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  • {♫Intro♫}

  • If you live in the U.S., you may have heard that the Pacific Northwest is supposedly overdue

  • for an earthquake of colossal, devastating proportions.

  • It is true that a so-calledmegaquakeis building beneath the region, and it's

  • something we'll someday have to deal with.

  • And while we're not exactlyoverduefor a disaster, the better we understand the

  • threat, the better we can prepare for the day it comes.

  • Earthquakes are caused by the movement of the Earth's tectonic plates, rigid slabs

  • of slowly-moving crust beneath the surface that fit together like puzzle pieces.

  • As the plates shift, they push and grind against one another, creating rifts and mountains,

  • and putting a lot of stress on the rock beneath our feet.

  • This mounting pressure eventually gives way,

  • and the rock slips along fractures called faults.

  • All at once, the stress is relieved and energy

  • gets released in waves that ripple outward

  • in all directionsthat's an earthquake!

  • Small earthquakes happen literally every day,

  • but they don't make the news because we

  • don't actually feel most of them.

  • The size of an earthquake is expressed using

  • a relative scale called magnitude, and people

  • generally can't feel earthquakes

  • below magnitude 2 or 3.

  • These everyday earthquakes tend to originate

  • on fault lines less than 70 kilometers underground,

  • so they're called shallow earthquakes, and usually they're pretty harmless.

  • Some shallow earthquakes can be pretty severe,

  • though, like the magnitude 6.4 quake that

  • hit Coalinga, California in 1983.

  • The vibrations caused damage to hundreds of

  • buildings and set off fires that added to

  • the destruction.

  • In all, it caused about 50 million dollars' worth of damage.

  • In general, the magnitude of an earthquake

  • depends on the size of the fault and the amount

  • of movement in the crust.

  • The bigger the fault area, the bigger the earthquakes it can produce.

  • The famous San Andreas Fault in California is particularly impressiveit's more than

  • 1,200 kilometers long and about 16 kilometers deep.

  • And it's capable of producing extremely powerful events like the catastrophic San

  • Francisco earthquake in 1906, a magnitude 7.9 quake that shook all of California.

  • But there's a limitit's extremely unlikely

  • that this fault would ever cause an earthquake

  • stronger than magnitude 8.3, at worst.

  • The very biggest earthquakes happen at the most dramatic faults.

  • These exist at places called subduction zones,

  • where, instead of two plates sliding horizontally

  • past each other, an oceanic plate runs into

  • a continental plate and slides down underneath it.

  • This motion causes a lot of stress on rocks,

  • and these are the only places where you can

  • get what are called megathrust earthquakes.

  • These are massive earthquakes that release

  • truly ridiculous amounts of energy.

  • These earthquakes are rare, but they include

  • the most powerful earthquakes in history,

  • such as the 2011 Tōhoku earthquake in Japan

  • and the 19 60 Valdivia earthquake in Chile,

  • which both caused catastrophic damage.

  • Thehoku earthquake reached a magnitude

  • of 9.1, and the Valdivia quake was the biggest

  • in recorded history at magnitude 9.5.

  • A magnitude of 9.5 might not sound like a huge jump from, say,

  • Coalinga's 6.4 magnitude

  • quake, but earthquakes aren't measured on a linear scale.

  • They're measured on a logarithmic scale.

  • That means that every time the magnitude increases

  • by one, the amplitude of the waves increases

  • tenfold, and the earthquake releases 32 times

  • more energy!

  • That makes the Chile megathrust earthquake

  • more than 44,000 times stronger than the quake

  • in Coalinga.

  • And right now, an earthquake of this size is brewing beneath the United States.

  • The Pacific Northwest sits right on top of the Cascadia Subduction Zone,

  • where the Juan

  • de Fuca plate is sliding beneath the North American plate.

  • Tectonic movement in this area has created

  • lots of faults and caused many earthquakes

  • and tsunamis.

  • It's also created the volcanoes in the Cascades.

  • But the biggest fault in any subduction zone

  • is the boundary between the tectonic plates themselves.

  • The Cascadia subduction fault is 1,000 kilometers

  • long, running from under southern British

  • Columbia down to northern California,

  • and dipping hundreds of kilometers down below

  • the surface.

  • It's a megathrust fault.

  • In the deepest parts of the fault,

  • the plates move slowly and steadily as warmer temperatures

  • make the rock more flexible.

  • But in the upper 30 kilometers of the fault,

  • friction causes the rock to get stuck.

  • The plates lock and stress builds and builds

  • until it finally slips, releasing a huge megathrust earthquake.

  • Something like this is building in Cascadia

  • and we know because it's happened before.

  • The last Cascadian megaquake happened in the year 1700.

  • No one recorded that earthquake

  • on any sort of scientific equipment, but it left behind

  • enough evidence

  • on both sides of the Pacific Ocean

  • that geologists have been able to

  • learn a lot about it,

  • including the date and time that it happened.

  • In North America, geologic evidence along the coasts of Oregon and Washington points

  • to a dramatic event just over 300 years ago.

  • At that time, the stacked layers of peat left behind in coastal forests are interrupted

  • by a layer of ocean sediment that washed ashore with one or more large tsunamis.

  • And on the seafloor, geologists find layers called turbidites, massive piles of sediment

  • and debris left behind by underwater landslides.

  • These simultaneous tsunamis and landslides point to a massive earthquake that shook up

  • the entire coastline.

  • Scientists have even found the remains of trees that died in the event, and they've

  • used tree-ring dating to narrow down the timing of the event to late 1699 or early 1700.

  • This lines up with stories from native people about shaking and flooding in that area.

  • And across the ocean, that time frame also lines up with Japanese legends of anorphan

  • tsunami.”

  • In Japan, tsunamis generally come after people feel the earth shake, but one day, a tsunami

  • struck with no warning.

  • That happened right around midnight on January 28, 1700.

  • Knowing that it takes about ten hours for a tsunami to travel from the Cascades to Japan,

  • geologists estimate that the earthquake hit at around 9pm local time on January 26, 1700.

  • There are records of the damage that tsunami caused in Japan, and they tell us the earthquake

  • was a big one, most likely magnitude 8 or 9—a megaquake.

  • And it's not the only one!

  • Similar geological evidence in the Pacific Northwest shows that several huge earthquakes

  • have struck the region over the last several thousand years.

  • For millennia, the Cascadia Subduction Zone has been going through cycles of slowly building

  • up stress and then cracking and releasing enormous earthquakes.

  • For more than 300 years, that energy has been building, and someday it will give again.

  • But earthquakes don't exactly happen like clockwork.

  • On average, these quakes have struck every 500 years or so, but the intervals vary from

  • 200 to 1,000 years.

  • That makes it very difficult to predict when the next huge quake will be, and we can't

  • exactly say we're overdue for the next one.

  • But those who think we have some predictive power give it a roughly 10% chance of happening

  • within the next 50 years.

  • Whenever it happensit won't be pretty.

  • A magnitude 9 earthquake would cause several minutes of intense vibrations throughout the

  • Pacific Northwest, taking down buildings and causing as much as several billion dollars

  • of damage.

  • Then, after the main earthquake, large aftershocks would rattle the coast again, possibly knocking

  • down weakened structures that survived the first round.

  • Within 30 minutes, tsunamis up to several meters high would wash ashore, threatening

  • over 71,000 people currently living in the inundation zone.

  • And over the following hours, the waves could reach Alaska, Hawaii, and even Japan.

  • The megaquake would damage roads and airports, making it difficult to evacuate people or

  • get emergency assistance where it's needed.

  • And the Pacific Northwest isn't nearly as prepared as we'd hope.

  • Since the history of these huge Cascadian earthquakes was only uncovered in the past

  • few decades, the cities in the region weren't built to handle such an event.

  • Many of their buildings, for example, are built of heavy brick and concrete without

  • steel beams for reinforcement, which makes them likely to collapse during strong vibrations.

  • And, historically, these communities haven't set up standard earthquake evacuation procedures.

  • So that's slightly terrifying, but I have good and bad news.

  • This inevitable catastrophe isn't actually the biggest earthquake threat to the Pacific

  • Northwest.

  • The megaquake will certainly be a disaster when it happens, but in the meantime, shallow

  • earthquakeslike the ones that happen along the San Andreas faultpose a more immediate

  • threat.

  • These earthquakes happen more frequently, and while the Cascadian megaquake would most

  • likely happen offshore, smaller earthquakes often strike farther inland, which makes them

  • a serious danger for big cities like Seattle.

  • But don't go thinking that the Pacific Northwest is just doomed!

  • Even though we can't predict with precision when earthquakes will hit, there's actually

  • a lot we can do to prepare for quakes, mega and otherwise.

  • For one thing, scientists are working hard to understand hazard areas.

  • That includes scoping out flood zones that might be most vulnerable in a tsunami, roads

  • that might be in the paths of landslides, and unstable soils that could weaken during

  • an earthquake.

  • This knowledge helps emergency management groups prepare for the impacts of an earthquake.

  • Already, building codes in the Pacfiic Northwest have been adjusted to account for the threat

  • of earthquakes.

  • Older buildings are being reinforced with steel bracing to prevent structural collapse,

  • and newer buildings are often built on flexible bearings called base isolators, which can

  • absorb a lot of the energy from a quake.

  • At the same time, evacuation routes are being mapped out to allow citizens to get to safety

  • in the case of disaster.

  • Scientists are also working on early warning systems.

  • A study published in early 2019, for example, found that it might be possible to predict

  • the magnitude of an earthquake from its very first minor waves.

  • We're only talking a few seconds' warning, but even so, this early detection could make

  • it possible to get information out to the public.

  • Other groups are working on ways to fast-track these warnings to the people who need to hear

  • them.

  • For instance, an early-warning system called ShakeAlert is working on detecting earthquakes

  • and alerting people seconds before the shaking even reaches them.

  • The system is currently being tested in places like hospitals and transportation facilities

  • along the West Coast of the United States.

  • A few seconds may not sound like much, but it's enough time for people to take cover,

  • move away from dangerous objects like glass or chemicals, and to slow down cars, trains,

  • and taxiing planes.

  • Even with all this technology, the most important safety measure is still public education.

  • Communities that understand the risks can do much more to keep themselves safe.

  • For instance, in 2010, a magnitude 8.8 earthquake struck Chile, and by the time the tsunami

  • reached the coast, most residents were already heading for higher ground, saving their own

  • lives.

  • Now, in the Cascadian region, people are practicing safety measures like evacuation drills and

  • tsunami siren tests.

  • There's also plenty of information on what you can do to prepare yourself.

  • So if you live in the Pacific Northwestor anywhere with a significant earthquake riskfind

  • out what you can do!

  • Check out your local geological agency resources, and look at the links in the description of

  • this video.

  • The next Cascadian megaquake is brewing beneath the surface right now, and so are similar

  • earthquakes all around the world.

  • But we know more about earthquakes now than we ever have, and as we keep learning, we'll

  • be better prepared to protect ourselves from the next one.

  • Thanks for watching this episode of SciShow!

  • And a big thanks to our patrons on Patreon, who make it possible for us to keep making

  • episodes like these.

  • If you're not a patron but would like to support SciShow, head over to Patreon.com/SciShow

  • to learn more.

  • {♫Outro♫}

{♫Intro♫}

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Are We Overdue for a Megaquake?

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    林宜悉 posted on 2020/04/13
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