San Francisco does not sit directly on the San Andreas Fault (SAF), but the city’s identity is linked to its proximity to this major geologic boundary. The SAF is a massive transform fault running nearly 800 miles through California, marking the sliding boundary between the Pacific Plate to the west and the North American Plate to the east. The Pacific Plate grinds slowly northwestward relative to the North American Plate, a motion that builds tremendous stress released periodically in powerful earthquakes.
The San Andreas Fault’s Path Relative to San Francisco
The San Andreas Fault does not cut through the main urban core of San Francisco but instead traces a path just west of the city limits along the San Francisco Peninsula. This segment of the fault is visible as a prominent linear valley that contains both the San Andreas Lake and Crystal Springs Reservoir. The fault runs up the peninsula before diving into the Pacific Ocean near Mussel Rock in Daly City, which is immediately southwest of the city proper.
Once offshore, the fault continues its trajectory as a submarine feature, passing approximately six to seven miles (about 10 kilometers) west of the Golden Gate. This offshore location means that San Francisco sits on the North American Plate, while the nearby Farallon Islands are part of the Pacific Plate. The fault then returns onshore north of the city at Bolinas Lagoon in Marin County. From there, the fault trace is clearly visible as it cuts through the landscape of the Point Reyes National Seashore.
The movement along this boundary is a right-lateral strike-slip motion, where the land west of the fault moves north relative to the land on the east. This lateral sliding motion, averaging between 0.79 to 1.38 inches (20 to 35 millimeters) per year, is the primary mechanism driving seismic activity in the region. Its extreme proximity means San Francisco is built upon ground that directly abuts the main plate boundary.
The Greater Bay Area Fault System
The San Francisco Bay Area is crisscrossed by a complex network of active faults that accommodate the overall plate movement. Since the San Andreas Fault curves slightly in this region, the stress is distributed across several parallel faults that pose a more direct threat to the dense urban centers. These faults absorb a significant portion of the total plate-boundary motion.
The Hayward Fault runs along the base of the East Bay hills through highly populated cities like Oakland, Berkeley, and Fremont. It is located directly beneath major infrastructure and residential areas, with over two million people living in close proximity. The Calaveras Fault is another major branch that separates from the SAF near Hollister and runs eastward, eventually linking to the Hayward Fault system.
These faults form the larger San Andreas Fault system, relieving immense tectonic strain. The proximity of the Hayward and Calaveras faults to the East Bay means that a major earthquake on either of these can produce intense, close-range shaking for a large population center. Geologic studies suggest the Hayward and Calaveras faults may be connected at depth, meaning a rupture could involve both, leading to an even larger magnitude earthquake than previously anticipated.
Major Earthquakes and Seismic Risk
The history of the Bay Area is punctuated by destructive earthquakes. The most famous was the 1906 Great San Francisco Earthquake, caused by a massive rupture along the northern segment of the San Andreas Fault. This event ruptured approximately 296 miles (477 kilometers) of the fault, both on land and offshore, and is estimated to have been a magnitude 7.8.
The 1989 Loma Prieta Earthquake registered a magnitude 6.9 and occurred on the San Andreas Fault system in the Santa Cruz Mountains, about 56 miles south of San Francisco. While this earthquake caused significant damage, its relatively distant epicenter spared the city from the worst possible shaking. Scientists warn that a moderate-magnitude earthquake occurring closer to the urban center on a fault like the Hayward could produce ground shaking five to twelve times stronger than felt during the 1989 event.
Current seismic hazard assessments indicate a high likelihood of a major earthquake striking the region in the coming decades. There is a 72% chance of a magnitude 6.7 or greater earthquake occurring somewhere in the Bay Area within the next 30 years. Among the individual faults, the Hayward-Rodgers Creek system has the highest probability of producing this next major event, estimated at 31%, followed by the San Andreas Fault itself at 21%.