The Earth is always moving, and not just cosmically, the tectonic plates that make up the Earth’s crust are constantly grinding against each other and rearranging themselves over millions of years. Ideally, they would inch along over the centuries without us ever noticing. But these plates often get stuck, and when they get stuck, they create faults, and where there’s faults, there’s earthquakes. The famous San Andreas Fault in California is caused by the North American and Pacific Plates moving parallel in opposite directions and getting jammed against one another. When enough pressure builds, the plates snap forward in what’s called a Strike-Slip Fault.

However, the San Andrea Fault’s popularity is a result of its effects being more noticeable due to it being closer to the surface, this inherent fame has led to its inevitable earthquake being referred to simply as The Big One. When “The Big One” is finally unleashed, it’s estimated to reach between 8 – 8.6 on the Richter Scale, and while that is indeed big, it’s considered bare minimum for Cascadia.

Cascadia Subduction Zone

Off the coast of Oregon, the Juan de Fuca Plate is borrowing its way underneath the North American Plate in a process known as subduction. And as tectonic plates do, they’ve gotten stuck. As a result, the landmass of the North American plate is being compressed by about 1 ½ inches every year. As the plate compress’, pressure builds, and when it finally becomes too much for the Earth the bare, the plate will suddenly shift, and all that energy will be unleashed.

The good news is that faults don’t normally let loose all at once. It’s anticipated that the next Cascadia quake will only occur in the southern section of the fault in Northern California, and measure around a magnitude 8.0 (San Andreas). But if the entire Cascadia Fault were to give way all at once in what’s known as a Full Margin Rupture, it could produce an earthquake as high as a magnitude 9.2. And because the Richter Scale is logarithmic, this means a Cascadia Megaquake would be around 30 times bigger than anything the San Andreas Fault could possibly produce. It would tie for both the biggest quake in US history, and the second biggest quake the world has ever seen.

By analyzing deposits left by the tsunamis, researchers were able to calculate how many times in history Cascadia has ruptured: 41 megaquakes over 10,000 years. That averages to about 1 every 250 years. The last time a Cascadia event happened was around 1700, meaning that it is believed we are overdue for another megaquake. In 2009, geologists predicted that sometime in the next 50 years, there is a 10-14% chance that Cascadia will produce an earthquake of a magnitude 9.0 or higher, and a 37% chance of a magnitude 8.0 or higher.

The Big One

The following is a speculative worst-case scenario if Cascadia were to unleash all of its 9.2 fury.

Over 13,000 people will be killed outright, immediately making it the deadliest quake in American history, with tens of thousands more being trapped in the rubble. The electrical grid will fail; dams will give way causing floods; chemical spills; ruptured gas lines; and hundreds of simultaneous fires. Thousands of deadly landslides will trigger, and up to half of all highway bridges will fall. It’s estimated that over 75% of all buildings will be compromised, with over a million collapsed, including over 3,000 schools and 60% of hospitals. An estimated 1/3 of public safety workers will not respond to the disaster due to a collapse in infrastructure and concern for their own safety. All in a span of 5 to 6 minutes.

Taken alone, any one of these could be considered one of the worst disasters in American history, but all together… there are no words.

However, as bad as the quake will be, it will have nothing on the tsunami. When it arrives, it will be impossible to miss. The water will range anywhere from 20ft to over 100ft high and will be travelling a roughly 12mph. As the water moves inland, over 100,000 mi.² will be flooded, with entire towns being drowned and erased off the map. When all is said and done, the Pacific Northwest will be unrecognizable.

When The New Yorker interviewed FEMA Regional Director, Kenneth Murphy, in 2015 about the tsunami’s aftermath, he replied, “Our operating assumption is that everything west of Interstate 5 will be toast.”

The Scary Part

It’s been known for a while that the Cascadia and San Andreas Faults are connected, brushing together just offshore of Mendocino County. But by dating the geological remains of previous San Andreas quakes and comparing them to the remains of Cascadia quakes, researchers were able to make a frightening discovery. Over the past 3000 years, a major Cascadia quake has caused the San Andreas Fault to also rupture between 9 and 11 times. That means that almost every time Cascadia unleashes The Big One, it’s followed by San Andreas also bringing about the earthquake apocalypse.