depends on the size of the fault that breaks.

And there's something unique about subduction zone earthquake faults.

With most earthquakes, only one small part of a fault line shifts, the part that's snagged.

The section that breaks can cause violent shaking and devastation in the immediate area.

But when a subduction fault ruptures,

something quite different happens.

It can unzip along the entire length of the fault line, for hundreds of kilometres.

The effect goes way beyond the reach of any normal earthquake event.

The Cascadia subduction zone is almost 1000 kilometres long.

If it does rupture along its full length, scientists believe

the next Cascadia earthquake will be one of the largest ever recorded.

A magnitude nine, or greater.

That's a terrifying prospect for the people of the Pacific region.

When the Cascadia fault ruptures, the shock waves will fan out

across the whole north-west coast of the continent.

And lying directly in its path are major cities like Vancouver, Seattle and Portland.

But it's not just its power that would be so devastating.

As the fault ruptures it will unzip at over 10,000 kilometres an hour.

Even at that speed,

it will take five minutes to travel the entire length of the fault line.

A five-minute earthquake is far longer than normal.

The duration of the event is very unusual.

In that sense alone it can cause more damage.

A quake that goes on for longer causes more damage than one that is over within 10 or 20 seconds.

And the big question is, when will it happen?

Forecasting earthquakes is notoriously difficult.

And no-one can say when Cascadia will rupture again.

But it's possible to look back at the geological record

and see how frequently they've happened in the past.

Sure enough, the Washington coast does hold traces of several past mega-thrust earthquakes.

In the United States, tree rings, Native American legends and geological evidence

all indicate that the Cascadian fault line has ruptured five times in the last 2,500 years.

So when will it happen next?

You can calculate that according to when it last ruptured.

And, according to the geological record, the last time that happened was in 1700.

That's 300 years ago. And in the past, it has ruptured at roughly 300-year intervals.

So the next Cascadia earthquake could be just around the corner.

We just don't know.

And, for the people of the Pacific coast, that's a pretty troubling thought to live with.

It seems to me that earthquakes will always bring devastation to our planet.

We certainly can't prevent them.

We still can't even predict them.

Earthquake science has still a long way to go if that's ever going to change.

In the meantime, all we can do is build for earthquakes and learn to live with them.

Subtitles by Red Bee Media Ltd.

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