The San Andreas Fault (SAF) is a massive geological feature in California, marking the active boundary between the Pacific and North American tectonic plates. Extending for roughly 750 miles (1,200 km), this fracture zone is a continental right-lateral strike-slip transform fault where the plates grind past one another horizontally. The Pacific Plate, carrying coastal California, moves northwestward relative to the North American Plate. This constant, slow movement creates immense stress that periodically releases in large earthquakes, making the fault a primary concern for millions of residents.
Identifying Directly Affected Urban Zones
The San Andreas Fault rarely cuts directly through the dense downtown centers of California’s largest metropolitan areas, instead bisecting numerous smaller cities and suburban communities. In Southern California, the fault runs through towns in the high desert and mountain communities northeast of the Los Angeles basin. Specific communities like Palmdale in the Antelope Valley and the mountain town of Frazier Park are situated directly on the fault line.
The fault trace also runs through the Inland Empire, traversing the northern edge of San Bernardino. Wrightwood, a mountain resort town, and the desert community of Desert Hot Springs are also positioned squarely on the fault’s surface trace. In Central California, Hollister is famously bisected by the fault, where observable offsets demonstrate the continuous, slow movement known as aseismic creep.
Moving north, the fault enters the San Francisco Bay Area and runs along the San Francisco Peninsula. It is mostly offshore or runs through less-developed areas like the Crystal Springs Reservoir valley. However, the fault trace is directly adjacent to coastal towns like Daly City and Point Reyes Station. These communities face the immediate hazard of surface rupture, where the ground is physically torn apart along the fault line.
The Major Segments of the San Andreas Fault
Geologically, the San Andreas Fault is categorized into three distinct segments, each exhibiting different behaviors and risk profiles.
Northern Segment
The Northern Segment extends from Hollister north to the Mendocino triple junction. This segment was responsible for the devastating 1906 San Francisco earthquake. It runs through the Santa Cruz Mountains and along the San Francisco Peninsula before heading offshore.
Central Segment
The Central Segment stretches from Parkfield to Hollister and is characterized by “aseismic creep.” In this region, the fault moves continuously and slowly, releasing tectonic strain gradually without major earthquakes. This section runs through the sparsely populated Carrizo Plain, where the fault’s path is visibly etched into the landscape.
Southern Segment
The Southern Segment extends from Parkfield down to the Salton Sea and is considered the most dangerous due to its proximity to major population centers and its long period of seismic quiescence. This segment runs through the Transverse Ranges, including the “Big Bend” area where the fault changes direction. It crosses the Cajon Pass and runs along the base of the San Gabriel and San Bernardino Mountains. The lack of major ruptures on this southern section since 1857 indicates significant accumulated strain, posing a substantial risk to Southern California.
Seismic Risk and Defining Proximity
The term “on the San Andreas Fault” refers either to the immediate danger of surface rupture or the broader threat of seismic hazard. Being literally on the fault line means a structure is at risk of being physically torn apart by lateral ground displacement. However, the primary risk for major metropolitan areas is intense ground shaking and secondary effects, not surface rupture from the main fault.
Cities like Los Angeles and San Francisco are situated within a vast Seismic Hazard Zone, even though the main SAF trace does not run through their downtown areas. The risk is compounded by numerous secondary faults, such as the Hayward, Calaveras, and San Jacinto faults, which run parallel to the SAF. These secondary faults can produce major earthquakes and pose a direct threat to the dense urban cores of the Bay Area and the Los Angeles basin.
The geometry of the fault dictates the extent of the risk zone. The Northern Segment is relatively straight, limiting earthquake activity to a narrow band, generally within about 10 miles of the fault trace. In contrast, the Southern Segment features the “Big Bend,” which generates significant compressional stress. This allows earthquakes to occur further away, potentially up to 30 miles from the main fault line. This broader zone of risk means major population centers are heavily exposed to secondary hazards like ground shaking, liquefaction, and landslides.