The term “The Big One” refers to the maximum credible earthquake expected on a major fault line, typically the Cascadia Subduction Zone (CSZ) or the San Andreas Fault (SAF). This event represents a catastrophic failure of a major tectonic boundary, far exceeding a typical earthquake. Understanding the disaster requires looking beyond the initial shaking to the cascading failures that would fundamentally disrupt modern society, involving massive ground deformation, secondary hazards, and the collapse of essential services.
Defining the Scale of the Disaster
The two most discussed “Big One” scenarios involve events releasing energy far exceeding typical earthquakes. The Cascadia Subduction Zone can generate a megathrust earthquake (magnitude 9.0 or greater) where the Juan de Fuca plate is forced beneath the North American plate. The San Andreas Fault, a strike-slip boundary, is expected to produce a maximum event in the magnitude 7.9 to 8.3 range, similar to the 1906 San Francisco earthquake.
A Cascadia event is distinct in the duration of its ground motion, estimated to last between three and five minutes, significantly longer than typical earthquakes. This prolonged movement subjects structures to sustained stress, increasing the likelihood of failure even in modern buildings. The massive rupture along the fault zone, stretching over hundreds of miles, causes widespread physical changes to the landscape.
Ground deformation is a direct geological outcome of the plate movement. In the Cascadia scenario, the coastal edge of the North American plate would snap back upward, while areas immediately inland would experience massive subsidence (sinking) as the plate relaxes. The 1700 Cascadia event caused land to sink by two to six feet over a 55,000 square kilometer region, instantly drowning coastal forests and marshes.
The San Andreas Fault event, being a strike-slip fault, would primarily result in horizontal ground displacement. During the 1906 earthquake, parts of the fault moved laterally by as much as 21 feet. A modern event could see infrastructure, such as roads, pipelines, and rail lines that cross the fault, instantly severed and offset by up to 20 feet, creating a permanent, impassable rupture zone.
Secondary Hazards Following the Initial Shock
Once the ground shaking subsides, the energy released triggers secondary hazards often more destructive than the initial seismic event. For the Cascadia scenario, the most immediate danger is the generation of a near-field tsunami. The sudden vertical displacement of the seafloor from the megathrust rupture shoves the entire water column upward, creating massive waves.
These tsunami waves would reach the Pacific coastline in as little as seven to 30 minutes after the shaking begins, leaving virtually no time for official warnings to reach the public. Expected wave heights can vary dramatically depending on local topography, with models suggesting initial waves between three and 80 feet high. The inundation zone would extend up to a kilometer inland in open coastal areas and several kilometers up river valleys, with multiple waves continuing for 10 to 24 hours.
Prolonged shaking also triggers widespread soil liquefaction in areas composed of loose, water-saturated sediments, such as reclaimed land or river deltas. During the earthquake, the strong ground motion increases the water pressure between the soil particles, causing the ground to temporarily lose its strength and behave like a liquid. This loss of bearing capacity causes buildings, bridges, and other structures to tilt, sink, or collapse as their foundations fail.
The sustained energy from a major earthquake destabilizes hillsides and mountain slopes, leading to extensive landslides and rockfalls. In mountainous regions, shaking causes massive blocks of earth and rock to tumble down, blocking major roadways and valleys. Historical earthquakes have demonstrated that such failures can generate localized tsunamis in lakes and fjords.
A major urban earthquake creates numerous simultaneous fires, overwhelming the capacity of any fire department. These fires start from compromised infrastructure, such as broken natural gas lines, ruptured fuel tanks, and electrical system failures. Ruptured gas lines and electrical faults are major factors in post-quake fires. The combination of multiple ignition sources and an impaired water supply due to broken water mains can quickly lead to uncontrollable firestorms.
The Failure of Essential Lifelines and Services
The sheer scale of damage from “The Big One” would ensure the systemic failure of the integrated systems that support modern life, known as lifelines. The power grid would experience a complete and prolonged shutdown due to damage to transmission substations and distribution lines across a vast area. This widespread blackout would trigger cascading failures across all other sectors, as nearly every utility relies on electricity.
Communication would be severely impaired, with cellular networks and internet services failing due to power loss and physical damage to fiber optic cables. Even satellite phones would struggle to function reliably under simultaneous emergency communication attempts, isolating communities and complicating relief efforts. The lack of reliable communication would cripple the ability of local and federal agencies to coordinate their response.
Transportation infrastructure would be rendered largely unusable. Major bridges and elevated highways would suffer widespread structural damage or collapse, while rail lines and port facilities would be incapacitated by ground failure and liquefaction. This destruction prevents the organized deployment of emergency personnel, medical supplies, and food into the affected region for days or weeks.
The distribution of potable water and natural gas would cease entirely. Water mains, brittle and decades old in many urban areas, would fracture in thousands of locations, leading to a complete loss of pressure and supply. The failure of gas lines and the necessary manual shutdown of regional distribution systems would leave millions without heat or cooking fuel for an extended period, creating a severe humanitarian crisis.