The San Andreas Fault (SAF) is a major geological feature in California, marking the sliding boundary between the Pacific Plate and the North American Plate. This boundary extends nearly 800 miles across the state. The primary concern for populations along this boundary is the inevitable release of accumulated geological stress as a major earthquake. Assessing the risk requires understanding the fault’s mechanics, the nature of direct surface rupture, and the secondary hazards that affect wider regions.
The Mechanics of the San Andreas Fault System
The San Andreas Fault is classified as a transform, or strike-slip, fault, meaning the two sides move horizontally past each other. The Pacific Plate moves northwest relative to the North American Plate, but this motion is not constant. Instead, the friction between the plates causes the fault to become “locked” in segments, preventing smooth movement.
Continuous tectonic movement drives the accumulation of elastic strain in the rocks surrounding the fault. When the stress exceeds the strength of the rocks, the fault suddenly slips. This releases the stored energy as seismic waves, which constitutes the earthquake itself. The severity of the resulting ground shaking is determined by the earthquake’s magnitude and the distance from the rupture point.
The southern portion of the fault has not experienced a major rupture since 1857. It is currently accumulating strain at a rate of approximately 25 millimeters per year. This suggests the fault is approaching the end of its typical cycle of stress buildup. The release of this energy will generate the most intense shaking near the rupture, but the effects will extend hundreds of miles into populated areas.
Cities Bisected by the Fault Trace
Communities built directly atop the fault trace face the most immediate risk: ground rupture. This hazard involves the physical tearing or offset of the ground surface. Ground rupture can instantly shear foundations, roads, and utility lines that cross the fault. The damage is localized but severe for any structure that straddles the rupture zone.
In Northern California, the fault trace passes through several smaller communities. These include Point Reyes Station and areas near Bodega Bay on the northern segment. Further south, the town of Hollister is known for structures visibly offset by slow, continuous fault movement, or creep. Parkfield sits directly on the central section of the fault.
The southern segment of the fault runs through several population centers. The high desert communities of Palmdale and Frazier Park are situated directly in the path of the fault’s movement. The fault also cuts through the Cajon Pass and the eastern edge of the San Bernardino Mountains. This places communities like Wrightwood and the eastern parts of San Bernardino at risk of direct surface rupture.
In the Coachella Valley, the fault trace runs along the eastern flank. This directly affects the communities of Desert Hot Springs and areas near Indio.
Regional Impact Zones and Secondary Hazards
Intense ground shaking affects major metropolitan areas, including the Los Angeles and San Francisco Bay regions. Although these areas are not directly on the SAF, they would experience severe and prolonged shaking causing widespread structural failure. Damage in these wider zones is often amplified by two hazards: basin effects and liquefaction.
Basin effects occur when seismic waves enter deep, sedimentary basins, such as the Los Angeles Basin. The soft, deep soils trap the seismic energy, causing the waves to bounce around. This increases both the amplitude and the duration of the ground shaking, subjecting distant buildings to prolonged stress. The Los Angeles metropolitan area is highly vulnerable due to this deep sedimentary structure, even though it is approximately 30 miles from the fault.
Liquefaction occurs in areas with loose, saturated, sandy soils. During intense shaking, water pressure increases, causing the ground to temporarily lose its strength and behave like a liquid.
In the San Francisco Bay Area, extensive areas of artificial fill and Bay muds are highly susceptible, especially along the shoreline and in parts of San Jose and Palo Alto. Coastal areas and riverbeds throughout Southern California also face a high probability of ground failure. This includes portions of Orange County and the Los Angeles coastal plain.
Assessing the Probability of a Major Event
Geological studies show that major earthquakes on the San Andreas Fault are inevitable, driven by the steady motion of the tectonic plates. Scientists use paleoseismology and GPS measurements of current strain to calculate recurrence intervals. The southern segment of the fault has a recurrence interval estimated between 100 and 300 years. It is currently well past the average time since its last major rupture in 1857.
The Uniform California Earthquake Rupture Forecast (UCERF) provides comprehensive probability estimates for the state. The current model projects about a 7% chance that California will experience an earthquake of magnitude 8.0 or greater in the next 30 years. Focusing on the Southern San Andreas Fault, the probability of a magnitude 6.7 or larger earthquake occurring on that segment within the next three decades is approximately 19%. This represents the highest probability for any single fault section in the state.
The northern segments, which ruptured most recently in 1906, have a lower current probability for a major event on the SAF itself. However, the overall tectonic forces continue to tighten the fault system. Large, damaging earthquakes remain a certainty within the coming decades.