San Francisco is situated in one of the most seismically active regions globally, positioned directly along the boundary between the Pacific and North American tectonic plates. The city’s location means it is constantly subjected to the forces generated by the slow, relentless movement of the Pacific Plate. This geographic reality results in perpetual seismic activity, ranging from daily, imperceptible tremors to rare, powerful events. Understanding earthquake frequency requires distinguishing between constant, low-level seismicity and the long-term forecasts for truly damaging events.
The Primary Fault Systems Affecting San Francisco
Earthquakes occur because the Pacific Plate is grinding northwestward past the North American Plate at a rate of approximately two inches per year. This slow motion causes stress to build up along the San Andreas Fault (SAF) system, a complex network of fractures. The SAF is the principal plate boundary, a right-lateral strike-slip fault that runs hundreds of miles through California and passes near the San Francisco Peninsula.
The overall plate motion is distributed across several parallel faults that pose direct threats to the Bay Area. The Hayward Fault, located in the densely populated East Bay, is a dangerous secondary fault due to its proximity to urban centers. Further east, the Calaveras Fault also absorbs some accumulated strain.
All of these fractures are strike-slip faults, meaning the land on either side moves horizontally past the other. This movement causes rocks to bend and store energy until friction is overcome, resulting in a sudden slip that generates seismic waves. The presence of multiple active faults means seismic risk is dispersed across the entire Bay Area.
Measuring Earthquake Frequency and Defining Magnitude
The San Francisco Bay Area experiences earthquakes frequently, though most are too small for residents to notice. Seismologists track thousands of events annually as the crust continuously adjusts to tectonic stress. On average, the Bay Area records approximately 1,600 quakes each year across all magnitudes, but the vast majority fall below the threshold of human perception.
The frequency of felt earthquakes is much lower. About 211 events of magnitude 2.0 or greater occur annually, while earthquakes of magnitude 3.0 or higher occur around 19 times per year. Scientists use the Moment Magnitude Scale (Mw) to quantify the size or strength of an earthquake based on its seismic moment, which measures the energy released.
Unlike the older Richter scale, the Moment Magnitude Scale is the standard for reporting larger events because it accurately represents the total energy released. It considers the area of the fault that ruptured and the amount of slip. Because the scale is logarithmic, an increase of one whole number, such as from 5.0 to 6.0, represents a release of about 32 times more energy. This distinction separates constant, small-scale seismicity from rare, high-energy events that cause damage.
Probabilistic Forecasts and Major Event Recurrence
While small earthquakes occur daily, hazard planning focuses on the long-term probability of a major, damaging event. Scientific models calculate the recurrence interval, which is the estimated average time between large ruptures on a specific fault segment. For example, the San Andreas Fault segment that ruptured in 1906 has an estimated recurrence interval of approximately 200 years for a similar-sized event.
The U.S. Geological Survey (USGS) provides consensus forecasts predicting the likelihood of significant earthquakes within a specified timeframe. The most recent comprehensive forecast indicates a 72% probability of at least one earthquake of magnitude 6.7 or greater striking the San Francisco Bay region before 2043. Magnitude 6.7 is considered the threshold for widespread damage.
This high overall probability reflects the combined risk from all major fault systems, not just the San Andreas. The Hayward Fault is a major contributor to this forecast, with the potential for a magnitude 6.7 or greater event due to stress accumulated since its last large rupture in 1868. These forecasts highlight that the region’s seismic hazard is defined by the eventual recurrence of massive, high-magnitude ruptures.