The question of “when” the next massive earthquake, often called “The Big One,” will strike a major population center is one of the most pressing and least satisfying questions in earth science. This anticipated seismic event typically refers to a magnitude 7.5 or greater earthquake that will cause widespread devastation in a heavily populated region. While the anxiety surrounding the timing is understandable, scientists can only provide a framework of likelihood and location, not a specific date. Understanding the geological forces and the limitations of current technology is the first step toward effective preparation.
Defining the Major Seismic Threats
The threat of a catastrophic earthquake in the United States is concentrated in three distinct geological regions. The most commonly discussed threat is the San Andreas Fault in California, a continental strike-slip fault forming the boundary between the Pacific and North American plates. Scientists anticipate a potential rupture on the southern segment of the San Andreas could release a magnitude 7.8 or greater earthquake, directly impacting the greater Los Angeles area.
The Cascadia Subduction Zone (CSZ) lies off the coast of the Pacific Northwest, stretching about 700 miles from northern California to British Columbia. The Juan de Fuca tectonic plate is slowly sliding beneath the North American plate, building up immense strain. A full-margin rupture of the CSZ is expected to unleash a megathrust earthquake possibly reaching magnitude 9.0, which would also generate a devastating tsunami.
A third major risk is the New Madrid Seismic Zone (NMSZ) in the central United States, running through parts of Illinois, Missouri, Arkansas, Kentucky, and Tennessee. This intraplate zone is far from a tectonic plate boundary, but it produced a series of powerful earthquakes estimated at magnitude 7.0 to 8.0 in 1811 and 1812. Due to the dense, rigid rock structure of the central and eastern US, the shaking from a major NMSZ event would be felt over an area up to 20 times larger than a comparable earthquake in California.
Why Exact Timing Remains Impossible to Predict
The short answer to “when” is that seismologists cannot predict the exact date, time, and location of any future earthquake. This inability stems from the inherent complexity of fault mechanics and the physical limitations of monitoring underground stress. Scientists can measure how quickly stress is accumulating along a fault line, but they lack a mechanism to determine the precise breaking point of the rock.
Prediction, which specifies a narrow window for an event, is distinct from forecasting, which deals in long-term probabilities. Fault systems are heterogeneous, containing locked segments where strain builds up and creeping sections that release pressure slowly. The transition from stress accumulation to sudden rupture remains hidden deep beneath the surface, making short-term warnings impossible.
Earthquakes do not happen on a predictable schedule, even on faults that have historically ruptured at relatively regular intervals. Past attempts to forecast quakes based on short-lived precursors like foreshocks or changes in ground chemistry have proven unreliable. The most accurate scientific information available is the long-term statistical assessment of risk, based on historical patterns and geological modeling.
Assessing the Long-Term Likelihood
While a precise date is unattainable, scientists use complex models to calculate the probability of a major event over decades. The Uniform California Earthquake Rupture Forecast (UCERF) model estimates a greater than 99% chance of one or more magnitude 6.7 or greater earthquakes striking California in the next 30 years. The likelihood of a magnitude 8.0 or greater event along the San Andreas Fault system in that period is about 7%.
The Cascadia Subduction Zone last ruptured in 1700 and operates on a longer recurrence interval, with major events estimated every 300 to 500 years. Current forecasts suggest a 16% to 22% chance of a full-margin magnitude 8.7 to 9.2 earthquake within the next 50 years. A smaller event on the southern segment of the CSZ has a 37% to 43% chance in the same timeframe.
In the New Madrid Seismic Zone (NMSZ), the probability of a repeat of the massive 1811–1812 earthquakes (M7.0 to M8.0) over the next 50 years is low, at 7% to 10%. However, the chance of a smaller but damaging magnitude 6.0 or greater earthquake in the NMSZ within that period is significantly higher, estimated between 25% and 40%. These figures are derived from studying the average time between large past events, using geologic evidence like ancient soil liquefaction features.
Necessary Steps for Personal Safety
Since the timing of a major earthquake cannot be known, preparation focuses entirely on mitigating risk before the ground begins to shake. The most immediate safety action is to “Drop, Cover, and Hold On”: immediately getting low, taking cover under a sturdy desk or table, and holding onto it until the shaking stops. This action protects the body from falling debris and nonstructural hazards, which cause the majority of injuries.
Preparation also involves securing the home environment to prevent nonstructural damage and injury. Heavy furniture, such as bookcases, dressers, and water heaters, should be anchored to wall studs using flexible straps or L-brackets. Falling objects cause a high percentage of injuries, a hazard that can be reduced by moving heavy items to lower shelves and using museum putty for smaller valuables.
Every household should assemble a comprehensive emergency kit containing enough supplies to be self-sufficient for at least 72 hours (two weeks is preferable). This kit must include:
- A minimum of one gallon of water per person per day.
- Non-perishable food.
- A hand-crank or battery-powered radio.
- A first-aid kit.
Families should also establish an out-of-state contact person to check in with after a disaster, as local communication lines are often overloaded.