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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-called “megaquake” is building beneath the region, and it's something we'll someday have to deal with. And while we're not exactly “overdue” for 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 directions—that'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 impressive—it'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 limit—it'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. The Tōhoku 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 an “orphan 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 happens… it 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 earthquakes—like the ones that happen along the San Andreas fault—pose 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 Northwest—or anywhere with a significant earthquake risk—find 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.

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