Cascadia: The Mother of All Earthquakes
The Earth is in constant motion—not just on a grand cosmic scale, but right beneath our feet. The planet’s outer shell is divided into massive slabs of rock known as tectonic plates, and these plates are always shifting, grinding, and reshaping the surface over spans of millions of years. Ideally, these movements would be so gradual that we'd never notice. But the reality is far less peaceful.
When tectonic plates get stuck instead of sliding smoothly, stress begins to accumulate. This tension creates fractures in the Earth's crust known as faults. And where there are faults, there are earthquakes. One of the most infamous of these is California’s San Andreas Fault—a massive scar where the Pacific and North American Plates are locked in a slow-motion tug of war, sliding past one another in opposite directions. Over time, pressure builds until it suddenly releases in a violent lurch known as a strike-slip fault movement.
The San Andreas Fault has become a household name, partly because its activity occurs relatively close to the surface, making its effects more dramatic and visible. This notoriety has turned its long-overdue earthquake into a cultural specter known simply as “The Big One.” When it does finally rupture, seismologists expect a magnitude somewhere between 8.0 and 8.6 on the Richter scale—massive by any measure. Yet in the Pacific Northwest, that kind of quake is just the starting point. For the Cascadia Subduction Zone, 8.0 is merely the baseline.
Cascadia Subduction Zone
Just off the coast of Oregon, the Juan de Fuca Plate is slowly diving beneath the North American Plate in a geologic process called subduction. But as tectonic plates do, they’ve gotten stuck. As a result, the North American plate is being compressed at a rate of roughly 1.5 inches per year. As the plate compress’, pressure builds. Eventually, when the pressure exceeds what the Earth's crust can contain, the fault will snap, unleashing a massive burst of seismic energy.
The somewhat reassuring news is that major fault lines don’t usually rupture all at once. Scientists currently expect that the next significant Cascadia earthquake will likely originate in the fault’s southern section, near Northern California. That event is projected to register around magnitude 8.0—similar in scale to what’s expected from the San Andreas Fault.
However, if the entire 700-mile Cascadia Subduction Zone were to break at once—a scenario known as a Full Margin Rupture—the outcome would be catastrophic. Such an event could generate a magnitude 9.2 earthquake. Due to the logarithmic nature of the Richter Scale, this wouldn’t just be slightly stronger than an 8.0—it would release nearly 30 times more energy. A quake of that size would rival the most powerful in U.S. history and rank as the second most intense ever recorded globally.
By studying sediment left behind by ancient tsunamis, scientists have traced the Cascadia Fault’s violent past. Over the last 10,000 years, it’s ruptured at least 41 times—roughly once every 250 years. The last major event struck in 1700. That means, by geologic standards, we’re overdue. In 2009, researchers estimated there’s a 10–14% chance that Cascadia will produce a magnitude 9.0 or greater earthquake within the next 50 years, and a 37% chance of a magnitude 8.0 or higher. Either scenario would be devastating—not just for the Pacific Northwest, but for the entire continent.
The Bigger One
The following is a speculative worst-case scenario if Cascadia were to unleash all of its 9.2-magnitude fury:
In the span of just five to six minutes, over 13,000 lives are expected to be lost—instantly making it the deadliest earthquake in U.S. history. Tens of thousands more will be buried beneath the rubble, many critically injured or trapped with no immediate rescue. The region’s electrical grid will collapse. Dams will fail, unleashing devastating floods. Hazardous materials will spill, gas lines will rupture, and hundreds of fires will ignite simultaneously across cities and towns.
The shaking will also trigger thousands of lethal landslides, while up to half of all highway bridges are expected to crumble. The structural toll will be staggering: an estimated 75% of all buildings will sustain serious damage or outright collapse. Over a million structures will be reduced to ruins—including more than 3,000 schools and approximately 60% of the area’s hospitals. In the chaos, it’s estimated that nearly one-third of emergency personnel will be unable—or unwilling—to respond, due to collapsed infrastructure and concern for their own families and survival.
Any one of these outcomes would rank among the worst disasters the country has ever faced. Together, they defy imagination.
And yet, the earthquake will only be the beginning.
The tsunami that follows will dwarf the destruction. It will arrive like a wall of fury, with waves ranging from 20ft to over 100ft high, traveling inland at roughly 12mph. In its path, more than 100,000 square miles will be submerged. Entire coastal towns will be swept away—drowned, flattened, and erased from the map. When the waters recede, what remains of the Pacific Northwest will be unrecognizable.
In a 2015 interview with The New Yorker, FEMA’s Regional Administrator Kenneth Murphy offered a stark forecast of what to expect in the tsunami’s wake:
“Our operating assumption is that everything west of Interstate 5 will be toast.”
The Scary Part
Scientists have long understood that the Cascadia Subduction Zone and the San Andreas Fault intersect just off the coast of Mendocino County. But recent geological research has revealed a far more unsettling connection. By analyzing sediment layers and other remnants left behind by ancient earthquakes, researchers compared the timelines of major events on both faults and made a frightening discovery.
Over the past 3,000 years, a powerful rupture along the Cascadia Fault has triggered a subsequent quake along the San Andreas Fault between 9 and 11 times. In other words, when Cascadia unleashes its full force, it often sets off a catastrophic chain reaction—one massive quake followed by another. Almost every time the Pacific Northwest has endured its version of “The Big One,” California hasn’t been far behind, echoing the destruction with a seismic disaster of its own.
