The Next Megaquake (2005) BBC Horizon

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NARRATOR (JACK FORTUNE): Boxing Day, 2004. The world was shocked by one of the worst natural disasters of all time. Over two hundred and fifty thousand people died. The cause of this devastation was the most powerful kind of earthquake on the planet, called a megathrust. This event made us realise how poorly prepared we are to face these huge geological catastrophes. And scientists are now trying to work out where else is at risk. They have discovered that a megathrust as large as the Sumatra quake could hit North America.

NARRATOR: Everyone knows that America is going to be struck by a devastating earthquake. For years the people of California have been waiting for the day when the San Andreas fault unleashes the big one. But all the time an even more powerful hazard has lain undiscovered. A giant megathrust earthquake, just like the one that hit Indonesia, threatens America's Pacific Northwest. A huge area from northern California all the way to Canada is at risk, including major cities like Seattle and Vancouver.


NARRATOR: The authorities have started to prepare for this catastrophe. Rehearsals like this one help to train the emergency services to deal with such an event. Children are now taught lessons in survival that could mean the difference between life and death. The source of all this danger lying under water off the Pacific Northwest coast is a huge gash in the earth's crust, a subduction zone. The earth's crust is made up of huge plates of rock that are constantly in motion. Where two of these giant plates meet head to head one of them can get pushed down under the other, this is a subduction zone. It was a subduction zone of the coast of Sumatra that caused the Boxing Day earthquake. And worldwide there are many other similar faults.

Prof BILL McGUIRE (Benfield Hazard Research Centre): There are subduction zones all over the planet, but mainly they occur around the rim of the specific, so called ring of fire. And lots of big earthquakes occur there, most of the world's really destructive earthquakes.

NARRATOR: 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 creating a megathrust earthquake.

TIM WALSH (Washington State Geological Survey): A megathrust earthquake happens when the subducting slab, which is diving under the overriding plate, is locked and causes the overriding plate to bulge upward. And then when that becomes unlocked it slides suddenly creating a huge earthquake. Worldwide these are the biggest earthquakes, they range in magnitude up to nine and a half.

Prof BILL McGUIRE: Generally speaking, if you have two great masses of rock and you're scraping them one underneath the other, they're not going to move very easily, you're going to get a lot of friction there. And I, I liken it to sort of two cheese graters pushing past one another. Very, very difficult to get any smooth sort of movement there.

NARRATOR: But as the 26th December showed, megathrust earthquakes have another devastating consequence.

Prof BILL McGUIRE: In a case of a, of a megathrust earthquake the overlying plate which has been bent pings back upwards in to position. And it's that sort of pinging motion that transmits an enormous amount of energy to the seabed, that energy is then transmitted to the water above it which oscillates up and down and then moves out as a series of huge ripples. And that essentially is what a tsunami is.

NARRATOR: A tsunami is very different from a normal wave. In normal waves on the water on the surface is moving. But a tsunami involves the movement of the whole water column. Millions of tonnes of water.

Prof BILL McGUIRE: Normal wind driven waves have very small wave lengths, that's from crest to crest, there may only a few tens of metres and then the wave has crashed and it's gone. Tsunami have wave lengths that can be hundreds of kilometres long. So when that initial wave comes in, the water behind it is pretty much at the same level, and that can keep coming for five or ten minutes as a huge flood. And that's why they're so destructive, it's not a simple wave that you can hold your breath under and survive.

NARRATOR: The combination of massive earthquakes and tsunamis makes subduction zones a deadly geological hazard. And so it should have been a cause for concern that the Cascadia subduction zone, a six hundred mile long fault, lies right off the Pacific Northwest coast. The strange thing was Cascadia didn't seem to be a danger at all. For years scientists had been monitoring seismic activity along the Cascadia subduction zone. They found like that unlike other subduction zones it was virtually silent.

ROBERT MUIR-WOOD (Earthquake Hazard Specialist): People saw the Cascadia had many of the features of a subduction zone, it had an oceanic trench, it had a line of volcanoes above a subduction zone. It simply didn't seem to have big earthquakes. And so they put it in a category of its own, the subduction zone that doesn't have big earthquakes.

NARRATOR: And there was a simple explanation for why Cascadia wasn't creating earthquakes. If the plates were moving smoothly past each other there would be no strain being built up and no earthquakes. This theory was backed up by over two hundred years of historical record. For as long as the Europeans have lived here there's no record of any significant earthquakes from Cascadia. But in this region there is another kind of history, a kind that isn't written down. For centuries, before the Europeans arrived, this land was home to native peoples. Viola Riebe is a member of the Ho Nation on the northern Washington coast. As a child she was taught the legend of the Thunderbird.

VIOLA RIEBE: The Thunderbird lives in the glacier at the headwaters of the Ho river. When he comes out the ground would start to shake and he would even make the waters trouble.

NARRATOR: Could this legend be describing a real event? A megathrust earthquake that occurred long ago. Most geologists have no time for such speculation, but one decided to take a closer look. Brian Atwater wondered whether the native legends might be a warning that the Pacific Northwest could be at risk from giant earthquakes. So he took to his canoe and started exploring the marshes and rivers of the Washington coast. He was hoping that the layers of mud here, laid down over centuries, might provide a clue to the events of the past. And buried in the marsh he did find evidence of an unusual event.

BRIAN ATWATER (United States Geological Survey): We're on an ordinary coast, standing on a salt marsh like this one. Underneath we find salt marsh deposits, salt marsh deposits, salt marsh deposits, just steadily on down. But here we have something completely different. We've got a spruce forest here, underneath the salt marsh. We can dig out here the bark, the bark of Cyprus spruce.

NARRATOR: These trees can only grow on dry land. Yet this layer of trees was covered with mud that must have been deposited by water. So the land here must once have been higher and then at some point it dropped down, plunging the forest underwater.

BRIAN ATWATER: At the time that the spruce forest dropped down sand was laid down, it was the first thing that covers the peat, it's a little skin of sand. So that's a mystery, how did that happen?

NARRATOR: Since there's no sand nearby there must have been a sudden rush of sea water that carried the sand in with it. This was no gradual change in land level, it must have been a violent collapse.

BRIAN ATWATER: The easiest explanation for that is that you had an earthquake that caused the land here to drop and also warped the sea floor, that warping of the sea floor set off a tsunami and the tsunami then lays down the sand on the freshly down dropped land surface.

NARRATOR: Carbon dating of the buried trees showed that this event had occurred roughly three hundred years ago, before Europeans had arrived. So the native legends might indeed be about a real event. But it would need more than just layers of mud to prove that there had been a devastating earthquake here. The next piece of evidence was to come from thousands of miles away. As the Indonesia earthquake has shown, megathrust earthquakes cause damage at astonishing distances, because they create tsunamis. If such an earthquake really had occurred in Cascadia it should have created a tsunami capable of travelling right across the Pacific, to countries like Japan. Kenji Satake is a geologist who studies earthquakes and tsunamis. When Satake heard about Brian Atwater's theory he realised Japan could hold the answer.

Dr KENJI SATAKE (Geological Survey of Japan): Three hundred years ago it's prehistoric time for Americans, but in Japan we have documents that would record a tsunami from America, from Cascadia, three hundred years ago, so that's why we started looking for the records.

NARRATOR: What Satake was looking for was a very special kind of tsunami. Most tsunamis in Japan are caused by nearby earthquakes, so they're accompanied by shaking of the ground. But a few tsunamis arrive without shaking because the parent earthquake is far away. When there's no known earthquake that could have caused the wave it's called an orphan tsunami. So Satake started hunting for records of an orphan tsunami that could have come from the Pacific Northwest. And in the coastal town of Neho, southwest of Tokyo, there's a document that describes just such a tsunami.

Dr KENJI SATAKE: On this page describes a tsunami of January 28th of 1700. On that day from morning, tsunami arrived this town, and like a high tide, and the receding wave was like a big river, and it continued at seven times until the noon of that day.

NARRATOR: The account told how the villagers took refuge in a shrine that still exists today. The author also recorded that this tsunami was unlike any that he had experienced before.

Dr KENJI SATAKE: Writer note that there was no earthquake but the tsunami arrived so he was surprised, and he says such strange thing should be passed to the future generation.

NARRATOR: Crucially the same tsunami was recorded in four other accounts, from different parts of Japan. So this couldn't be a local event. Satake thought this tsunami might indeed have come from a huge megathrust earthquake, five thousand miles away, in Cascadia. But still there was no proof that the tsunami had come from North America. The carbon dating only showed that the Cascadia event had happened at roughly the same time as the orphan tsunami. The final piece of evidence would be found in a mysterious corner of the Pacific Northwest. A hundred miles south west of Seattle, in a remote area of the Washington coast, is the ghost forest. These are trees that died hundreds of years ago, but remain standing to this day.

DAVID YAMAGUCHI (University of Washington): Some time in the past this would have been an intact red seeded forest. Large trees standing a hundred feet or more in the air. And this landscape was filled with them. And then one day something killed the trees here in place, and the mystery is what killed them. You know what could kill an entire forest along sixty miles of Washington coast, just like this.

NARRATOR: Tree specialist David Yamaguchi has spent years trying to solve the mystery of what happened to the trees. He wanted to work out when they had died by looking at their tree rings.

DAVID YAMAGUCHI: Most people know that trees have annual rings, so depending on the climate from year to year, the tree rings are either wide or narrow or wide or narrow. And so the developing of bark code going back in time, that's unique in time.

NARRATOR: Using this pattern David was able to work out exactly when the trees had died, and he found out that all of them had died around the early months of 1700.

DAVID YAMAGUCHI: The summer before the tsunami hit Japan these trees were just growing happily in the forest here. Then the winter came along and by the following summer they were all dead. And so the tree ring story matched the Japanese tsunami records perfectly.

NARRATOR: There was now no doubt that the same catastrophe that had killed the ghost forest had also sent the tsunami across to Japan. And from the Japanese records Kenji Satake could work out exactly when it had happened. On the 26th January, 1700, at nine pm. On that winter's night a megathrust earthquake, just like the Boxing Day earthquake of 2004, struck the Pacific Northwest. It drowned forests and turned land in to sea. It sent a tsunami hurtling across the Pacific. And it spawned a legend that would be passed down to a dozen generations. The scientists knew that if it happened here once it would happen again. One day the people of the Pacific Northwest will face a megathrust earthquake. So how big will it be? What damage will it cause? And when will it happen? The first question is how large will the earthquake be. The power of an earthquake depends on the size of the fault that breaks. In the case of the Boxing Day earthquake it was huge. Over six hundred miles of fault ruptured. The Cascadia subduction zone is almost exactly the same length. So it's likely that it will create an equally powerful earthquake.

ROBERT MUIR-WOOD (Risk Management Solutions): Now we do not know exactly when the next Cascadia earthquake is going to occur, but we do know that the impact of that earthquake in terms of the ground shaking, the huge area impacted, the extent of land level changes, the size of the tsunami which will be generated, will be very comparable to that which was seen on December 26th in 2004.

NARRATOR: Scientists believe the next Cascadia earthquake will be one of the largest on the planet. Up to magnitude nine. The Kobe earthquake, which killed six thousand people and devastated the Japanese economy, was a magnitude six point eight. The terrible Mexico City earthquake which killed over ten thousand people was eight point one. But a magnitude nine releases many times more energy than those.

TIM WALSH: The magnitude scale is logarithmic, that is each one is ten times bigger than the previous number, but that's the amount of displacement. When you do that in terms of energy release each one is thirty to forty times bigger than the previous one. So a magnitude nine is, has a thousand times more energy release than does a magnitude seven. Thirty thousand more than a magnitude six. So to put that in perspective the Kobe earthquake that was so damaging in Japan was about a magnitude six point eight. So a Cascadia event that would reach magnitude nine is more than a thousand times bigger than that one.

NARRATOR: Just as happened in the Indian ocean this huge earthquake will cause a sudden uplift of the sea floor, and that will create a tsunami. The Boxing Day tsunami devastated the densely populated northwest coast of Sumatra, and almost totally destroyed the town of Banda Aceh. The cities of Seattle, Portland and Vancouver will at least be spared that fate.

ROBERT MUIR-WOOD: One of the fortunate things about Cascadia in comparison with northern Sumatra is that the big towns and cities aren't located right out on the open ocean coast. The complex of waterways in Washington state means that the big ports are actually located someway in land.

NARRATOR: However, thousand of people do live on the Pacific Northwest coast. And in summer the beaches are a major draw to tourists.

TIM WALSH: A lot of the population on the Washington coast is vacationers. The population can grow from just a few thousand permanent population to tens of thousands of visitors. And if we have a Cascadia subduction zone earthquake and tsunami the wave crest would arrive at places like Ocean Shores and Long Beach within about a half hour. And that's a very short period of time to be able to move a lot of people off those Peninsulas to high ground.

NARRATOR: So even though there are no major cities on the coast there will still be many thousands of people at risk from the tsunami. But far more people will be affected by the earthquake itself. All the major cities in Washington, Oregon and British Columbia are going to experience strong ground shaking. And this megathrust earthquake will be very different from a normal quake.

ROBERT MUIR-WOOD: Magnitude nine earthquakes have these special characteristics. One of them is that it takes several minutes for the, the fort to break from one end to the other, the fort rupture spreads out at a few kilometres a second. But it still may take two or three minutes to get from one end to the other. And that means the earthquake shaking goes on for a very long period of time.

NARRATOR: If the full six hundred mile length of the Cascadia subduction zone ruptures it will mean the earthquake will continue for as long as five minutes, just like the Indonesian earthquake did.

Prof BILL McGUIRE: The duration of the event is very unusual, and in that sense alone it can cause more damage. A quake that goes on for longer causes more damage generally than one that is over within ten or twenty seconds.

NARRATOR: So what damage will several minutes of shaking do to cities like Seattle? Even though the Boxing Day earthquake and the next Cascadia earthquake may be very similar they could have very different effects. In Indonesia most of the damage was caused by the tsunami not the earthquake itself.

ROBERT MUIR-WOOD: Most people's houses are built out of wood, there are some more modern concrete, concrete construction, but typically only one or two storey buildings. So these buildings are not sensitive to the very long period ground motions we can expect from, from a magnitude mine earthquake.

NARRATOR: But the modern high rise structures of the Pacific Northwest may react very differently. Tom Heaton is an earthquake engineer from California. He was brought in to advise on the construction of a nuclear power station near the Washington coast. In the end the project ran out of money and was never completed. But ever since Heaton has been concerned by the question of what damage a Cascadia earthquake could do, particularly to sky scrapers.

Prof TOM HEATON (California Institute of Technology): My theory is that in a Cascadia event these buildings may sway some large distance and as we get a very long duration of shaking that the swaying may grow in intensity and the buildings may begin to be damaged.

NARRATOR: But not everyone agrees. John Hooper is a buildings engineer who has worked on many of Seattle's tallest buildings. He believes that the modern skyscrapers at least should be strong enough to avoid serious damage.

JOHN HOOPER (Magnusson Klemencic Associates): The majority of the high risers here they'll move and they'll move a lot, but they're designed to withstand that motion and that energy absorption, and they go through that eight or ten foot drift back and forth during the earthquake for several minutes, scaring a lot of people probably but the damage should be related mainly to the non-structural components and not to the major structural elements themselves.

NARRATOR: The reality is no one knows for sure. Because there has never been a megathrust earthquake near a modern high rise city.

Prof TOM HEATON: These very large earthquakes don't happen often enough for us to understand what it is we need to do in the first place. So the building codes have never really been tested by an earthquake of this nature, at least not for tall buildings. The lessons haven't been learnt yet, so what concerns me is that we may learn the lesson in a very difficult way.

NARRATOR: But there is a type of building that everyone agrees will be at risk. The older brick buildings, known as un-reinforced masonry, or URMs.

JOHN HOOPER: These buildings we see around here around the square like mini cities on the west coast. They're constructed with un-reinforced masonry, bricks stacked upon bricks separated by mortar, and so if an earthquake shaking happens those bricks end up sliding past one another, they lift apart.

NARRATOR: URM buildings are very weak and very brittle, so the long duration of shaking that a megathrust earthquake will produce could cause many to collapse.

JOHN HOOPER: URM buildings have been noticeably not very resistant to earthquakes in general, and if you don't do some renovations and start connecting the pieces together they're very susceptible to damage, especially in the long event like the Cascadia. So even those that do have some improvements made to them they still might be challenged. But those that don't have any their chances of surviving is probably fairly limited.

NARRATOR: There are thousands of these un-reinforced masonry buildings in the earthquake zone. They are used as homes, offices and schools. The collapse of such buildings is likely to be a major cause of death and injury when the next Cascadia earthquake occurs. So the big question is, when will it happen? Predicting earthquakes is impossible. No one could have known that the Indonesian earthquake was about to happen. And no one can say when Cascadia will strike. But it is possible to look back at the geological record and see how frequently earthquakes occur on a particular fault. Sure enough, the Washington coast does hold traces of several past megathrust earthquakes. From even before seventeen hundred.

BRIAN ATWATER: About two thousand five hundred years of earthquake history, one, two, three, four events recorded. Radio carbonate does show that this event happened about 600 years BC, and that this event happened about AD 400. So something about a thousand years between this event and this event, a very, very long time. This event's from about AD 700. There are only about three centuries between this event and this event. It's about the same amount of time as between here and today. So this is why it would not be surprising if while we're standing here another one of these great Cascadia earthquakes happen and we have to run to high ground.

NARRATOR: And that is the problem. The next megathrust earthquake may not happen for centuries, or it could be imminent. No one knows. We don't know whether the entire Cascadia fault will rupture like it did in 1700. We don't know how badly effected the modern cities will be. But Yumei Wang, director of Geo Hazards for Oregon, believes we must still take action.

YUMEI WANG (Oregon State Department of Geology): We know that a Cascadia earthquake is inevitable, we can't prevent earthquakes, but one thing that we can do is prevent a lot of the damage. We can save lives if we prepare now.

NARRATOR: That preparation must be based on our current understanding of what the next Cascadia earthquake will be like. What follows is a reconstruction based on the knowledge of leading experts of what may happen. What it would look and feel like to experience a megathrust earthquake.

TIM WALSH: We don't know what actually sets the earthquake off but typically it would probably start at some rough spot on the fault.

NARRATOR: The rupture is most likely to start at one end of the fault. It would then spread along the fault at over 7000 miles per hour. As it tears, the North American plate which has been pushed inwards would spring back releasing the strain.

ROBERT MUIR-WOOD: There may be a region four or five hundred kilometres long where the, the sea floor has suddenly risen up by two or three metres. It happens so fast that it lifts up the whole body of the water on top of it. And as a result suddenly the sea surface finds itself two or three metres higher than it was before, over a large area, and that sets off a wave.

NARRATOR: This is the tsunami which would radiate out in all direction. Part of it would head out in to the Pacific, and part would head directly for the coast of North America.

TIM WALSH: It travels at the same speed roughly as an airliner out in the open ocean, perhaps 600 miles an hour.

NARRATOR: Even at that speed it would take many hours to reach the other Pacific nations. It would take five hours to reach Hawaii and more than ten hours to reach Japan. Thanks to the sophisticated Pacific tsunami network those countries would get a warning.

Prof BILL McGUIRE: The quake would be detected by a network of seismographs, the tsunami, if they form, will be spotted and identified and tracked by sea bed sensors, which will send via buoys on the surface a radio message via satellite to the emergency authorities in the countries around the Pacific rim who might be affected. It's then their job to tell their populations to evacuate the coastal region.

NARRATOR: This warning system should make the distant effects of a Cascadia earthquake very different from the events of Boxing Day.

Prof BILL McGUIRE: I think that the loss of life remote from the actual location of the Cascadia earthquake will be, will be small when the next big event occurs. And this is because although the waves travel at the speed of a jumbo jet, maybe eight or nine hundred kilometres an hour across the Pacific, it's a huge ocean basin and it will take many hours for the waves to reach places like Hawaii and Japan, which will probably be badly hit but they will have plenty of time to evacuate people to, to safe ground.

NARRATOR: But the situation in the Pacific Northwest would be very different. The tsunami would arrive their in half an hour, and they'd have the earthquake to deal with first. The seismic waves which carry the shaking would be travelling through the earth at over ten thousand miles per hour, much faster than the tsunami. In just a few seconds the earthquake would reach the land. The earthquake would be at its most violent here on the coast.

JOHN HOOPER: They're right at ground zero, the shaking, so the shaking they feel will be the largest of anybody because they're nearest to the fault rupture.

NARRATOR: But the shaking wouldn't have reached the inland cities yet, people here wouldn't even know that an earthquake had started. However news would have reached the emergency services. This is the Washington State Emergency Operation Centre. It will be one of the first places to receive an alert from the Tsunami Warning Centre.

WASHINGTON STATE EMERGENCY OPERATIONS CENTRE EMPLOYEE: We're on a magnitude 9, we're activating our ELC to a phase 3, 4 a tsunami.

NARRATOR: Horizon filmed them rehearsing for a major earthquake. The two on duty officers would immediately activate the centre and start calling in staff.


NARRATOR: Their job would be to coordinate the emergency response. But there would be no time to issue a public warning before the earthquake hits the big cities. Up to two minutes after the start of the earthquake the seismic waves would reach the city of Seattle. Because of the distance, the different types of seismic wave would have separated out, with the faster compression waves reaching the city first.

JOHN HOOPER: The first thing you sense is a vertical acceleration, you get pushed up a little bit, and you think it's maybe a jolt of a train going by, or something of that type.

Prof TOM HEATON: But then later maybe, twenty seconds even later, you might feel, start to feel the shear waves coming in which are shearing motions in the earth, the kind of motion that does most of the damage.

NARRATOR: These shear waves would move the earth from side to side by as much as a metre. There would also be surface waves, like ocean waves rippling through the solid earth.

YUMEI WANG: If you are in a parking lot it's likely that you see waves rolling across the parking lot, like if you took a carpet and shook it.

NARRATOR: As the shaking becomes more and more intense people would realise that this was no ordinary earthquake.

JOHN HOOPER: That shaking will continue to build, you'll feel the first sway and it'll start to build and build and build and you'll wonder when it's going to stop.

NARRATOR: Indoors objects and furniture would be hurled around the room, parts of the building may start to fall.

YUMEI WANG: Right when you feel the earthquake shaking, what we train people to do is to duck cover and hold.

NARRATOR: Schools and offices now practice this life saving manoeuvre, going under a strong desk and holding on to it.

YUMEI WANG: Anything that might fall won't fall on you directly, it will fall on the table and the whole time you protect your head.

NARRATOR: For the people outside, the major hazard would be falling debris and shattering glass.

YUMEI WANG: If you're outside somewhere the best thing to do is to move quickly in to open space, such as away from a building where you might have falling objects.

NARRATOR: Buildings would now be exposed to huge forces as their shunted back and forth. The un-reinforced masonry buildings would be the first to suffer damage.

YUMEI WANG: The weakest point starts to fail and in most cases because they're older structures it's the mortar.

JOHN HOOPER: The elements that support the building vertically, if they start to come down, the floors themselves potentially can come down.

NARRATOR: Collapsing URMs could cause many fatalities throughout the region. The shaking in Seattle would now have been going on for two minutes, but we'd only be half way through.

YUMEI WANG: For a typical earthquake if a building gets damaged in the first twenty, thirty seconds, it very likely can remain standing. But if that damaged building is shaken for another three minutes then that damage can propagate in to collapse.

NARRATOR: Meanwhile the large movements of the ground would be making skyscrapers bend further and further.

Prof TOM HEATON: You may see the buildings begin to sway more and more violently to the point where they start to perhaps lose their windows. They may in addition start to have some fracturing of welds and steel frame buildings. What happens after that is, is anybody's guess.

NARRATOR: The worst case scenario would be the total collapse of a high rise building. Meanwhile buildings on higher ground would be suffering their own problems.

TIM WALSH: Earthquakes this large can generate landslides at distances up to hundreds of miles away.

JOHN HOOPER: The classic worst case scenario where you're on a hill, a landslide and your house goes with it and the house will obviously be destroyed.

NARRATOR: Five minutes after the start of the earthquake the rupture would have reached the northern end of the fault, Vancouver would still be experiencing powerful shaking. But in Seattle the earthquake would finally be subsiding. For people in buildings that have suffered structural damage now would be time to evacuate. What would have felt like the longest few minutes of people's lives will finally be over. But on the coast the ordeal would have only just begun. The tsunami unleashed by the earthquake would be minutes away. For the Pacific Northwest the tsunami warning system, that should save lives across the world, would be virtually useless.

TIM WALSH: There won't be time for the Tsunami Warning Centre to detect that earthquake, make a determination whether or not it was tsunamagenic, then send a warning down to emergency managers in Washington who will then send it to the people. That would waste valuable time. People need to know that when they feel strong shaking if they're on the coast they need to go to high ground and or inland.

NARRATOR: The tsunami would have started out as a wave of only a metre or two high. Travelling at huge speed. But as it nears the coast it starts to rise up.

TIM WALSH: Those waves can grow, they can amplify as more and more water piles up in shallow water, and all of that energy then causes the wave to slow down and grow in amplitude and create waves that have been known to be hundreds of feet high.

ROBERT MUIR-WOOD: That first wave is often simply a step in the water level, and the water level then stays up high for five or ten minutes before it eventually drains away again.

NARRATOR: Just as happened in Indonesia, within half an hour of the earthquake the tsunami would rush on to the land. More like an ever growing tide than a normal wave. Anyone who doesn't manage to get inland on to high ground in time would be unlikely to survive. The tsunami would devastate hundreds of miles of coast. In total more than fifty thousand square miles would be effected by the earthquake.

YUMEI WANG: Unfortunately I don't think people understand that a Cascadia earthquake is going to be so very different than the other types of earthquake that we've all experienced, or many of us have experienced. One of the main differences is that it's going to effect such a large region.

JOHN HOOPER: It's not just going to be City of Seattle, or City of Portland, it could be an eight hundred mile stretch of Washington, Oregon and California that is effected.

NARRATOR: Until recently many people would have found it difficult to imagine that scale of devastation. But the Boxing Day disaster changed all that.

Prof BILL McGUIRE: The Indian Ocean earthquake and tsunami will remind those people that are living in the Pacific Northwest that this is something they will have to face in the future. And the window of opportunity that now exists should be used to make sure that the people that live in that part of the world are educated in terms of how to respond when the earthquake happens.

NARRATOR: The simple knowledge that after an earthquake people should move away from the ocean and to high ground can save lives. The scientists who discovered this threat are now playing their part in spreading the word to as many people as possible, before the next earthquake.

YUMEI WANG: We know that this Cascadia earthquake is imminent, it's imminent in geologic time, so basically we're in a race against time and the more we can get done now the more lives we'll save.

DAVID YAMAGUCHI: If we have ten years, is that enough? Probably not. If we have fifty years? Maybe, you know. If we have a century? You know maybe we'll really be ready. But do we have a century? We don't know.

NARRATOR: The Indonesian earthquake has given the people of the Pacific Northwest a glimpse of what they will one day face. Now they must heed that warning.

Documentary Description

A huge megathrust earthquake and tsunami, just like the one that occurred on Boxing Day, will one day hit America. On Boxing Day 2004 the world was shocked by one of the worst natural disasters of all time. The cause of so much devastation was the most powerful kind of earthquake on the planet - a megathrust. Megathrust earthquake only occur on a particular kind of fault. Scientists have now discovered that just such a fault could cause a huge megathrust earthquake and tsunami right off the coast of North America.

The subduction zones

The surface of the earth is divided into giant plates of rock - and most earthquakes occur at faults where two of the plates meet. Where the plates are colliding one of the plates usually gets pushed down under the other - this is subduction.

Not surprisingly, the process of subduction can be very violent. The two plates can get stuck together and the result is that the area where subduction is occurring (the subduction zone) gets compressed. Eventually the strain on the fault becomes too much. The plates suddenly slip past each other. The result is a megathrust earthquake.

Subduction zones are mainly found in South East Asia (like the subduction zone that caused the Indian Ocean Tsunami) and around the Pacific Rim. It has long been known that a subduction zone runs along the Pacific northwest coast, from northern California all the way to Vancouver Island in Canada. It's called the Cascadia Subduction Zone, and is linked to the Cascade Range of volcanoes that includes Mt St Helens.

Quiet in Cascadia

No one, however, thought that this area was at significant risk from earthquakes, largely because there was no historical record of large earthquakes there. The first suggestion of a problem came when plans were drawn up to build a number of nuclear power stations near the coast of Washington state.

Tom Heaton, a geophysicist and engineer from the California Institute of Technology, was brought in to examine the proposal from a geohazards perspective. Heaton pointed out the possibility that the Cascadia Subduction Zone might be capable of producing a megathrust earthquake.

The lack of historical record might be explained if the last such earthquake had occurred before Europeans arrived in the region during the 18th century. Heaton pointed to the existence of Native American legends that might be describing a megathrust earthquake that had happened before written records began. In the end, the nuclear power station project ran out of money and the reactors were never completed. Heaton's concerns were still just theoretical - there was no scientific evidence that such earthquakes had actually occurred.

When Brian Atwater, a specialist in marshes and estuaries, heard about Heaton's theory he decided to take a look himself. He started investigating in the very areas where the Native legends had been recorded. He found evidence that some time in the past there had been a sudden change in land level. The coast had dropped down, drowning forests under layers of mud. Other geologists soon found similar evidence all along the Pacific northwest coast. The simplest explanation was that there had been a huge megathrust earthquake in the past. This evidence, however, still wasn't enough to convince.

The orphan tsunami

The most intriguing piece of evidence came from Japan. As the Indonesian earthquake has shown, megathrust earthquakes can create tsunamis capable of crossing entire oceans. If there had been a giant earthquake in Cascadia it should have sent a tsunami across to Japan. So Japanese geologist Kenji Satake looked in old Japanese texts for any record of such a tsunami.

What he was looking for was an orphan tsunami, a wave the comes out of nowhere, with no local earthquake recorded. Satake and his colleagues found several such records from January 1700. The final proof came when scientists in America were able to date the land level change there by looking at the tree rings of drowned red cedar trees. They found that entire forests had been killed in the winter of 1699/1700, matching the Japanese records perfectly.

This evidence enabled scientists to put together a picture of the last Cascadia megathrust earthquake. It occurred around 9pm on 26 January 1700, and would have had a devastating effect on over 600 miles of coast. It's little wonder that the Native people of the region passed on legends of that event that survive to this day.

A deadly warning

Before the 1700 event was discovered, the people of the Pacific north-west would have had little awareness of the threat of earthquakes and even less of tsunamis. Now that is starting to change. Tsunami evacuation signs are being installed along the coast. The region's building codes are now some of the strictest in the world. All new buildings are designed to withstand a powerful earthquake.

However, building to survive a megathrust earthquake is a major challenge. These earthquakes tend to last several minutes - much longer than other quakes - and there is little knowledge about how modern buildings will react to such shaking.

As the events of Boxing Day 2004 showed, the tsunami unleashed by the sudden movement of the sea floor can be even more devastating than the earthquake. Unlike the Indian Ocean, the Pacific has a warning system that should give the other nations of the Pacific Rim many hours of warning.

On the Pacific northwest coast, however, a tsunami from Cascadia will arrive in some places in under half an hour. In such regions the key to saving lives is education. People there have to know that if they feel an earthquake they should move inland and to high ground. This knowledge could save many lives.

The terrible events of 26 December are a warning to the world that we should be better prepared for these huge geological catastrophes. This message has particular significance for the people of the Pacific northwest. One day that region will be hit by a megathrust that will probably be very similar to the Indonesian earthquake. At least, thanks to the work of many scientists, they have been forewarned.

Source: BBC


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