The “Big One” is the earthquake expected to strike the Pacific Northwest coast. This future event is not a theoretical possibility but a certainty rooted in geological history, tied to the Cascadia Subduction Zone. This boundary is capable of generating the planet’s largest type of earthquake, known as a megathrust event. Understanding the event requires examining the mechanics of plate movement, the scale of the expected rupture, and the cascading effects that will follow the initial ground shaking.
The Geological Mechanism Driving the Event
The mechanism for this earthquake lies offshore, where the oceanic Juan de Fuca tectonic plate is slowly sliding beneath the continental North American plate. This process, known as subduction, involves one plate diving underneath the other at a rate of approximately 40 millimeters per year. The boundary forms the Cascadia Subduction Zone, a fault that stretches for about 1,000 kilometers from northern California to British Columbia.
Friction locks the shallow portion of the fault, creating a “locked zone.” Here, the Juan de Fuca plate drags the edge of the North American plate, causing the continent to compress and slowly deform. This continuous drag accumulates tremendous stress and strain in the crust over centuries, storing potential energy.
When the built-up strain exceeds the frictional strength of the locked fault, the overriding North American plate will instantaneously snap back toward the ocean. This sudden rebound triggers a megathrust earthquake, releasing the stored energy in one catastrophic rupture. The movement involves a massive section of the fault, explaining the expected large scale of the seismic event.
Projecting the Maximum Magnitude
A full-margin rupture of the Cascadia Subduction Zone is projected to generate an earthquake in the range of Moment Magnitude (M) 8.7 to 9.2. Seismologists use the Moment Magnitude Scale (MMS) because it accurately reflects the total energy released by the largest earthquakes. The historical precedent is the estimated M9.0 Cascadia earthquake that occurred on January 26, 1700, which ruptured the entire fault length.
The maximum magnitude is directly proportional to the total length and area of the fault that ruptures. Achieving a magnitude near 9.0 requires the rupture to extend for hundreds of kilometers along the subduction zone. Geological evidence indicates that the entire 1,000-kilometer fault segment is capable of moving simultaneously.
Geological records show that major events have happened on average every 243 to 500 years. Since the last great event was over 300 years ago, stress has been building throughout the zone. Scientists estimate the probability of a magnitude 9.0 or higher event occurring in the next 50 years to be between 7% and 14%.
Immediate Ground-Level Consequences
The immediate consequence of the rupture will be intense ground shaking. Along the coast, the shaking is estimated to persist for four to seven minutes, a duration that maximizes the potential for structural failure. The intensity of this shaking will be measured using the Modified Mercalli Intensity (MMI) scale, which describes the effects on people, buildings, and the environment.
The severe, prolonged shaking will trigger liquefaction in areas with saturated, loose soil, such as river deltas. During liquefaction, shaking causes water pressure to increase between soil particles, temporarily transforming the solid ground into a slurry. Structures built on these unstable soils may sink, tilt, or collapse as their foundations lose support, and underground utility lines can float to the surface.
Simultaneously, the sudden tectonic rebound will cause permanent land subsidence along the coast. The edge of the North American plate will abruptly drop by an estimated 0.5 to 2 meters (1.6 to 6.6 feet) in many coastal areas. This sudden drop will instantly lower the elevation of coastal communities, causing immediate saltwater inundation and making them vulnerable to future flooding.
Secondary Hazards and Long-Term Effects
The primary secondary hazard is the tsunami, generated the moment the seafloor is vertically displaced by the megathrust rupture. The sudden upward movement of the ocean floor pushes the water column above it, forming powerful waves that move outward. Since the earthquake occurs close to the shore, the first tsunami waves will arrive with little warning.
Coastal residents will have as little as 15 to 30 minutes to reach high ground after the shaking stops. The resulting waves could reach heights of up to 30 meters (100 feet) in some areas, sweeping inland for miles. The wave sequence will consist of multiple surges arriving over several hours; the first wave is not necessarily the largest.
The earthquake will also trigger widespread landslides across the mountainous terrain. These landslides will sever critical transportation links, including major highways and rail lines, isolating many communities. The destruction of infrastructure, such as power grids, water systems, and communication networks, will severely complicate recovery efforts, leaving the region without basic services for weeks. Furthermore, the long-term effect of coastal subsidence will result in the permanent loss of low-lying coastal land and wetlands, increasing the baseline flood risk for decades.