The term “The Big One” is a shorthand for a catastrophic earthquake capable of widespread, life-altering destruction. This phrase generally refers to an event of moment magnitude (Mw) 8.0 or greater that strikes a densely populated region. The public interest in this topic is driven by the potential for infrastructure collapse, massive casualties, and the complete disruption of daily life. For scientists, this concept represents the maximum credible event for a given fault system, driving modern seismic hazard planning and preparedness efforts.
How Scientists Define Catastrophic Earthquakes
The severity of an earthquake is measured using two distinct concepts: magnitude and intensity. Magnitude is a single value that quantifies the energy released at the source of the earthquake rupture, calculated using seismograph readings. Intensity, conversely, describes the strength of ground shaking at a specific location and is based on observed effects on people, buildings, and the natural environment. A single earthquake can have multiple intensity values that decrease with distance.
The potential for a “Big One” depends heavily on the type of fault involved. The most powerful earthquakes globally occur on megathrust faults, which are found in subduction zones where one tectonic plate slides beneath another. These environments are capable of producing earthquakes exceeding magnitude Mw 9.0 because they allow for immense strain to build up over a very large contact area. Strike-slip faults, where plates slide past each other horizontally, typically produce events up to a maximum of about Mw 8.3.
North America’s Subduction Zone Threat
The Cascadia Subduction Zone (CSZ) represents the greatest potential for a truly massive earthquake in North America. This 1,100-kilometer fault runs offshore from Northern California up to mid-Vancouver Island, British Columbia, where the Juan de Fuca plate is subducting beneath the North American plate. The geological setting is primed for a megathrust event, which is the mechanism responsible for the planet’s largest quakes.
Historical evidence confirms this capacity, pointing to a massive event that occurred on January 26, 1700, estimated to be around Mw 9.0. The precise date was determined by correlating geological evidence, such as buried “ghost forests” that subsided into the tidal zone, with records of an “orphan tsunami” that struck the coast of Japan hours later. This fault is believed to rupture roughly every 500 years on average.
A rupture of the CSZ presents a dual hazard: intense, long-duration ground shaking and a devastating tsunami. Because the fault lies so close to the coast, a megathrust quake would generate a massive wave, potentially up to 100 feet high. This wave would strike coastal communities with less than 30 minutes of warning, resulting in a significant inundation risk to the Pacific Northwest coastline.
The Seismic Risk in Southern California
The San Andreas Fault (SAF) is the system most commonly associated with “The Big One” in popular culture, and it poses a severe, immediate threat to major population centers. It is a continental right-lateral strike-slip transform fault, marking the boundary where the Pacific and North American plates grind past one another. The maximum credible earthquake for a full rupture of the most critical segment is estimated to be approximately Mw 8.3.
Historically, the fault has ruptured in major segments, most notably the 1906 San Francisco earthquake (Mw 7.9) and the 1857 Fort Tejon earthquake (Mw 7.9). The highest current risk is concentrated on the southernmost segment, particularly the area underlying the Coachella Valley. This section has not experienced a major rupture since around 1680, meaning it has accumulated over 300 years of strain, making it significantly overdue compared to its estimated recurrence interval.
The primary hazard from the San Andreas is the direct result of the fault’s movement: severe ground shaking, surface rupture that can tear through infrastructure, and liquefaction in saturated soils. The proximity to Los Angeles and other major cities means that the shaking alone would cause immense damage. The unique danger of this fault is the direct exposure of major metropolitan areas to high-magnitude strike-slip motion.
Major Fault Lines Outside the West Coast
Seismic risk is not exclusive to the Pacific coast, as demonstrated by active intraplate zones in the central and eastern United States. The New Madrid Seismic Zone (NMSZ) is a prime example, running through the central Mississippi Valley across parts of Arkansas, Missouri, Tennessee, and Kentucky. This zone is an ancient rift system deep within the North American plate, and it remains seismically active despite its distance from a plate boundary.
The NMSZ is famous for a series of three powerful earthquakes that struck in the winter of 1811–1812, with estimated magnitudes in the Mw 7 to Mw 8 range. The stable, dense bedrock of the central and eastern U.S. crust is incapable of dampening seismic waves as effectively as the fractured crust of the West Coast. This difference means that a major earthquake in the NMSZ would transmit damaging shaking over an area up to 20 times larger than a comparable event in California.
Further east, the Charlevoix Seismic Zone in Quebec, Canada, also poses a significant intraplate hazard along the St. Lawrence River. This region has experienced five large earthquakes since 1663, including a destructive event estimated to be between Mw 7.3 and Mw 7.9.
Understanding Earthquake Probabilities
Scientists cannot predict the exact time or day an earthquake will strike, as this would require deterministic knowledge of when a fault’s built-up strain will finally overcome friction. Instead, seismologists rely on earthquake forecasting, which involves calculating the probability of an event occurring within a specific region and timeframe, typically over the next 30 years. This probabilistic approach is based on a collection of geological and seismic data.
These forecasts are calculated through methods like paleoseismology, which involves digging trenches across fault lines to find and date evidence of past ruptures. This historical record allows researchers to determine the average recurrence interval for large earthquakes on a specific fault segment. Combining this recurrence data with current rates of strain accumulation, measured by GPS and other instruments, allows scientists to assign a numerical risk. For example, recent forecasts estimate a 7% probability that the San Andreas Fault will produce an earthquake of Mw 8.0 or greater in the next three decades.