The term “The Big One” describes a future catastrophic earthquake capable of causing widespread destruction across major metropolitan areas of the West Coast. This phrase commonly refers to two distinct, massive-scale seismic events that threaten millions of people. The first involves a mega-rupture along the southern segment of the San Andreas Fault in California, a strike-slip boundary. The second, and potentially larger, event is a full-margin rupture of the Cascadia Subduction Zone (CSZ), an offshore fault that runs from Northern California up to Vancouver Island.
The Southern California Scenario
A rupture of the southern San Andreas Fault is often modeled as a magnitude 7.8 to 8.1 earthquake. This event would bring the most intense shaking to cities closest to the fault line, specifically the metropolitan areas of San Bernardino and Riverside in the Inland Empire. The fault runs directly underneath or in very close proximity to communities such as San Bernardino and Highland, subjecting structures there to the highest levels of ground acceleration.
The Coachella Valley, which includes cities like Palm Springs, Indio, and Coachella, also lies perilously close to the fault’s southern terminus near the Salton Sea. A rupture would cause severe shaking and potentially significant surface displacement that could cut across infrastructure like highways and utility lines.
While the city of Los Angeles is located about 35 to 50 miles away from the main San Andreas Fault trace, the scale of a magnitude 8.1 event means intense, prolonged shaking would still reach the greater metropolitan area. The shaking could last several minutes, capable of damaging a vast number of buildings and transportation networks across the Los Angeles Basin.
The Pacific Northwest Scenario
The Cascadia Subduction Zone (CSZ) poses a different and even larger threat, capable of producing a megathrust earthquake estimated at magnitude 9.0 or greater, as occurred in the year 1700. This massive offshore fault’s full-margin rupture would cause violent shaking lasting an estimated four to seven minutes along the coast. Coastal towns in Oregon and Washington, such as Crescent City, Seaside, and Aberdeen, would experience the strongest shaking, immediately followed by the secondary hazard of a tsunami.
Tsunami waves could reach coastal communities within 15 to 30 minutes of the shaking stopping, with projected heights of 30 to 40 feet in some locations. This rapid inundation would devastate communities built at low elevations along the Pacific shoreline, including those in Northern California and extensive portions of the Oregon and Washington coasts. The earthquake itself would also cause the coastline to subside by several feet, making the area more vulnerable to the incoming waves.
Inland metropolitan areas like Seattle, Tacoma, and Portland would experience significant damage from the prolonged ground shaking. The shaking duration from a CSZ event is significantly longer than a typical strike-slip earthquake, greatly increasing the potential for structural fatigue and collapse in tall buildings. Portland, situated in the Willamette Valley, and the cities surrounding the Puget Sound are built on geological features that would magnify the seismic waves as they travel inland.
Geological Factors Magnifying City Damage
Local geology plays a determining role in the severity of damage, often magnifying the shaking intensity far from the fault line. One of the most significant factors is soft-sediment amplification, which occurs in deep geological basins.
The Los Angeles Basin, a massive trough filled with soft sedimentary rock up to nine kilometers deep, is a prime example of this phenomenon. As seismic waves from the San Andreas Fault enter the basin, the soft materials trap and slow the waves, causing their amplitude to dramatically increase. This amplifies the ground motion by a factor of up to four to six times in the deepest parts of the basin, significantly increasing the duration of shaking by a minute or more. This prolonged, amplified motion would impact cities across the basin, including downtown Los Angeles and parts of the San Fernando and San Gabriel Valleys.
A second critical factor is liquefaction, the process where saturated, loose, granular soils temporarily lose their strength and behave like a liquid during intense shaking. This threat is particularly acute in areas built on reclaimed land, such as the Seattle waterfront, Pioneer Square, and the Duwamish industrial area. Similarly, much of Portland is built on soft alluvial silts along the Willamette and Columbia Rivers that are highly susceptible to liquefaction and lateral spreading.
Liquefaction vulnerability in Portland threatens major infrastructure, including the critical fuel tank farm and the many bridges that cross the Willamette River, which could suffer severe damage and failure. Areas with high water tables and loose soils, such as coastal plains and river valleys, face a compounded risk of destruction in both the California and Pacific Northwest scenarios.