California sits on a geological fault line, prompting the question of when, not if, the next massive seismic event will strike. A “major earthquake” is generally defined as one registering a magnitude of 7.0 or greater, capable of causing widespread destruction. The concern over California being “due for” such an event is rooted in the constant scientific monitoring of accumulated tectonic stress. This seismic threat drives ongoing efforts in forecasting and public safety.
The Geological Engine Driving California’s Seismic Risk
The underlying cause of California’s seismicity is the relentless movement of two massive crustal blocks: the Pacific Plate and the North American Plate. California straddles the boundary where these plates meet, forming a right-lateral strike-slip transform fault system. The Pacific Plate, carrying parts of the state including Los Angeles and San Diego, is slowly grinding northwestward relative to the North American Plate.
This horizontal motion occurs at an average rate of approximately two inches (50 millimeters) per year. The rough, irregular edges of the plates do not slide smoothly past each other, causing friction to lock portions of the boundary. Over time, this constant motion builds up enormous amounts of strain in the crustal rocks, similar to stretching a giant rubber band. When the accumulated stress overcomes the resistance holding the fault locked, the stored energy is suddenly released, causing the ground-shaking known as an earthquake.
Assessing the Threat: Major Fault Systems and Seismic Gaps
The primary geological feature accommodating this immense plate movement is the San Andreas Fault system, a vast network of fractures extending over 800 miles across the state. This system is a distributed boundary with different segments posing unique dangers to major population centers. The southern segment of the San Andreas Fault, which runs near the densely populated Los Angeles region, is currently considered the most likely to host the next “Big One.”
Beyond the main San Andreas, other faults present a severe threat, such as the Hayward Fault in the San Francisco Bay Area and the Puente Hills Fault beneath the Los Angeles basin. The Hayward Fault is an example of a “seismic gap,” a segment that has not ruptured in a long time compared to its historical recurrence interval. The last major event on the Hayward Fault occurred 157 years ago, exceeding its average return period of approximately 130 years. These gaps represent sections of the fault that are locked and accumulating stress, making them prime candidates for future rupture.
Probability vs. Prediction: What Scientists Mean by “Due For”
When scientists discuss California being “due for” a major earthquake, they are referring to a high probability established through forecasting models, not an absolute prediction of time and place. Earthquake forecasting relies on complex data, including the historical record of large quakes and the rate of strain accumulation measured by GPS and other instruments. The primary tool for this assessment is the Uniform California Earthquake Rupture Forecast (UCERF).
The UCERF model provides a long-term outlook, stating with near-certainty—greater than 99%—that a magnitude 6.7 or larger earthquake will strike somewhere in California within the next 30 years. While M6.7 matches the magnitude of the destructive 1994 Northridge earthquake, the most profound risk is from larger events. The current forecast indicates a 7% likelihood that California will experience a magnitude 8.0 or greater earthquake in that same 30-year period.
Forecasting differs fundamentally from prediction; it communicates the statistical likelihood of an event based on observed geological processes and the elastic-rebound theory. Scientists cannot predict the exact day or year of the next major rupture, but the calculated probabilities confirm that California resides in a state of perpetually high seismic hazard. These forecasts serve as the scientific justification for building codes, insurance planning, and public preparedness efforts.
Essential Steps for Earthquake Preparedness
Given the confirmed high probability of future major earthquakes, personal preparedness is the most effective form of risk mitigation. The immediate response during shaking is universally taught as “Drop, Cover, and Hold On.” This action involves immediately dropping to the floor, taking cover under a sturdy piece of furniture, and holding onto it until the shaking ceases.
Preparation also requires assembling an emergency supply kit designed to support a household for a minimum of 72 hours following a disaster. This kit should contain essential supplies. Personal documents, medications, and some cash should also be kept in an easily accessible, portable container.
Emergency Kit Contents
- At least one gallon of water per person per day.
- Non-perishable food.
- A battery-powered or hand-crank radio.
- A comprehensive first aid kit.
Structural mitigation is a necessary preventative measure to reduce injury and property damage from falling objects. Homeowners should secure water heaters by strapping them to wall studs. They should also fasten tall, heavy furniture, like bookcases and cabinets, directly to the walls to decrease the risk of injury from falling hazards.