0:00:04 > 0:00:08The world is full of stunning and dramatic landscapes,
0:00:08 > 0:00:12all formed by the complex history of our planet.
0:00:15 > 0:00:20A country like Greece may seem like the perfect holiday destination -
0:00:20 > 0:00:25beautiful scenery, gorgeous beaches and a fascinating history -
0:00:25 > 0:00:30but, for geologists like me, Greece has other attractions.
0:00:30 > 0:00:32It's the earthquake capital of Europe.
0:00:38 > 0:00:42Earthquakes are one of the most destructive natural forces on our planet
0:00:42 > 0:00:46and, as a geologist, it's my job to try and understand them.
0:00:46 > 0:00:53I'm here in Greece, following this dramatic earthquake fault line as it slices through the landscape.
0:00:53 > 0:00:56On the way, I'm going to tell you ten of the most remarkable
0:00:56 > 0:01:00earthquake stories from around the world.
0:01:03 > 0:01:07There will be plenty of surprises along the way.
0:01:07 > 0:01:12So, if you don't know how Cold War spying gave scientists a crucial clue to understanding earthquakes...
0:01:12 > 0:01:18where quakes last 60 times longer than anywhere on the planet...
0:01:18 > 0:01:25or which earthquake fault line causes hallucinations, then stay around, as I reveal:
0:01:31 > 0:01:34DEEP RUMBLING
0:01:48 > 0:01:52About an hour west of Athens, nestling in the hillside
0:01:52 > 0:01:54and surrounded by olive trees,
0:01:54 > 0:01:57lies the sleepy little village of Pisia.
0:01:57 > 0:02:04It might not be on the tourist trail, but it's a fascinating place.
0:02:04 > 0:02:07It sits right on top of an earthquake fault line.
0:02:07 > 0:02:11The fault itself is best seen just outside the village.
0:02:13 > 0:02:16This is a pretty special place for me...
0:02:16 > 0:02:20first came here in 1986 when I was a student studying earthquakes and
0:02:20 > 0:02:25this is the very earthquake fault I came to study.
0:02:25 > 0:02:29This smooth, polished rock face is the actual fault surface
0:02:29 > 0:02:34that gets violently pushed out the ground during an earthquake.
0:02:34 > 0:02:39It's impossible to imagine how destructive those forces are.
0:02:39 > 0:02:43And it was here that I first came face-to-face with
0:02:43 > 0:02:47people who had actually experienced the devastation of an earthquake.
0:02:59 > 0:03:03In February 1981, a major earthquake struck this region.
0:03:03 > 0:03:08More than 15 people died and hundreds were seriously injured.
0:03:17 > 0:03:20When I came to study this fault a few years later,
0:03:20 > 0:03:25all the villagers wanted to know was - when will it happen again?
0:03:25 > 0:03:29Could I calculate when the next earthquake would happen?
0:03:29 > 0:03:33For them, you know, it was a case of life and death.
0:03:33 > 0:03:35I hadn't thought about it until then, but then
0:03:35 > 0:03:41you realise how important the work is that geologists do, how it really affects people's daily lives.
0:03:47 > 0:03:51Earthquakes usually last for just a few seconds...
0:03:52 > 0:03:58yet in those few seconds they can cause destruction on a massive scale.
0:04:00 > 0:04:03Greece is the most active earthquake zone in Europe.
0:04:03 > 0:04:09Pretty bad luck, then, that this was also the birthplace of western civilisation.
0:04:09 > 0:04:12And that's the subject of my first story.
0:04:12 > 0:04:15The effect of earthquakes on our ancient heritage.
0:04:22 > 0:04:29Here in Greece, we're surrounded by the remnants of one of the greatest of all civilisations.
0:04:29 > 0:04:33Everywhere you look you see the ruins of a world that's been lost to us.
0:04:33 > 0:04:40Of course, many of these magnificent buildings were destroyed by war and pillage, but an astonishing amount
0:04:40 > 0:04:46were destroyed by earthquakes - even whole cities have all but vanished.
0:04:46 > 0:04:49The list is endless.
0:04:49 > 0:04:53The beautiful city of Helike, on the coast, lost to an earthquake.
0:04:53 > 0:04:59The wonderful Temple of Apollo at Corinth, destroyed by an earthquake.
0:04:59 > 0:05:01What must the ancient Greeks have thought when they saw their
0:05:01 > 0:05:06magnificent monuments shaken to destruction in a matter of seconds?
0:05:06 > 0:05:11How did they explain the terror of earthquakes?
0:05:11 > 0:05:16Well, not surprisingly, the Ancient Greeks thought it was all down to the anger of the gods.
0:05:19 > 0:05:24They blamed Poseidon, Earth shaker and god of the sea, for all the carnage and ruin.
0:05:29 > 0:05:34But in the fourth century BC, the Greek philosopher Aristotle
0:05:34 > 0:05:39came up with one of the first rational explanations of how earthquakes happened.
0:05:40 > 0:05:45He thought that there were deep, underground winds that caused the ground to fracture and shake.
0:05:49 > 0:05:52He may have been wrong, but at a time when every aspect
0:05:52 > 0:05:59of life was explained by the actions of the gods, this was at least a rational explanation.
0:05:59 > 0:06:04Today, we can explain most earthquakes through the theory of plate tectonics.
0:06:06 > 0:06:12The surface of our entire planet is made up of a number of moving plates.
0:06:12 > 0:06:17These tectonic plates fit together like giant pieces of a jigsaw puzzle.
0:06:17 > 0:06:23They sit on top of a hot, plasticky mantle of rock that's in constant motion.
0:06:25 > 0:06:31The cold, rigid plates move slowly over this hot, soft interior just a couple of centimetres a year,
0:06:31 > 0:06:37but as they move, they grate and tug and get caught up with each other at the plate boundaries.
0:06:37 > 0:06:41And that's how earthquakes begin.
0:06:41 > 0:06:47You get a real sense of how plates move when you come up to a fault line like this.
0:06:47 > 0:06:52As the vast plates slide past each other, they get snagged along their edges.
0:06:52 > 0:06:56When they get stuck, the pressure builds up along fault lines.
0:06:56 > 0:07:00Eventually the strain gets so great that the fault ruptures, releasing
0:07:00 > 0:07:05huge amounts of energy which shakes the ground for miles around.
0:07:09 > 0:07:15We measure that shaking on a scale of magnitude, sometimes called the Richter scale.
0:07:15 > 0:07:20Each number on the scale registers an earthquake 10 times larger than the last.
0:07:20 > 0:07:28So a magnitude 6 earthquake is 10 times larger than a 5, and an 8 is an earthquake 1,000 times larger.
0:07:30 > 0:07:35Greece lies close to the edge of one of these plates, in what's probably
0:07:35 > 0:07:39one of the most geologically complex parts of our planet.
0:07:39 > 0:07:41That's why it's so earthquake prone.
0:07:41 > 0:07:46You can find dramatic earthquake fault lines like this all over the country.
0:07:49 > 0:07:53It's no surprise, then, that so much of the ancient civilization that
0:07:53 > 0:07:58flourished here 2,000 years ago has disappeared forever.
0:08:05 > 0:08:08I wonder how different this country's architectural heritage
0:08:08 > 0:08:13would have been if it hadn't been in an earthquake zone?
0:08:13 > 0:08:18And what other unseen treasures from around the world have we lost to earthquakes?
0:08:21 > 0:08:26Without earthquakes, we might even have some of the great wonders of the world still standing,
0:08:26 > 0:08:34because we know that three of the seven Ancient Wonders of the World were devastated by earthquakes.
0:08:34 > 0:08:36The Colossus of Rhodes.
0:08:36 > 0:08:41In 226 BC, a powerful earthquake struck this region.
0:08:41 > 0:08:48The poor Colossus snapped at the weakest point, his knees, and fell to the ground, lost forever.
0:08:50 > 0:08:52And, of course, it's not just Greece.
0:08:52 > 0:08:57Over in Egypt, another wonder, the Lighthouse of Alexandria.
0:08:57 > 0:09:02Probably the tallest building in the world when it was built in 300BC.
0:09:02 > 0:09:07But it disappeared, toppled by earthquakes.
0:09:07 > 0:09:11And over in Turkey, there was the Mausoleum of Halicarnassus,
0:09:11 > 0:09:17a tragic and romantic monument built in 351 BC.
0:09:17 > 0:09:24The wife of King Mausolus was so distraught at his death that she built this beautiful tomb for him.
0:09:24 > 0:09:29Built for love, it was destroyed by earthquakes.
0:09:29 > 0:09:34You don't often think about it, but if it weren't for the awesome destructive power of earthquakes,
0:09:34 > 0:09:39many of the greatest lost treasures from the Ancients would still be with us today.
0:09:39 > 0:09:42Not just in Greece, but throughout the world.
0:09:42 > 0:09:47Earthquakes have dramatically changed our legacy from the past.
0:09:49 > 0:09:54My next story takes me back to the Great San Francisco earthquake of 1906.
0:09:54 > 0:09:59This was one of the worst natural disasters in American history.
0:09:59 > 0:10:03But it was also a turning point for geologists because it led to a real
0:10:03 > 0:10:08breakthrough in our understanding how earthquakes happen.
0:10:12 > 0:10:131906.
0:10:13 > 0:10:18San Francisco was a boom town, the largest city in the American West,
0:10:18 > 0:10:21thanks to the Californian Gold Rush.
0:10:24 > 0:10:31But while its 400,000 inhabitants were chasing the American Dream, they had no idea that the city was
0:10:31 > 0:10:38perched on one of the most unstable fault lines in the world, the San Andreas Fault.
0:10:38 > 0:10:44But on the 18th April 1906, they discovered the horrible truth.
0:10:53 > 0:10:59An earthquake of magnitude 7.9 devastated San Francisco.
0:11:02 > 0:11:08In the first few seconds of the quake, many people died in their beds
0:11:08 > 0:11:13as brick chimneys crashed through walls and crushed them as they were waking.
0:11:13 > 0:11:17The shaking set church bells ringing.
0:11:17 > 0:11:19It must have seemed like the end of the world.
0:11:21 > 0:11:26The quake lasted for just 45 seconds, but that's all it took to destroy the city.
0:11:28 > 0:11:30Gas mains burst
0:11:30 > 0:11:33and fires burned out of control for three days.
0:11:33 > 0:11:42The earthquake, and its aftermath, killed up to 6,000 people and left more than 200,000 homeless.
0:11:53 > 0:11:55This earthquake stunned everyone.
0:11:55 > 0:12:00Not even the scientists understood what had happened.
0:12:00 > 0:12:05In 1906, the study of earthquakes was still in its infancy.
0:12:05 > 0:12:09Very little was understood about how and why earthquakes occurred.
0:12:11 > 0:12:13But this was about to change.
0:12:13 > 0:12:17The scale of the devastation in San Francisco was so shocking
0:12:17 > 0:12:21that it stirred the scientific community into action.
0:12:21 > 0:12:26Our understanding of earthquakes was about to take a giant leap forward.
0:12:28 > 0:12:35Within a week of this event, a team of nine scientists met for the first time,
0:12:35 > 0:12:40determined to revolutionise our understanding of earthquakes.
0:12:40 > 0:12:44First, they studied the way the ground had shifted throughout the area.
0:12:44 > 0:12:47And it was through this detailed analysis that they made
0:12:47 > 0:12:49- the crucial link between the earthquake
0:12:49 > 0:12:53and the fact that the San Andreas fault was somehow the cause.
0:12:58 > 0:13:01The San Andreas Fault is one of California's
0:13:01 > 0:13:07most distinctive natural landmarks, a gigantic gash across the state.
0:13:07 > 0:13:12We now know that this fault is where two massive tectonic plates meet,
0:13:12 > 0:13:16and they're slowly moving past each other all the time.
0:13:16 > 0:13:21But back in 1906, no-one knew anything about these plates.
0:13:24 > 0:13:31Even so, the scientists did give us a lasting legacy that came from this study.
0:13:31 > 0:13:36One member of the team, Henry Fielding Reid, became fascinated by
0:13:36 > 0:13:42the way features like farm fences had been shoved out of line by the earthquake.
0:13:42 > 0:13:46The fences showed that the two sides of the fault line had shifted
0:13:46 > 0:13:49violently past each other in a consistent pattern.
0:13:51 > 0:13:53And, from this simple observation,
0:13:53 > 0:13:58Reid came up with the first real theory about how earthquakes happened.
0:14:03 > 0:14:07Reid was the first person to work out that, in some places,
0:14:07 > 0:14:12the Earth's crust is being strained and stretched to breaking point
0:14:14 > 0:14:16until finally, it snaps.
0:14:16 > 0:14:19And that's what an earthquake is.
0:14:25 > 0:14:29After this breakthrough, the whole study of earthquakes and geology took off
0:14:29 > 0:14:35and eventually revealed the secrets of how our planet works.
0:14:35 > 0:14:40So, out of the ashes of the terrible San Francisco earthquake came the first glimmerings
0:14:40 > 0:14:45of scientific understanding about this terrifying force of nature.
0:14:53 > 0:14:56My next story is an intriguing tale about
0:14:56 > 0:15:03one of the eureka moments in earthquake science, though it reads rather like a Cold War spy plot.
0:15:03 > 0:15:09The 1960s was a time when America and the Soviet Union were locked in a nuclear arms race.
0:15:09 > 0:15:14And it was America's paranoia with secret Soviet nuclear testing
0:15:14 > 0:15:17that helped to prove one of the biggest theories in geology.
0:15:17 > 0:15:22So my next story's not an earthquake site, but a bomb site.
0:15:31 > 0:15:38This is Novaya Zemlya Island, in Russia, one of the Soviet Union's secret nuclear test sites.
0:15:40 > 0:15:46The Soviets began testing their weapons in this remote place in 1954 at the height of the Cold War.
0:15:48 > 0:15:54Over a period of 35 years, hundreds of nuclear devices were detonated.
0:15:56 > 0:16:02But, of course, the Americans were determined to find out exactly what the Russians were up to.
0:16:03 > 0:16:09Rather than send in their spies, the Americans came up with a brilliant plan.
0:16:09 > 0:16:14They decided do some long-distance spying using seismometers.
0:16:18 > 0:16:21Seismometers are instruments that can register and measure shaking ground
0:16:21 > 0:16:27from thousands of miles away, even from the most secret locations.
0:16:32 > 0:16:36And, of course, nuclear explosions shake the ground,
0:16:36 > 0:16:38so seismometers would be able
0:16:38 > 0:16:42to record every explosion happening deep inside Russia.
0:16:46 > 0:16:53So, in 1961, the Americans installed 120 seismometers
0:16:53 > 0:16:59at sites all around the world and soon they were recording every Soviet nuclear test.
0:17:06 > 0:17:11But all the time this network of seismometers was recording nuclear explosions,
0:17:11 > 0:17:18it was also monitoring the natural phenomenon that shakes the ground all over the world - earthquakes.
0:17:28 > 0:17:34And that earthquake data produced a major surprise for science back in the '60s.
0:17:34 > 0:17:38Earthquakes weren't scattered chaotically across the planet.
0:17:38 > 0:17:41They followed a narrow set of lines around the globe.
0:17:43 > 0:17:45In their search for nuclear explosions,
0:17:45 > 0:17:51they'd coincidentally created an accurate map of the world's earthquake zones.
0:17:52 > 0:17:55And what was the significance of this map?
0:17:55 > 0:18:01Well, the earthquake lines revealed the hidden edges of the Earth's tectonic plates.
0:18:04 > 0:18:10In the 1960s, the theory of plate tectonics was a revolutionary new idea.
0:18:10 > 0:18:13But it was still just a theory.
0:18:13 > 0:18:17Now there was proof because the seismometers were able
0:18:17 > 0:18:21to measure the direction the plates were moving in during an earthquake.
0:18:27 > 0:18:31For the first time, geologists had solid evidence that showed
0:18:31 > 0:18:37how all these plates were moving against each other at the fault lines and causing earthquakes.
0:18:50 > 0:18:55Who could have imagined that spying for nuclear secrets would end up providing
0:18:55 > 0:19:01key evidence for the most important geological theory ever developed,
0:19:01 > 0:19:03the theory of plate tectonics.
0:19:07 > 0:19:11This discovery was a major breakthrough for earthquake science,
0:19:11 > 0:19:13and one of the few good things to come out of
0:19:13 > 0:19:17the bitter Cold War rivalry between America and the Soviet Union.
0:19:29 > 0:19:33My next story is about the deadliest earthquake in recorded history.
0:19:33 > 0:19:37It happened in China in 1556.
0:19:37 > 0:19:43In a matter of seconds, this one earthquake killed nearly a million people.
0:19:43 > 0:19:47The reason it was so devastating is because, in this particular region,
0:19:47 > 0:19:50people had always lived in man-made caves.
0:19:50 > 0:19:53And this would prove lethal in the earthquake.
0:20:03 > 0:20:06The world's deadliest earthquake happened
0:20:06 > 0:20:15four centuries ago in central China, on a vast, fertile plateau rising above the Yellow River.
0:20:15 > 0:20:20This is a region with freezing winters and baking hot summers,
0:20:20 > 0:20:26and the Chinese have come up with an ingenious solution to cope with these extremes of weather.
0:20:28 > 0:20:32Instead of building traditional houses, the locals have
0:20:32 > 0:20:36burrowed into the hillsides, creating caves, or Yaodongs.
0:20:36 > 0:20:41It's a tradition that goes back at least 2,000 years.
0:20:43 > 0:20:48Tunnelling through the soft soil to depths of hundreds of metres,
0:20:48 > 0:20:53these amazing structures are cool in the summer and warm in winter.
0:20:57 > 0:21:02Probably the most famous caves are in Yan'an.
0:21:02 > 0:21:07It's here that Mao Zedong and his troops holed up during the Communist Revolution.
0:21:12 > 0:21:20What I find so surprising is that even today, up to 40 million Chinese still live in these kinds of caves.
0:21:20 > 0:21:24That's two-thirds of the entire population of Britain.
0:21:25 > 0:21:30The fine, silty soil here is easy to dig out to make caves.
0:21:30 > 0:21:36But that means it's also highly unstable, and collapses easily.
0:21:36 > 0:21:42And it's this characteristic that proved to be so fatal in 1556.
0:21:46 > 0:21:51On the 23rd of January, a huge earthquake struck the region.
0:21:51 > 0:21:55Modern estimates put the earthquake at a magnitude 8.
0:21:55 > 0:21:58It shook a vast area of ground.
0:21:58 > 0:22:03Everything within 1,300 square kilometres was destroyed.
0:22:03 > 0:22:08But it's the loss of life that makes this the deadliest earthquake in history.
0:22:08 > 0:22:13Nearly 1 million people perished in this disaster.
0:22:13 > 0:22:18And it was the very structures they relied on for shelter that killed them.
0:22:18 > 0:22:23Their Yaodongs collapsed like a pack of cards as the soft soil gave way
0:22:23 > 0:22:26under the huge shockwaves, burying everyone inside them.
0:22:28 > 0:22:31A Chinese account from the time gives us a vivid description of how
0:22:31 > 0:22:35the force of the earthquake rearranged the landscape.
0:22:37 > 0:22:42Mountains and rivers changed places, and roads were destroyed.
0:22:42 > 0:22:46In some places, the ground suddenly rose up and formed new hills,
0:22:46 > 0:22:50or it sank in abruptly and became new valleys.
0:22:52 > 0:22:59The Shaanxi earthquake may not have been the biggest ever recorded, but it was certainly the deadliest.
0:22:59 > 0:23:03And when you think that today 40 million people still live
0:23:03 > 0:23:09in the same sort of caves as their ancestors, you realise that there could be much worse to come.
0:23:19 > 0:23:23It's not earthquakes that kill people, buildings do.
0:23:23 > 0:23:28And the ancient Greeks were the first to try and design their buildings against earthquakes.
0:23:28 > 0:23:33Some of their temples had metal rods inserted in the columns.
0:23:33 > 0:23:38In our next story, we'll see how innovative building design can save lives.
0:23:38 > 0:23:43And it turns out, surprisingly, that one particular kind of skyscraper
0:23:43 > 0:23:46might be the safest place to be when an earthquake strikes.
0:23:55 > 0:24:03The teeming metropolis of Mexico City lies in one of the worst earthquake zones in the world.
0:24:03 > 0:24:08What's more, large parts of the city are built on the site of an old lake bed,
0:24:08 > 0:24:13drained by the Spanish after they conquered the Aztecs.
0:24:13 > 0:24:16It's a highly unstable site.
0:24:16 > 0:24:22The soil is a mixture of soft sands and clays, which are full of water.
0:24:22 > 0:24:28Yet this is where developers decided to build the tallest skyscraper in Latin America.
0:24:28 > 0:24:30It's called the Torre Mayor.
0:24:30 > 0:24:36Completed in 2003, the building is 225 metres high.
0:24:39 > 0:24:45And yet the people who come to work here are confident that it will keep them safe in an earthquake.
0:24:45 > 0:24:51I would rather be in Torre Mayor than anywhere else in the city whenever there's an earthquake.
0:24:51 > 0:24:54That's a pretty astonishing claim.
0:24:54 > 0:24:59But the Torre Major is probably the strongest building in the world,
0:24:59 > 0:25:03built to withstand an earthquake of a magnitude of 8.5.
0:25:03 > 0:25:06That's one colossal earthquake.
0:25:06 > 0:25:09So what makes this building so earthquake-proof?
0:25:09 > 0:25:15And what on Earth made them reach for the sky in the most unstable part of the city?
0:25:15 > 0:25:21The answers lie long before the Torre Mayor was built, in the events of 1985.
0:25:23 > 0:25:29Early in the morning of September the 19th, Mexico City was struck by a massive earthquake.
0:25:36 > 0:25:42At a magnitude of 8.1, it was the worst earthquake in Mexico's history.
0:25:42 > 0:25:47At least 9,000 people died, with 30,000 injured.
0:25:49 > 0:25:54Despite the scale of the tragedy, there were some amazing miracles.
0:25:54 > 0:25:59Three days after the quake, 58 newborn babies were pulled alive
0:25:59 > 0:26:01from the wreckage of a maternity ward.
0:26:01 > 0:26:06Rescuers continued to find survivors up to a week later.
0:26:10 > 0:26:14The greatest destruction happened in the area of the old lake bed.
0:26:14 > 0:26:20That's because when the shockwaves hit the lake bed, something strange happened.
0:26:20 > 0:26:23The soft sediments under the buildings
0:26:23 > 0:26:27actually amplified the shockwaves, making them far more destructive.
0:26:27 > 0:26:31These powerful waves cracked the foundations of buildings,
0:26:31 > 0:26:34setting up vibrations that shook them into rubble.
0:26:34 > 0:26:41And that's not all. Something else even stranger happened to the soil beneath these buildings.
0:26:43 > 0:26:45It's called liquefaction.
0:26:45 > 0:26:49Liquefaction is when apparently solid ground disintegrates
0:26:49 > 0:26:52and water trapped in the soil starts leaking out.
0:26:55 > 0:26:59When wet ground gets shaken, the land takes on the properties of a liquid.
0:26:59 > 0:27:05Buildings simply sink into the ground, regardless of how solid their foundations,
0:27:05 > 0:27:08and torrents of water get erupted out of the surface.
0:27:10 > 0:27:16Liquefaction has only been caught on camera once, during an earthquake in Japan in 1964.
0:27:16 > 0:27:20It's an amazing sight.
0:27:20 > 0:27:22Buildings just slip into the ground.
0:27:27 > 0:27:30It wasn't as severe as this in Mexico City,
0:27:30 > 0:27:38but in 1985, liquefaction did add to the devastation in the drained lake area.
0:27:38 > 0:27:43So it's astonishing that it was in this very area of maximum destruction
0:27:43 > 0:27:47that developers decided to build the tallest building in Mexico.
0:27:53 > 0:27:58It sounds crazy, but engineers were confident that in the Torre Mayor they could construct a building
0:27:58 > 0:28:03in this vulnerable place to withstand almost anything that nature could throw at it.
0:28:06 > 0:28:09Their first challenge was the foundations.
0:28:09 > 0:28:14Without solid foundations, this building could become an enormous glass and steel coffin.
0:28:14 > 0:28:19To get past the loose marshy sediments of the lake bed
0:28:19 > 0:28:24and anchor the building in solid bedrock, engineers drilled down 60 metres.
0:28:24 > 0:28:30That's three times as deep as the foundations of the Empire State Building.
0:28:30 > 0:28:35Over 250 piles secured the main structure to solid ground.
0:28:38 > 0:28:41But what really helps the tower ride out an earthquake
0:28:41 > 0:28:46is an altogether different and unique piece of engineering.
0:28:46 > 0:28:50Behind the oceans of glass lies the strongest possible skeleton -
0:28:50 > 0:28:55a network of super diagonal diamond braces.
0:28:55 > 0:29:01Where the super diagonals overlap, they form three smaller diamonds.
0:29:01 > 0:29:06At each junction rests four huge shock absorbers.
0:29:06 > 0:29:08And here's the magic of the design.
0:29:08 > 0:29:14They look and operate just like a regular shock absorber in your car.
0:29:14 > 0:29:17The only difference is that they're about the size of your car.
0:29:19 > 0:29:21And this is how they work.
0:29:23 > 0:29:25The moment a seismic wave strikes the tower,
0:29:25 > 0:29:30giant pistons inside the shock absorbers are forced inward.
0:29:30 > 0:29:34These dampen the severe vibrations from the shock waves,
0:29:34 > 0:29:36and absorb their energy.
0:29:36 > 0:29:42Instead of crashing to the ground, the tower flexes safely.
0:29:45 > 0:29:48I think Torre Mayor is a benchmark.
0:29:48 > 0:29:55Everybody would like to build buildings as important, as safe, as Torre Mayor is today.
0:29:57 > 0:30:01In the past, Mexico's been struck by massive earthquakes,
0:30:01 > 0:30:05five times more powerful than anything Torre Mayor has faced.
0:30:05 > 0:30:09Yet the impressive team of engineers that built this skyscraper
0:30:09 > 0:30:13are confident that it would survive an earthquake of that size.
0:30:13 > 0:30:17They believe that in one of the planet's worst earthquake zones,
0:30:17 > 0:30:20this could remain one of the world's safest buildings.
0:30:26 > 0:30:29Earthquakes usually last for a few seconds.
0:30:29 > 0:30:35Occasionally they go on for minutes, but there's one place where quakes can last for up to one hour.
0:30:35 > 0:30:41But no matter how long they last or how violent they are, these quakes pose no threat at all.
0:30:41 > 0:30:46The reason that you've never heard of them is that they're out of this world.
0:30:46 > 0:30:47INDISTINCT
0:30:47 > 0:30:49The engines are on.
0:30:49 > 0:30:51My next earthquake story
0:30:51 > 0:30:55is all about our close encounter with the moon.
0:30:55 > 0:31:01On July 16th, 1969, when Apollo 11 blasted off towards the moon,
0:31:01 > 0:31:05it wasn't just carrying a trio of astronauts.
0:31:05 > 0:31:08Apart from its valuable human cargo,
0:31:08 > 0:31:11Apollo 11 also carried a number of scientific instruments.
0:31:14 > 0:31:19What's extraordinary to realise now is that in these early flights to
0:31:19 > 0:31:23the moon, these science experiments were almost an afterthought.
0:31:23 > 0:31:27NASA scientists were so intent on simply getting the craft to the moon
0:31:27 > 0:31:29that they didn't give too much thought
0:31:29 > 0:31:34to what Buzz Aldrin and Neil Armstrong would do once they got there.
0:31:34 > 0:31:38So, in something of a rush, they put together instruments that
0:31:38 > 0:31:43would measure the Moon's magnetic fields and investigate solar winds.
0:31:43 > 0:31:48But the one that interests me is a specially designed seismometer,
0:31:48 > 0:31:52placed carefully on the moon's surface by Buzz Aldrin.
0:31:52 > 0:31:58These seismometers would one day reveal something totally unexpected about the moon.
0:32:00 > 0:32:04Considering how rushed the NASA boffins had been,
0:32:04 > 0:32:07the seismometer was a very smart bit of kit.
0:32:07 > 0:32:12It was powered by solar panels, which would work for as long it was in sight of the sun.
0:32:13 > 0:32:21Once the lunar night kicked in, after about two of our weeks, the seismometer would close down.
0:32:21 > 0:32:26To operate properly, it had to be perfectly level.
0:32:26 > 0:32:28The astronauts didn't have time to do this,
0:32:28 > 0:32:35so the seismometer was fitted with motors, so that it could be levelled by remote control from Earth.
0:32:35 > 0:32:42For two weeks, the seismometer made the first ever recordings of the moon's jolts and tremors.
0:32:45 > 0:32:52It even picked up the vibrations made by Buzz Aldrin climbing up the ladder into the lunar module.
0:32:57 > 0:32:59But that wasn't all.
0:32:59 > 0:33:04As more seismometers were sent up, the data sent back showed,
0:33:04 > 0:33:10to everyone's amazement, that the moon had thousands of quakes every year...
0:33:10 > 0:33:11moonquakes.
0:33:18 > 0:33:22This came as a surprise to scientists, because the moon has
0:33:22 > 0:33:25none of the geological features that cause earthquakes here on Earth.
0:33:28 > 0:33:31The moon is almost entirely solid rock.
0:33:31 > 0:33:35It has no tectonic plates rubbing up against each other.
0:33:35 > 0:33:39So what was the cause of these moonquakes?
0:33:39 > 0:33:42Well, they finally worked it out.
0:33:42 > 0:33:48And amazingly, many of these quakes, the really deep ones, are caused by the Earth.
0:33:48 > 0:33:52In just the same way that the moon's gravity pulls at our oceans,
0:33:52 > 0:33:57creating tides, Planet Earth exerts a gravitational pull on the moon.
0:33:57 > 0:34:03Now, the moon doesn't have oceans or tides, but as the Earth pulls at its rocky structure,
0:34:03 > 0:34:11there's a build up of strain as these rocks deform, and this strain is then released through moonquakes.
0:34:11 > 0:34:16And what came as a real surprise is that, unlike the short, sharp events we have on Earth,
0:34:16 > 0:34:19moonquakes can last for up to an hour or more.
0:34:19 > 0:34:23That's 60 times longer than most quakes on Earth.
0:34:28 > 0:34:33But it turns out there's a very simple reason for this.
0:34:35 > 0:34:39When seismic waves travel through the Earth's crust, they die away pretty quickly.
0:34:39 > 0:34:43Even the biggest earthquake shakes the ground for just a couple of minutes.
0:34:43 > 0:34:46And that's because there's water on the Earth.
0:34:46 > 0:34:49And rather than acting like solid rock, the crust acts
0:34:49 > 0:34:54like a giant sponge, soaking up the energy of the seismic waves.
0:34:57 > 0:35:00But the moon has no water on its surface,
0:35:00 > 0:35:06and when a moonquake strikes it sets the whole sphere vibrating like a tuning fork.
0:35:06 > 0:35:09So that's why quakes last so long up there.
0:35:09 > 0:35:14There's no liquid to absorb the vibrations, so they just keep going and going.
0:35:23 > 0:35:30Just imagine the destruction an hour-long earthquake would produce in the average city here on Earth.
0:35:33 > 0:35:40Of course, if we ever went to live on the moon we'd have to make sure our houses were moonquake-proof.
0:35:40 > 0:35:46It's one small step for man, one giant leap for mankind.
0:35:48 > 0:35:51My next story couldn't be more different.
0:35:51 > 0:35:56It's about an earthquake fault that's destroyed empires and deposed kings.
0:35:56 > 0:35:59But not because of the violence of its shaking,
0:35:59 > 0:36:04but rather because of its extraordinary powers of prediction.
0:36:09 > 0:36:13This is Delphi, on the slopes of Mount Parnassus.
0:36:13 > 0:36:17The centre of the universe for the Ancient Greeks.
0:36:17 > 0:36:24Here stood one of the most famous of all Greek temples, dedicated to the sun god, Apollo.
0:36:24 > 0:36:31Built in the 7th century BC, this temple has a surprising dependence on earthquakes.
0:36:31 > 0:36:37For centuries, this was the site of the Delphi Oracle, THE place to come
0:36:37 > 0:36:41if you wanted to ask Apollo what the future held in store.
0:36:41 > 0:36:46His answers came through the medium of a priestess known as the Pythia.
0:36:46 > 0:36:52She would enter into the temple's inner sanctum, a small, enclosed basement chamber,
0:36:52 > 0:36:59sit on a three-legged stool, and begin to issue prophecies to her paying customers.
0:36:59 > 0:37:05People came from all over the Mediterranean to ask if they'd be lucky in love or successful in war.
0:37:05 > 0:37:10Her answers determined the fate of people and nations.
0:37:12 > 0:37:15But things could go horribly wrong.
0:37:15 > 0:37:20For instance, legend has it that in the 6th century BC,
0:37:20 > 0:37:25the Pythia was consulted by Croesus, the last King of Lydia.
0:37:25 > 0:37:28One of the ancient world's super rich,
0:37:28 > 0:37:33he ruled over a huge empire and had his sights on the Persian one, too.
0:37:33 > 0:37:38The Oracle at Delphi told Croesus that if he attacked the Persians,
0:37:38 > 0:37:41he would destroy a great empire.
0:37:44 > 0:37:49So he did attack the Persians, but to his great surprise, lost the war.
0:37:50 > 0:37:55So the oracle came true, but not in the way he wanted.
0:37:57 > 0:38:00An empire WAS destroyed -
0:38:00 > 0:38:01unfortunately, it was his own.
0:38:05 > 0:38:11By now you're probably wondering, what's the connection between earthquakes and prophecies?
0:38:11 > 0:38:17Well, Delphi actually sits on top of two intersecting earthquake faults.
0:38:17 > 0:38:19Geologists have recently found
0:38:19 > 0:38:22that these faults pass right through the inner sanctum,
0:38:22 > 0:38:25where the Pythia was thought to have sat.
0:38:25 > 0:38:33Now, we know that some faults emit noxious, and even hallucinogenic gases.
0:38:33 > 0:38:37And according to ancient writers, before going into a trance
0:38:37 > 0:38:41the Pythia would drink from a spring running through the site.
0:38:41 > 0:38:45She would inhale intoxicating vapours from deep within the Earth.
0:38:45 > 0:38:48Vapours which would loosen her tongue.
0:38:50 > 0:38:59Geologists have found that water from a spring near the Delphi Oracle contains a gas called ethylene.
0:38:59 > 0:39:04This sweet-smelling gas is a narcotic, so when the Pythia went
0:39:04 > 0:39:07to her enclosed, subterranean chamber to foresee the future,
0:39:07 > 0:39:12she could have been exposed to concentrations of ethylene,
0:39:12 > 0:39:16coming out of the fault line, strong enough to induce a trance.
0:39:16 > 0:39:20In other words, she was as high as a kite.
0:39:22 > 0:39:28It's thousands of years since people flocked to the Oracle at Delphi to have their futures foretold.
0:39:28 > 0:39:31These days we don't have to travel as far.
0:39:31 > 0:39:35Simply pick up a magazine and read your horoscope.
0:39:35 > 0:39:38My next earthquake is the most surprising of all.
0:39:38 > 0:39:42Because it happened in a place that should never have had an earthquake.
0:39:42 > 0:39:47It's nowhere near an active earthquake zone, in fact, it's nowhere near a plate boundary.
0:39:47 > 0:39:53To find out more about it, I need to pop over to the land of reindeer.
0:40:03 > 0:40:09I'm here in Lapland, in Northern Sweden, miles from any earthquake zone.
0:40:09 > 0:40:13So you'd think there would be no earthquakes here at all.
0:40:13 > 0:40:14But you'd be completely wrong.
0:40:16 > 0:40:19I'm standing on the Parve Fault.
0:40:19 > 0:40:24Parve in the Lapp language means wave, and when you look at it
0:40:24 > 0:40:27you can see it undulating away into the distance.
0:40:27 > 0:40:32And what's really impressive is it's length.
0:40:32 > 0:40:36It may only be a low cliff but it runs across the landscape
0:40:36 > 0:40:41for 150 kilometres, a great tear in the Earth's crust.
0:40:41 > 0:40:46There's only one thing that rips through the ground like this, and that's an earthquake.
0:40:46 > 0:40:49DEEP RUMBLING
0:40:57 > 0:41:02What gets me about this earthquake, what makes it so surprising,
0:41:02 > 0:41:05is that it's so far from any active earthquake zone.
0:41:05 > 0:41:12The nearest one is in Iceland, over 1,500 kilometres back there.
0:41:12 > 0:41:18So, at first sight, it seems that this earthquake in Northern Sweden is inexplicable.
0:41:18 > 0:41:19It shouldn't be here.
0:41:22 > 0:41:29The height of this cliff, about 10 metres, also reveals the strength of the earthquake.
0:41:29 > 0:41:33It's been calculated that the one that caused this uplift of the ground
0:41:33 > 0:41:37must have had a magnitude of around 8.2.
0:41:37 > 0:41:40That's an unbelievably large earthquake,
0:41:40 > 0:41:44in an area that's nowhere near a tectonic plate boundary.
0:41:44 > 0:41:47So what caused this mysterious earthquake?
0:41:47 > 0:41:50When geologists began to study the Parve Fault,
0:41:50 > 0:41:57they dated the earthquake event that thrust this cliff out of the ground at around 8,500 years ago.
0:41:57 > 0:42:02But while they had a date, they couldn't figure out the cause.
0:42:02 > 0:42:04And then it dawned on them.
0:42:04 > 0:42:08Something else had happened during this geological period.
0:42:08 > 0:42:13And that was the emergence of this entire region from the Ice Age.
0:42:19 > 0:42:26During the last Ice Age, this area was covered in ice to a depth of three kilometres.
0:42:26 > 0:42:31That's about 3,000 tons of ice pressing down on every square metre of land.
0:42:31 > 0:42:33Or think of it this way.
0:42:33 > 0:42:40The weight of two Eiffel Towers pressing down on an area the size of a snooker table.
0:42:42 > 0:42:45This exceptional weight of ice literally pushed the land
0:42:45 > 0:42:50down by many metres, for thousands and thousands of years.
0:42:51 > 0:42:57And when all this ice melted, the Earth's crust reacted in an unusually violent way.
0:42:57 > 0:43:00DEEP RUMBLING
0:43:00 > 0:43:04With the enormous weight of ice now suddenly lifted off the land,
0:43:04 > 0:43:07it began to spring up, back to its original height.
0:43:10 > 0:43:18Normally the land rises slowly and gradually, at about a centimetre or so a year. But not always.
0:43:18 > 0:43:24Sometimes, as the land is rebounding, stresses build up along a line of weakness in the rock
0:43:24 > 0:43:29until they reach such a point that an earthquake rips along and relieves the pressure.
0:43:36 > 0:43:40And that's exactly what happened here.
0:43:40 > 0:43:47All across Scandinavia, from Lapland to Norway, you can see these earthquake scars.
0:43:47 > 0:43:53They are the marks of the Earth's crust springing up after thousands of years of colossal pressure.
0:43:57 > 0:44:03We now believe that what happened in Norway and Sweden 8,500 years ago could happen again.
0:44:03 > 0:44:07And that's because of climate change.
0:44:07 > 0:44:14Some scientists believe that global warming could melt the ice caps that weigh down Antarctica and Greenland.
0:44:14 > 0:44:22And if that happens the land will rebound, producing big earthquakes in the most unexpected of places.
0:44:27 > 0:44:31Geologists know lots about earthquakes now, but surprisingly,
0:44:31 > 0:44:38even after decades of research, we still can't predict exactly when and where they'll happen.
0:44:38 > 0:44:41That's because the build up of pressure deep inside the Earth
0:44:41 > 0:44:47is so slow, and usually gives few warning signs that we can measure.
0:44:47 > 0:44:51But for some geologists, prediction remains the Holy Grail.
0:44:51 > 0:44:57And over in Turkey, there's one fault where we think we can make an earthquake forecast.
0:44:57 > 0:45:02We think we know exactly where the next earthquake will strike and that's an exciting breakthrough.
0:45:02 > 0:45:06The only trouble is, we don't know when.
0:45:14 > 0:45:22Here in Turkey, there's a fault line that slices through the country for 1,500 kilometres.
0:45:22 > 0:45:25It's one of the most treacherous faults on the planet.
0:45:25 > 0:45:28Millions of people live along its path.
0:45:28 > 0:45:32It's known as the North Anatolian Fault.
0:45:32 > 0:45:38Unlike most other faults, the North Anatolian Fault runs in a relatively straight, simple line.
0:45:38 > 0:45:45And that means it may be easier here for scientists to calculate where the next earthquake will happen.
0:45:45 > 0:45:51That's because scientists now know that after an earthquake, the stress from that one earthquake
0:45:51 > 0:45:56travels further down the same fault line and builds up in a new location along the fault.
0:45:56 > 0:46:01So in theory, if you can pinpoint where the stress has gone to
0:46:01 > 0:46:04after one earthquake, you know where the next earthquake will strike.
0:46:07 > 0:46:12The key is to work out where the stress from the last earthquake has travelled to.
0:46:13 > 0:46:20And one scientist, Geoffrey King, has been developing a computer model that can do just that.
0:46:22 > 0:46:29King and his team of scientists plot where all the aftershocks happen after a major earthquake.
0:46:29 > 0:46:34That tells you the broad areas where the stress has transferred to.
0:46:34 > 0:46:38But to pinpoint more precisely where the next earthquake might strike,
0:46:38 > 0:46:44this model analyses some key geological data about previous earthquakes.
0:46:44 > 0:46:47For instance, how deep underground the rupture was,
0:46:47 > 0:46:52the nature of the rocks in the area, and how much the fault had slipped.
0:46:56 > 0:47:01The calculations of where the stress had gone show up in red.
0:47:02 > 0:47:08To see if their model worked, in the 1990s, this team focused their attention on Turkey
0:47:08 > 0:47:14and fed all the geological data from the North Anatolian Fault into their computer model.
0:47:17 > 0:47:21Using this model, they then calculated exactly where
0:47:21 > 0:47:24earthquakes should happen along this particular fault line,
0:47:24 > 0:47:27according to how stress gets transferred.
0:47:27 > 0:47:32They came up with seven locations where they believe that earthquakes would happen.
0:47:32 > 0:47:37One after another, in a sequence running east to west.
0:47:37 > 0:47:42Then they compared these predictions with what had happened in the past.
0:47:42 > 0:47:44It was uncanny.
0:47:44 > 0:47:48They discovered that those seven sites, running from east to west,
0:47:48 > 0:47:52were almost exactly where earthquakes had actually occurred
0:47:52 > 0:47:55from 1939 to 1967.
0:47:55 > 0:47:57Their model worked.
0:47:59 > 0:48:01It was a triumph.
0:48:01 > 0:48:05The model confirmed that the stress generated in one earthquake
0:48:05 > 0:48:09was being transferred west along the fault line.
0:48:09 > 0:48:14The earthquakes seemed to be triggering each other like a set of falling dominoes.
0:48:18 > 0:48:24So the question was - where did the model predict that the next quake would happen?
0:48:26 > 0:48:30In 1998, the team fed all the necessary equations
0:48:30 > 0:48:38into their computer and calculated where the stress would travel next along the North Anatolian Fault.
0:48:38 > 0:48:40The area that showed up red
0:48:40 > 0:48:44was the Bay of Izmit, home to 500,000 people.
0:48:46 > 0:48:52The scientists couldn't say when, but they were sure that an earthquake would strike Izmit.
0:48:55 > 0:49:01Newspapers and journals printed this remarkable news, but surprisingly,
0:49:01 > 0:49:06the warning barely registered and life for the people of Izmit continued as normal.
0:49:09 > 0:49:13And then, just one year after they'd made their prediction,
0:49:13 > 0:49:17in August 1999, the scientists' forecast came true.
0:49:17 > 0:49:20DEEP RUMBLING
0:49:37 > 0:49:44The earthquake, measuring 7.4 in magnitude, lasted just 45 seconds.
0:49:44 > 0:49:51But that was enough to destroy much of the city and claim the lives of 25,000 people.
0:50:04 > 0:50:06But Izmit is not the end of the story.
0:50:06 > 0:50:11Everyone knows there will be another earthquake along this fault.
0:50:11 > 0:50:15Where next? Well, using the data from the Izmit quake,
0:50:15 > 0:50:22scientists have now calculated where the stress has transferred to along the fault line.
0:50:22 > 0:50:28The area they've pinpointed as the next earthquake site will be an area west of Izmit.
0:50:28 > 0:50:32It includes one of the greatest cities in the world...
0:50:32 > 0:50:34Istanbul.
0:50:50 > 0:50:54Scientists are sure that the earthquake that hits Istanbul
0:50:54 > 0:50:58will be as big or bigger than the one that hit Izmit.
0:50:58 > 0:51:02They're certain it will come. They just can't say when.
0:51:07 > 0:51:11More than 10 million people live in Istanbul.
0:51:11 > 0:51:16It contains some of the world's most treasured buildings.
0:51:16 > 0:51:21The scale of the catastrophe, when it happens, will be almost unimaginable.
0:51:29 > 0:51:34Being able to accurately forecast where an earthquake will happen is a major breakthrough.
0:51:34 > 0:51:38But even if we could take it a step further and predict the exact time
0:51:38 > 0:51:43and place, how can you evacuate a city of 10 million people
0:51:43 > 0:51:44in just a few days?
0:51:44 > 0:51:47It would be practically impossible.
0:51:49 > 0:51:56We've finally reached the end of our fault line here in Greece, where it plunges down into the sea.
0:51:56 > 0:52:01These lines etched into the cliff face are the former levels of the sea
0:52:01 > 0:52:05that is been left stranded as the land has risen in a series of earthquake jumps.
0:52:05 > 0:52:10Now here, those earthquake jumps are a matter of tens of centimetres.
0:52:10 > 0:52:14But in truly giant quakes, they are of the order of a few metres.
0:52:14 > 0:52:17And that brings me to my final story.
0:52:17 > 0:52:23The story of the most powerful and violent kind of earthquakes that can happen on Earth.
0:52:23 > 0:52:26They are called mega-thrust earthquakes.
0:52:26 > 0:52:30This particular story is about one coast where a mega-thrust earthquake
0:52:30 > 0:52:34happened in the past - and where we know one will happen again.
0:52:43 > 0:52:47The people of California live under a terrible shadow.
0:52:47 > 0:52:54They know that one day the San Andreas Fault will rupture, unleashing an enormous earthquake.
0:52:54 > 0:53:00But all this time, an even more powerful hazard lies just a little further north.
0:53:00 > 0:53:03It's a completely different fault.
0:53:03 > 0:53:07And it's going to unleash an earthquake up to 30 times more
0:53:07 > 0:53:10devastating than anything the San Andreas Fault can produce.
0:53:12 > 0:53:16The source of all this danger lies deep beneath the waters
0:53:16 > 0:53:18of the Pacific north-west coast.
0:53:18 > 0:53:21It's an huge gash in the Earth's crust
0:53:21 > 0:53:24that's nearly 1000 kilometres long.
0:53:24 > 0:53:26It runs from British Columbia in Canada
0:53:26 > 0:53:29and ends in northern California.
0:53:29 > 0:53:33It's called the Cascadia Fault.
0:53:37 > 0:53:42The Cascadia Fault lies on what geologists call a subduction zone.
0:53:42 > 0:53:46A subduction zone is where two giant plates meet head to head,
0:53:46 > 0:53:49and one of them gets pushed right down under the other.
0:53:49 > 0:53:54Subduction zones can produce the biggest earthquakes on the planet.
0:53:56 > 0:53:59Generally speaking, if you have two great masses of rock,
0:53:59 > 0:54:01and you're scraping them one underneath the other,
0:54:01 > 0:54:02they won't move very easily,
0:54:02 > 0:54:04we'll get a lot of friction there.
0:54:04 > 0:54:08And I liken it to sort of two cheese graters pushing past one another.
0:54:08 > 0:54:11Very difficult to get any smooth sort of movement there.
0:54:11 > 0:54:17Subduction zones cause earthquakes when the plate that's being pushed down gets stuck.
0:54:17 > 0:54:22As it pushes, the upper plate gets squeezed and distorted.
0:54:22 > 0:54:24Eventually, the strain becomes too much.
0:54:24 > 0:54:29The upper plate slips and that's what creates a rare event -
0:54:29 > 0:54:31a mega-thrust earthquake.
0:54:38 > 0:54:42The power of an earthquake depends on the size of the fault that breaks.
0:54:42 > 0:54:47And there's something unique about subduction zone earthquake faults.
0:54:47 > 0:54:53With most earthquakes, only one small part of a fault line shifts, the part that's snagged.
0:54:53 > 0:55:00The section that breaks can cause violent shaking and devastation in the immediate area.
0:55:00 > 0:55:02But when a subduction fault ruptures,
0:55:02 > 0:55:05something quite different happens.
0:55:05 > 0:55:10It can unzip along the entire length of the fault line, for hundreds of kilometres.
0:55:10 > 0:55:15The effect goes way beyond the reach of any normal earthquake event.
0:55:15 > 0:55:21The Cascadia subduction zone is almost 1000 kilometres long.
0:55:21 > 0:55:26If it does rupture along its full length, scientists believe
0:55:26 > 0:55:30the next Cascadia earthquake will be one of the largest ever recorded.
0:55:30 > 0:55:33A magnitude nine, or greater.
0:55:39 > 0:55:44That's a terrifying prospect for the people of the Pacific region.
0:55:44 > 0:55:47When the Cascadia fault ruptures, the shock waves will fan out
0:55:47 > 0:55:51across the whole north-west coast of the continent.
0:55:51 > 0:55:58And lying directly in its path are major cities like Vancouver, Seattle and Portland.
0:55:58 > 0:56:02But it's not just its power that would be so devastating.
0:56:02 > 0:56:09As the fault ruptures it will unzip at over 10,000 kilometres an hour.
0:56:09 > 0:56:10Even at that speed,
0:56:10 > 0:56:15it will take five minutes to travel the entire length of the fault line.
0:56:15 > 0:56:18A five-minute earthquake is far longer than normal.
0:56:23 > 0:56:26The duration of the event is very unusual.
0:56:26 > 0:56:29In that sense alone it can cause more damage.
0:56:29 > 0:56:34A quake that goes on for longer causes more damage than one that is over within 10 or 20 seconds.
0:56:37 > 0:56:40And the big question is, when will it happen?
0:56:43 > 0:56:46Forecasting earthquakes is notoriously difficult.
0:56:46 > 0:56:50And no-one can say when Cascadia will rupture again.
0:56:50 > 0:56:54But it's possible to look back at the geological record
0:56:54 > 0:56:57and see how frequently they've happened in the past.
0:56:57 > 0:57:04Sure enough, the Washington coast does hold traces of several past mega-thrust earthquakes.
0:57:05 > 0:57:12In the United States, tree rings, Native American legends and geological evidence
0:57:12 > 0:57:18all indicate that the Cascadian fault line has ruptured five times in the last 2,500 years.
0:57:18 > 0:57:21So when will it happen next?
0:57:21 > 0:57:26You can calculate that according to when it last ruptured.
0:57:26 > 0:57:32And, according to the geological record, the last time that happened was in 1700.
0:57:35 > 0:57:42That's 300 years ago. And in the past, it has ruptured at roughly 300-year intervals.
0:57:42 > 0:57:48So the next Cascadia earthquake could be just around the corner.
0:57:48 > 0:57:49We just don't know.
0:57:49 > 0:57:55And, for the people of the Pacific coast, that's a pretty troubling thought to live with.
0:58:03 > 0:58:08It seems to me that earthquakes will always bring devastation to our planet.
0:58:08 > 0:58:10We certainly can't prevent them.
0:58:10 > 0:58:12We still can't even predict them.
0:58:12 > 0:58:17Earthquake science has still a long way to go if that's ever going to change.
0:58:17 > 0:58:22In the meantime, all we can do is build for earthquakes and learn to live with them.
0:58:45 > 0:58:48Subtitles by Red Bee Media Ltd.
0:58:48 > 0:58:51E-mail subtitling@bbc.co.uk