The concept of “The Big One” has long been a fixture in discussions about the seismic future of the North American West Coast. It represents a widely anticipated, catastrophic earthquake event capable of inflicting widespread damage across densely populated regions. To understand the true hazard this event poses, it is necessary to move beyond generalized fears and examine the specific geological realities that determine the size and impact of such a massive tremor. This analysis requires distinguishing between two separate, high-magnitude scenarios and quantifying the energy they are expected to release.
Defining “The Big One” and Its Geological Sources
The name “The Big One” actually refers to two distinct, devastating earthquake scenarios originating from two fundamentally different types of fault systems. The first is the San Andreas Fault (SAF) system, which runs through California. This system is a continental right-lateral strike-slip fault, meaning the tectonic plates on either side slide horizontally past each other.
The San Andreas event is typically associated with a magnitude around M 7.8 or greater, with the most probable rupture anticipated on the southern segments near Los Angeles. The second and significantly larger scenario is the Cascadia Subduction Zone (CSZ), which stretches over 600 miles from Northern California to British Columbia.
Unlike the San Andreas, the CSZ is a subduction zone, where the oceanic Juan de Fuca plate is diving beneath the continental North American plate. This difference in plate movement is critical because the CSZ involves massive vertical displacement, capable of generating a much larger, megathrust earthquake. The San Andreas event would be characterized by intense, horizontal shaking across a major metropolitan area, while the Cascadia event would involve prolonged, widespread shaking across the Pacific Northwest.
Quantifying the Expected Magnitude
The measurement used by scientists to quantify the size of these potential quakes is the Moment Magnitude Scale (M_w), which has largely replaced the older Richter scale. The Moment Magnitude Scale is a logarithmic measure that relates directly to the total energy released by the earthquake, factoring in the size of the fault rupture and the amount of slip. Because the scale is logarithmic, each whole number increase represents approximately 32 times more energy released.
For the San Andreas Fault, the expected magnitude for a major rupture is typically M 7.8, consistent with historic events like the 1906 San Francisco earthquake. The Cascadia Subduction Zone, however, is capable of producing an M 9.0 or higher megathrust event. This difference between an M 7.8 and an M 9.0 is enormous, as the M 9.0 earthquake releases roughly 1,000 times more energy than an M 7.0 event.
The reason the Cascadia event is predicted to be so much larger is due to the fault’s geometry and massive scale. Subduction zones allow for a much larger area of the fault to rupture simultaneously, extending for hundreds of miles. This vast rupture area and the vertical nature of the plate movement are what give the Cascadia Subduction Zone the potential to generate the largest possible earthquakes on Earth.
The Scope of Potential Destruction
The magnitude of the earthquake directly determines the physical effects, particularly the intensity and duration of ground shaking.
Shaking Duration and Intensity
A major M 7.8 earthquake on the San Andreas Fault would produce intense, violent shaking, but the rupture process would likely last for about one to two minutes. This rapid, high-frequency shaking would cause widespread collapse of vulnerable structures near the fault line, particularly in basins where soft sediments amplify ground motion.
In contrast, an M 9.0 Cascadia megathrust event would shake the ground for a significantly longer period, potentially lasting three to five minutes, or even up to eight to ten minutes in some locations. This prolonged, lower-frequency shaking is particularly destructive to taller buildings and infrastructure. The longer duration of shaking increases the chance of secondary hazards, such as landslides in hilly areas and widespread liquefaction, where saturated, loose soil temporarily loses its strength and behaves like a liquid.
Tsunami Risk
The most dramatic difference in destruction relates to tsunami generation. The San Andreas Fault’s strike-slip motion does not typically displace enough ocean floor vertically to create a major tsunami threat to the immediate California coastline. However, the Cascadia Subduction Zone’s vertical plate displacement is a direct tsunami generator. A full rupture would immediately launch a massive tsunami, with wave heights potentially reaching 80 to 100 feet in some coastal areas, striking the Pacific Northwest shoreline within minutes of the shaking stopping.
Geological Recurrence and Probability
The history of these faults suggests that these large events are not a matter of if, but when, based on geological cycles. For the San Andreas Fault, scientific models estimate a probability of about 7% for an M 8.0 or greater event occurring somewhere along the fault in the next 30 years. This relatively low percentage reflects the complex segmentation of the fault, with some southern segments considered “overdue” based on their historical slip rates.
The Cascadia Subduction Zone operates on a much longer cycle. Geological evidence shows that full-margin M 9.0 events have occurred on average every 300 to 500 years. The last known full rupture occurred in January 1700, making the current interval approximately 325 years. This places the fault within the window of its recurrence interval, with current estimates suggesting a 10% to 15% probability of an M 9.0 event occurring in the next 50 years.