San Andreas Fault: Complete Guide to California's Most Famous Fault Line

No fault line on Earth is more studied, more monitored, or more culturally prominent than the San Andreas Fault. It cleaves California from the Mendocino Triple Junction near Cape Mendocino to the shores of the Salton Sea, passing through or near some of the most densely populated real estate in the Western Hemisphere.

The fault has produced two of the most destructive earthquakes in American history — the 1857 Fort Tejon earthquake and the 1906 San Francisco earthquake — and seismologists are certain it will produce more. The question is not whether "The Big One" will happen, but when, and how prepared California will be.

This article traces the entire length of the San Andreas, explains its geology and history, examines what scientists expect from future earthquakes, and details the monitoring systems tracking the fault in real time. For the science behind how faults produce earthquakes, see what causes earthquakes. For current earthquake activity in California, visit our California earthquake tracker.

Geography: Tracing the Fault Through California

The San Andreas Fault runs roughly 1,200 km (750 miles) through California, making it one of the longest fault systems in the world. The fault trace — the line where it intersects Earth's surface — crosses a remarkable variety of terrain, from dense forests in the north to arid desert in the south.

The Northern Trace

The fault enters California at the Mendocino Triple Junction near Cape Mendocino, where the Pacific, North American, and Gorda (Juan de Fuca) Plates meet offshore. The Cascadia Subduction Zone begins at this triple junction, extending northward along the Pacific Northwest coast. From here, the San Andreas fault trace runs southeast through the Coast Ranges, passing through or near:

• Point Arena — the fault runs offshore along the Mendocino coast before coming ashore
• Bodega Bay — Bodega Head sits on the Pacific Plate side of the fault
• Tomales Bay and Bolinas Lagoon — both are linear water bodies sitting directly in the fault trench
• Mussel Rock (Daly City) — visible fault trace on the Pacific coast just south of San Francisco
• San Andreas Lake and Crystal Springs Reservoir — the fault runs through the valley that holds these reservoirs on the San Francisco Peninsula. The fault was named after San Andreas Lake by geologist Andrew Lawson in 1895.
• Los Gatos and the Santa Cruz Mountains — the fault crosses through these mountains southeast of Silicon Valley

The Central Trace

• Hollister — the town famously sits atop the fault's creeping section, where slow continuous movement causes visible offsets in curbs, sidewalks, and building walls
• Parkfield — a tiny community straddling the fault that has been the site of the most intensive earthquake monitoring experiment in the world since 1985
• Cholame — where the 1857 Fort Tejon earthquake rupture began
• Carrizo Plain — a remote valley where the fault trace is spectacularly visible in the landscape, with offset stream channels clearly showing right-lateral displacement of up to 130 meters

The Southern Trace

• Frazier Park and Tejon Pass — the fault crosses through the Transverse Ranges
• Palmdale and Lancaster — the fault trace runs through the western Antelope Valley
• Wrightwood — a mountain community sitting directly on the fault
• Cajon Pass — the fault crosses through this critical transportation corridor (Interstate 15, BNSF railroad)
• San Bernardino — the fault passes along the northern edge of the city
• Banning Pass — the fault runs through this gap between the San Bernardino and San Jacinto Mountains, paralleling Interstate 10 and the wind turbines
• Palm Springs and the Coachella Valley — the fault trace is visible along the base of the San Jacinto Mountains
• Salton Sea — the fault terminates at the southern end of the Salton Sea, where it connects to the spreading centers of the Gulf of California

Geology: How the San Andreas Fault Works

Plate Boundary Setting

The San Andreas Fault is a right-lateral strike-slip fault forming the principal boundary between the Pacific Plate (to the west) and the North American Plate (to the east). In this context, "right-lateral" means that if you stand on one side of the fault and look across, the other side appears to move to the right.

GPS measurements from the USGS and the EarthScope GAGE facility show that the Pacific Plate moves northwest relative to the North American Plate at approximately 50 mm/year (about 2 inches/year). However, the San Andreas Fault itself accommodates only about 20–28 mm/year of this motion. The remaining plate motion is distributed across the broader San Andreas Fault system, including the Hayward, Calaveras, San Jacinto, Elsinore, and other faults, as well as deformation distributed across the Basin and Range Province to the east USGS — San Andreas Fault.

The Three Segments

The San Andreas Fault behaves very differently along its length, and seismologists divide it into three principal segments:

Northern Segment (Cape Mendocino to Parkfield, ~470 km)

The northern segment last ruptured in the 1906 San Francisco earthquake, producing approximately 296 miles (477 km) of surface rupture — one of the longest rupture lengths ever observed for a strike-slip fault. Maximum horizontal displacement was approximately 6.1 meters (20 feet) near the Mendocino coast, decreasing toward the south.

The northern segment is currently locked — meaning the fault surfaces are stuck together by friction while the plates on either side continue to move. Geodetic data shows strain is accumulating across the locked portion. The average recurrence interval for large earthquakes on this segment is estimated at 200–300 years based on paleoseismic trenching studies, though individual intervals have varied considerably.

Central Segment (Parkfield to Cholame, ~150 km)

The central segment is the creeping section of the San Andreas — it accommodates plate motion through continuous slow slip (aseismic creep) at rates of about 25–28 mm/year, rather than through sudden earthquakes. This is visible in Hollister, where curbs, building foundations, and fence lines crossing the fault trace are slowly and continuously deformed.

The central section does still produce moderate earthquakes. The Parkfield segment, which lies at the transition between the creeping and locked sections, has produced a series of M~6 earthquakes at quasi-regular intervals: 1857, 1881, 1901, 1922, 1934, 1966, and 2004. The average interval is about 22 years, though the 2004 event came 38 years after the 1966 earthquake — a delay that underscored the limits of time-predictable models.

Southern Segment (Cholame to Salton Sea, ~580 km)

The southern segment is locked and has not produced a major rupture since the 1857 Fort Tejon earthquake — over 165 years ago. Paleoseismic studies at Wrightwood, Pallet Creek, and other sites along the southern San Andreas show an average recurrence interval of roughly 100–150 years for major earthquakes (M7.5+) on this segment. The current hiatus exceeds this average, and geodetic measurements confirm that significant strain has accumulated.

The southern San Andreas carries the highest probability of a future major earthquake of any segment. This is the scenario that seismologists commonly refer to as "The Big One."

Earthquake History on the San Andreas Fault

1857 Fort Tejon Earthquake (M7.9)

On January 9, 1857, the southern San Andreas Fault ruptured in what remains one of the largest earthquakes in California's recorded history. The earthquake ruptured approximately 350 km (220 miles) of the fault from Cholame in the north to Wrightwood in the south. Maximum horizontal surface displacement reached about 9 meters (30 feet) at some locations along the Carrizo Plain.

Despite the enormous magnitude, the earthquake caused relatively few casualties — estimated at two deaths — because the region was sparsely populated. The town of Fort Tejon, a U.S. Army outpost in the Tehachapi Mountains, was the nearest significant settlement. The earthquake was felt from Sacramento to San Diego and produced significant ground deformation visible along the fault trace to this day.

The 1857 earthquake is critically important for seismic hazard assessment because its rupture area overlaps with the locked southern segment — the same segment scientists expect to rupture again.

1906 San Francisco Earthquake (M7.9)

The April 18, 1906 San Francisco earthquake is the most consequential natural disaster in California's history and one of the landmark events in the development of modern seismology. The northern San Andreas Fault ruptured along approximately 477 km (296 miles) from San Juan Bautista in the south to Cape Mendocino in the north.

The earthquake struck at 5:12 a.m. local time. Maximum horizontal displacement was about 6.1 meters (20 feet) near Shelter Cove on the Mendocino coast. Shaking lasted approximately 45–60 seconds, and ground rupture was visible along the entire surface trace.

Approximately 3,000 people died, primarily in San Francisco, where the earthquake and subsequent fires destroyed over 80% of the city. About 225,000 of the city's 400,000 residents were left homeless. The fire — fueled by broken gas mains and hampered by ruptured water lines — burned for three days and caused more damage than the shaking itself.

The 1906 earthquake led to Harry Fielding Reid's elastic rebound theory, published in 1910, which remains the fundamental model of earthquake mechanics. It also prompted the establishment of systematic seismic monitoring in California. For a complete account, see our 1906 San Francisco earthquake page USGS — 1906 San Francisco Earthquake.

1989 Loma Prieta Earthquake (M6.9)

On October 17, 1989, a M6.9 earthquake struck the Santa Cruz Mountains during the World Series at Candlestick Park in San Francisco, broadcast to a national television audience. The earthquake occurred on a fault that is a sub-branch of the San Andreas system — technically the Loma Prieta segment of the San Andreas, though some seismologists classify it as a separate structure with an oblique-slip component.

The earthquake killed 63 people, injured over 3,700, and caused an estimated $6 billion in damage. The collapse of the Cypress Viaduct (Interstate 880) in Oakland killed 42 of the 63 victims. Significant damage also occurred in San Francisco's Marina District, built on landfill that liquefied during shaking, and on the Bay Bridge, where a section of the upper deck collapsed.

The Loma Prieta earthquake was a turning point for seismic safety policy in California. It accelerated the retrofit of bridges, highways, and unreinforced masonry buildings statewide and led to the development of earthquake early warning research. For a complete account, see our 1989 Loma Prieta earthquake page.

Other Significant Earthquakes on or Near the San Andreas

Note: Pre-instrumental earthquakes (before ~1930) have estimated magnitudes based on paleoseismic evidence, historical accounts, and geological studies.

"The Big One": What Scientists Expect

Probability Estimates

The Third Uniform California Earthquake Rupture Forecast (UCERF3), published in 2015 by the USGS, the California Geological Survey (CGS), and the Southern California Earthquake Center (SCEC), provides the most authoritative probabilistic estimates for future California earthquakes.

Key findings from UCERF3:

• 72% probability of at least one M6.7+ earthquake in the San Francisco Bay Area between 2014 and 2043 (this includes the San Andreas, Hayward, Calaveras, and other Bay Area faults)
• 93% probability of at least one M6.7+ earthquake somewhere in Southern California between 2014 and 2043
• The Southern San Andreas Fault has the highest probability of producing a M7.5+ earthquake of any individual fault in California — approximately 19% in 30 years for the Mojave segment alone
• 7% probability of a M8.0+ earthquake anywhere on the San Andreas system

UCERF3 also introduced the possibility that earthquakes could rupture across multiple faults simultaneously — a scenario that earlier models did not consider. A multi-fault rupture involving the San Andreas and connecting faults could produce events exceeding M8.0 USGS Fact Sheet: UCERF3.

The ShakeOut Scenario

In 2008, the USGS led a comprehensive scenario study called the ShakeOut, modeling the effects of a M7.8 earthquake on the southern San Andreas Fault. This remains the most detailed disaster scenario ever produced for an American earthquake.

The ShakeOut scenario models a rupture beginning near the Salton Sea and propagating northwest along approximately 300 km (190 miles) of the southern San Andreas, through the San Gorgonio Pass, Cajon Pass, and into the western Mojave Desert. The estimated impacts:

• 1,800 deaths (with potential for significantly more depending on time of day and fire conditions)
• 50,000 injuries
• $200 billion in damage and economic losses
• 255,000 households displaced
• Disruption of all major transportation, water, power, and communications corridors crossing the fault — including Interstate 10, Interstate 15, the California Aqueduct, the Los Angeles Aqueduct, Southern California Edison transmission lines, and natural gas pipelines
• Fire following earthquake modeled as the single largest contributor to casualties and losses, with an estimated 1,600 large fires breaking out from ruptured gas lines and downed power lines

The scenario explicitly noted that much of the water supply for the greater Los Angeles area crosses the San Andreas Fault. A major rupture could sever these aqueducts and leave the region without adequate water for weeks to months.

The ShakeOut scenario is the basis for the annual Great ShakeOut earthquake drill, which draws over 10 million participants in California each October The Great ShakeOut Earthquake Drills.

For earthquake preparedness guidance, see what to do during an earthquake and best earthquake emergency kits.

Cities at Risk

The San Andreas Fault passes near some of the most populated areas in the United States. The following table lists major cities near the fault with their approximate distance to the nearest fault trace and population.

[MAP: Detailed Bay Area map showing San Andreas + Hayward + Calaveras faults] Data source: USGS Quaternary Fault and Fold Database; CGS Fault Activity Map of California Features: San Andreas Fault trace through San Francisco Peninsula; Hayward Fault along East Bay hills (labeled with major cities: Oakland, Berkeley, Hayward, Fremont); Calaveras Fault further east; Green Valley and Concord Faults; population density overlay; label San Andreas Lake, Crystal Springs Reservoir, major bridges

Current Monitoring and Research

GPS and Geodetic Measurements

The Plate Boundary Observatory, now part of the GAGE (Geodetic Facility for the Advancement of Geoscience) operated by EarthScope, maintains a network of over 1,100 continuously operating GPS stations across the western United States, with high density along the San Andreas Fault system. These stations measure crustal deformation with millimeter-level precision, tracking the accumulation of strain across locked fault segments and the rate of creep on the creeping section.

GPS data has confirmed that the southern San Andreas is accumulating a significant slip deficit — the fault has not slipped enough in recent centuries to keep up with the ~28 mm/year of plate motion. This deficit, estimated at over 5 meters on parts of the southern segment, represents stored elastic energy that will be released in future earthquakes.

Creepmeters and Strainmeters

The USGS operates creepmeters — instruments that directly measure the slow sliding of the fault — at numerous locations along the creeping section and at the boundaries between creeping and locked segments. These instruments typically consist of a wire or rod anchored on both sides of the fault, with the change in distance recorded continuously.

Strainmeters, including borehole tensor strainmeters installed at depth in the Plate Boundary Observatory, measure the deformation of the rock surrounding the fault with extraordinary sensitivity — capable of detecting changes of less than one part per billion. These instruments can detect the subtle strain changes associated with slow-slip events and can potentially identify changes in fault behavior before major earthquakes.

The Parkfield Earthquake Experiment

Parkfield, California — a hamlet with a population of about 18 — is the most intensively monitored earthquake zone in the world. The USGS established the Parkfield Earthquake Experiment in 1985, motivated by the apparent regularity of M~6 earthquakes on the Parkfield segment (events in 1857, 1881, 1901, 1922, 1934, and 1966) and the prediction that the next would occur around 1988.

The earthquake came 16 years late — on September 28, 2004 — but the experiment was a scientific success. The dense instrumentation at Parkfield recorded the most complete dataset ever captured for a moderate earthquake on the San Andreas, including pre-earthquake changes in ground deformation and water levels in nearby wells USGS — Parkfield Earthquake Experiment.

Earthquake Early Warning

California, Oregon, and Washington are protected by the ShakeAlert earthquake early warning system, developed by the USGS in partnership with Caltech, UC Berkeley, the University of Washington, and the University of Oregon. ShakeAlert uses real-time data from seismograph networks to detect earthquakes as they begin and issue warnings before strong shaking arrives at populated areas.

For a San Andreas earthquake, ShakeAlert could provide up to 50–60 seconds of warning for Los Angeles from a southern San Andreas rupture that initiates near the Salton Sea, and 10–20 seconds for San Francisco from a northern segment rupture.

The Broader San Andreas Fault System

The San Andreas is not a single isolated structure. It is the primary strand of a complex fault system that collectively accommodates the Pacific-North American plate boundary motion. Several parallel faults carry significant portions of the total slip and pose independent seismic hazards.

Hayward Fault

The Hayward Fault runs approximately 74 km (46 miles) along the eastern side of San Francisco Bay, passing directly through the cities of Oakland, Berkeley, Hayward, Fremont, and San Jose. It is considered by the USGS to be the single most dangerous fault in the United States because of its proximity to dense urban populations.

The last major earthquake on the Hayward Fault was the 1868 Hayward earthquake (estimated M6.8–7.0), which killed 30 people and heavily damaged communities along the fault. Paleoseismic studies suggest a recurrence interval of approximately 140–160 years, putting the fault at or past its average time between major earthquakes. UCERF3 assigns the Hayward Fault a 14.3% probability of producing a M6.7+ earthquake in the 30-year period from 2014 to 2043.

San Jacinto Fault

The San Jacinto Fault is the most seismically active fault in Southern California by earthquake rate. It runs approximately 230 km from its junction with the San Andreas near Cajon Pass southeast through the San Jacinto Valley to the Imperial Valley, passing near San Bernardino, Hemet, and the Salton Sea.

The San Jacinto has produced several significant earthquakes in the modern instrumental era, including the 1918 San Jacinto earthquake (M6.8) and the 1968 Borrego Mountain earthquake (M6.6). It accommodates approximately 12 mm/year of plate motion — about half the rate on the adjacent San Andreas. Paleoseismic evidence suggests the San Jacinto can produce earthquakes up to M7.5.

Calaveras Fault

The Calaveras Fault extends approximately 123 km (76 miles) from its junction with the San Andreas south of Hollister northward through the eastern San Francisco Bay Area, passing near Morgan Hill, San Jose, and ending near Danville. It produced the 1984 Morgan Hill earthquake (M6.2) and has an active creeping section.

Elsinore Fault

The Elsinore Fault runs approximately 250 km (155 miles) through southwestern Southern California, from the Mexican border near the Laguna Salada Fault northwest through Temecula, Lake Elsinore, and Whittier to the northern end near Claremont. It accommodates approximately 5 mm/year of plate boundary motion. Although less active than the San Andreas and San Jacinto in the historical record, paleoseismic studies indicate it is capable of producing M7+ earthquakes.

Garlock Fault

The Garlock Fault is a left-lateral strike-slip fault that extends approximately 250 km east-northeast from its junction with the San Andreas Fault near Frazier Park. It forms the tectonic boundary between the Sierra Nevada/Tehachapi Mountains and the Mojave Desert. The Garlock has not produced a major historical earthquake, but paleoseismic studies indicate a recurrence interval of about 1,000–2,000 years for large events.

Visiting the San Andreas Fault

The San Andreas Fault trace is visible and accessible at several locations, making it one of the few major tectonic features that the public can observe firsthand.

Carrizo Plain National Monument (San Luis Obispo County) offers the most dramatic surface expression of the fault anywhere in California. The arid landscape preserves offset stream channels — most famously at Wallace Creek, where a stream has been displaced approximately 130 meters to the right by accumulated fault motion over roughly 3,800 years. Wallace Creek is a classic field geology site and is accessible via a short interpretive trail. The Carrizo Plain is managed by the Bureau of Land Management and is best visited between March and May when the wildflower bloom adds color to the stark landscape.

Point Reyes National Seashore (Marin County) preserves direct evidence of the 1906 earthquake. The Earthquake Trail, a short paved loop near the Bear Valley Visitor Center, passes a fence that was offset approximately 4.9 meters (16 feet) by the 1906 rupture. Interpretive signs explain the geology and history. Point Reyes itself sits on the Pacific Plate, separated from the mainland (North American Plate) by the fault trace that runs through Tomales Bay and Olema Valley.

Parkfield (Monterey County), self-proclaimed "Earthquake Capital of the World," straddles the fault at the transition between the creeping and locked sections. A bridge on Highway 46 crosses directly over the fault trace, and a sign marks the plate boundary. The town is tiny — population approximately 18 — but is home to the USGS Parkfield Earthquake Experiment's dense array of instruments.

San Andreas Lake and Crystal Springs Reservoir (San Mateo County) sit in a linear valley carved along the fault trace on the San Francisco Peninsula. The reservoirs are visible from Interstate 280, though public access to the shoreline is limited. The Sawyer Camp Trail along the east side of the reservoirs follows the fault trace and offers accessible hiking.

Living on the San Andreas: Preparedness and Policy

Building Codes

California has among the most stringent seismic building codes in the world. The California Building Code requires new structures to be designed to withstand the level of shaking expected from the earthquake scenarios applicable to their location. Buildings are not designed to survive without damage — they are designed to not collapse, protecting the lives of occupants.

The Alquist-Priolo Earthquake Fault Zoning Act, enacted in 1972, prohibits the construction of most structures for human occupancy directly on the surface trace of active faults, including the San Andreas. Cities and counties along the fault maintain fault zone maps and require geological investigations before permitting construction within designated zones California Geological Survey — Alquist-Priolo Act.

Retrofit Requirements

Several California cities have enacted mandatory seismic retrofit ordinances for existing buildings. Los Angeles passed ordinances in 2015 requiring the retrofit of approximately 13,500 soft-story wood-frame apartment buildings and 1,500 non-ductile concrete buildings. San Francisco has a mandatory soft-story retrofit program that has addressed over 4,900 buildings since 2013.

Caltrans has spent over $13 billion since 1989 retrofitting bridges and highway structures across the state, prioritizing structures on or near major fault crossings. For more on this topic, see our seismic retrofitting guide.

Earthquake Insurance

The California Earthquake Authority (CEA) provides the majority of residential earthquake insurance in the state. As of 2024, only about 10–13% of California homeowners carry earthquake insurance — a figure that has declined from about 33% after the 1994 Northridge earthquake. The average annual premium for a CEA policy is approximately $800–$1,200, depending on location, building type, and chosen deductible. For detailed coverage information, see earthquake insurance in California.