A major rupture of the San Andreas Fault would directly threaten cities and communities along roughly 800 miles of California, from the Salton Sea near the Mexican border to Cape Mendocino on the northern coast. But the fault’s reach extends well beyond the towns that sit right on top of it. Los Angeles, despite lying some 30 to 40 miles from the nearest trace of the San Andreas, faces some of the most intense projected shaking of any metro area because of how seismic waves funnel through the sedimentary basins beneath it. San Francisco, San Bernardino, Palm Springs, and dozens of smaller cities in between are all in the crosshairs, each for slightly different geological reasons.
Cities Directly on or Near the Fault Trace
The San Andreas Fault does not follow a single clean line. It splits, bends, and steps over through a series of segments, and different cities sit close to different segments. In the south, the Coachella Valley communities of Palm Springs, Desert Hot Springs, Indio, and Coachella are practically on top of the fault. Desert Hot Springs sits between two converging fault strands, and research has shown that earthquakes nucleating at the junction of these strands would generate strong shaking from guided waves traveling along both fault zones and from amplified waves in the low-velocity sedimentary basin between them.1Bulletin of the Seismological Society of America. San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards and Groundwater
Moving northwest from the Coachella Valley, the fault threads through San Gorgonio Pass, where the geometry becomes complicated. The fault splinters into multiple strands with different slip rates. Geodetic and geologic measurements show that individual strands in this zone creep at rates ranging from about 2 to 12 millimeters per year depending on the strand and the measurement method.2Geosphere. New geodetic constraints on southern San Andreas fault-slip rates, San Gorgonio Pass, California San Bernardino sits just beyond the northwest end of this pass, positioned where the fault re-emerges as a more unified structure. Farther along, the fault runs through Cajon Pass and past the high-desert cities of Palmdale, Lancaster, and the Antelope Valley before continuing toward Parkfield and points north.
In central California, the fault passes through sparsely populated terrain but runs right by or through the towns of Parkfield, Hollister, and San Juan Bautista. These communities are small, but Hollister and San Juan Bautista have the distinction of sitting in a zone where the fault creeps almost continuously, producing frequent small earthquakes.
In northern California, the fault enters the San Francisco Peninsula from the south. It runs offshore near Daly City, passes close to San Francisco, and continues up the coast. The 1906 earthquake ruptured roughly 300 miles of the northern San Andreas, devastating San Francisco and severely damaging cities from San Jose to Santa Rosa.
Why Los Angeles Faces Severe Shaking Despite Being Off the Fault
Los Angeles sits a meaningful distance from the San Andreas Fault itself, which has occasionally led people to underestimate its risk. That would be a mistake. The geology beneath the Los Angeles Basin acts like a natural amplifier. Simulations of a large earthquake rupturing the southern San Andreas from southeast to northwest have consistently shown that seismic waves get channeled through a series of connected sedimentary basins, funneling energy directly into the metro area.
One detailed simulation of a magnitude 7.75 event predicted ground velocities near the fault of about 2.5 meters per second and velocities in the Los Angeles Basin of about 1.4 meters per second, with shaking in parts of the basin lasting more than 60 seconds. At some locations within the basin, the simulated shaking was up to ten times stronger than at sites outside the basin at comparable distances from the fault.3Science. Three-Dimensional Simulation of a Magnitude 7.75 Earthquake on the San Andreas Fault The phenomenon is sometimes called a waveguide effect: the fault zone and surrounding basin structure essentially steer seismic energy into the populated lowlands.
Subsequent research confirmed that earthquakes rupturing northward along the southern San Andreas would be especially dangerous for Los Angeles, producing unusually strong long-period ground motions across much of the metro area, with localized hot spots of intense shaking where the waveguide geometry changes.4Geophysical Research Letters. Strong shaking in Los Angeles expected from southern San Andreas earthquake This is a concern because the long-period waves are the ones most damaging to tall buildings and large structures. They make buildings sway for extended periods, and the deeper and softer the ground, the worse the effect.
There is a partial silver lining. More recent modeling that accounts for how rocks near the fault deform and yield under extreme stress suggests that this nonlinear behavior could absorb some of the seismic energy before it reaches the basin. One study found that accounting for this plasticity could reduce the earlier predictions of long-period shaking in Los Angeles by up to 70 percent compared to models that treat the ground as a simple elastic material.5Geophysical Research Letters. Expected seismic shaking in Los Angeles reduced by San Andreas fault zone plasticity That is a substantial reduction, but even a 70 percent cut from catastrophic levels still leaves dangerous shaking. And the reduction depends on conditions near the fault surface that are hard to measure ahead of time, so emergency planners have been cautious about banking on it.
San Francisco and the Bay Area
If any city in the world is synonymous with earthquake risk from the San Andreas Fault, it is San Francisco. The 1906 earthquake and fire killed an estimated 3,000 people and destroyed much of the city. But the fault’s threat to the Bay Area extends far beyond San Francisco proper. Historical records show that the 1838 earthquake caused damage throughout the Bay Area, from San Francisco south through the Santa Clara Valley and as far as Monterey, with effects unmatched by any event other than the 1906 earthquake. The pattern of aftershocks over the following three years suggests the fault ruptured over a stretch from San Francisco all the way to San Juan Bautista.6Bulletin of the Seismological Society of America. San Andreas Fault Zone, California: M ≥5.5 Earthquake History
San Jose and Silicon Valley sit in the Santa Clara Valley, flanked by mountains on their western side. Research analyzing river channels along those mountain fronts found that thrust faults bounding the valley could be capable of generating a magnitude 6.9 earthquake roughly every 250 to 300 years based on the rate at which strain is building up.7Geophysical Research Letters. Mountain Rivers Reveal the Earthquake Hazard of Geologic Faults in Silicon Valley These are not the San Andreas itself, but they are part of the broader fault system that accommodates the same tectonic motion. A major San Andreas rupture on the peninsula segment would shake many of these same communities.
An analysis of what would happen if the 1906 earthquake were to repeat today projected property losses to buildings alone of roughly $90 to $120 billion, with an estimated 7,000 to 10,000 commercial buildings closed due to serious structural damage and between 160,000 and 250,000 households displaced. Those numbers do not include losses from fires that would follow the earthquake or damage to utility and transportation systems.8Earthquake Spectra. When the Big One Strikes Again—Estimated Losses due to a Repeat of the 1906 San Francisco Earthquake Oakland, Berkeley, Hayward, and the other East Bay cities would also be severely affected, both from direct shaking and from the secondary hazards discussed below.
The Southern San Andreas Gap and Why Seismologists Are Worried
The southern portion of the San Andreas, from roughly San Bernardino southeast to the Salton Sea, has not produced a major earthquake in roughly 300 years. The northern portion last broke in 1906. But modeling of the fault’s earthquake cycle over the past thousand years has shown that present-day stress is high along the entire southern San Andreas precisely because of this long quiet period.9Journal of Geophysical Research. A model of the earthquake cycle along the San Andreas Fault System for the past 1000 years That accumulated stress has to go somewhere eventually.
Recent work focused on Cajon Pass, where the San Andreas and San Jacinto faults come close together northwest of San Bernardino, found that stress accumulation is highest just north of the pass, at about 1.8 megapascals per century. By 2025, estimated Coulomb stress on the Mojave South segment was about 2.8 megapascals, and on the San Jacinto’s Bernardino segment it was about 3.6 megapascals.10U.S. Geological Survey. Cajon Pass and the southern San Andreas Fault System: Earthquake cycle stress accumulation and present-day loading The stress numbers matter because this research found that when the imbalance between the two fault segments narrows, the faults tend to rupture together. That leads to a bigger earthquake and a wider area of damage than if one fault ruptured alone.
When One Fault Triggers Another
The San Andreas Fault is not a standalone feature. It is the largest member of a family of faults that together accommodate the motion of the Pacific and North American tectonic plates. The San Jacinto Fault, which runs through Riverside, San Jacinto, and Hemet, is one of the most active members of this family. The two faults come within a few miles of each other at Cajon Pass, and evidence shows they have ruptured together at least three times in the past 2,000 years, most recently during the 1812 earthquake.11Geology. Joint earthquake ruptures of the San Andreas and San Jacinto faults, California, USA
The interaction goes beyond simultaneous rupture. Analysis of historical seismicity suggests that the great 1857 Fort Tejon earthquake on the San Andreas may have triggered a sequence of earthquakes that migrated progressively along the San Jacinto Fault, consistent with a strain pulse moving southeastward at roughly 1.7 kilometers per year. The two fault systems also appear to share a similar average recurrence interval of about 130 to 150 years for large events, hinting that stress released on one fault can push the other closer to failure.12PubMed. Interaction of the san jacinto and san andreas fault zones, southern california: triggered earthquake migration and coupled recurrence intervals
This means cities that are not on or near the San Andreas itself can still be affected by its earthquakes. Riverside, Hemet, Temecula, and other Inland Empire communities sit along the San Jacinto system and face the risk of triggered events or joint ruptures. The Elsinore Fault, farther to the southwest, adds yet another potential pathway for earthquake energy to reach populated areas.
Fire, Water, and the Cascade of Secondary Hazards
The shaking itself is only the beginning. One of the least appreciated threats from a major San Andreas rupture is fire. Ruptured gas lines, downed power lines, and overturned appliances can start blazes across a wide area simultaneously, overwhelming fire departments that are already dealing with collapsed structures and impassable roads. A detailed analysis of the ShakeOut scenario, which modeled a magnitude 7.8 earthquake on the southern San Andreas, projected roughly 1,600 ignitions across southern California, with hundreds of large fires burning simultaneously in the central Los Angeles basin. The estimated fire losses included hundreds to perhaps a thousand lives and about 200 million square feet of destroyed building space, corresponding to as much as $100 billion in damage on top of the direct earthquake losses.13Earthquake Spectra. Fire following Earthquake Aspects of the Southern San Andreas Fault Mw 7.8 Earthquake Scenario
The fire problem is compounded by water supply. The California Aqueduct, the Los Angeles Aqueduct, and other major water-supply lifelines cross the San Andreas Fault. A large rupture could sever these systems, leaving firefighters without water in exactly the neighborhoods where they need it most. The ShakeOut fire analysis identified the construction of a seismically reliable regional saltwater pumping system as one key mitigation opportunity for central Los Angeles. Automated gas shutoff devices in densely built areas were also recommended.
Beyond fire, landslides are a major secondary threat in the mountainous terrain flanking the fault. Communities in the San Gabriel Mountains foothills, the Santa Cruz Mountains, and the coastal hills near San Francisco all face the risk of earthquake-triggered slope failures. Liquefaction, where loose, water-saturated soil behaves like a liquid during shaking, is a concern for low-lying areas in the Bay Area, parts of the Los Angeles basin, and the Coachella Valley.
Which Buildings Are Most Vulnerable
Not all neighborhoods face the same structural risk, even at equal shaking levels. The ShakeOut scenario analysis of wood-framed buildings found that residential construction from the 1960s through the 1980s is especially vulnerable. Homes built during that boom relied heavily on drywall sheathing and stucco for structural strength instead of plywood, making them weaker than both older homes (which used gypsum lath and plaster) and newer ones built to updated codes. Many apartment buildings from that era were built with “tuck-under” parking on the ground floor, which creates a weak, collapse-prone story. Heavy masonry chimneys were also common and are prone to toppling. The modeling estimated that more than 30,000 mostly older wood-framed buildings could be red-tagged or yellow-tagged in a single major event.14Earthquake Spectra. Earthquake Damage to Wood‐Framed Buildings in the ShakeOut Scenario
This matters because the geographic distribution of building vulnerability does not perfectly overlap with the distribution of shaking intensity. Older neighborhoods in Los Angeles, the San Fernando Valley, and parts of the Inland Empire have dense concentrations of exactly the building types most likely to suffer serious damage. Meanwhile, newer developments farther from city centers may be built to modern codes but sit on softer soils that amplify shaking. Knowing which city you live in is only part of the picture; the age and construction type of your building and the soil it sits on are equally important.
Early Warning and What It Can and Cannot Do
California’s ShakeAlert system, developed by the USGS and university partners, is designed to detect earthquakes as they begin and send warnings before the strongest shaking arrives at more distant locations. Along the West Coast, warning times could reach tens of seconds for moderate shaking levels, and potentially longer if a lower shaking threshold is used to trigger alerts. But there is a fundamental constraint: locations close to the earthquake source, the ones most likely to experience the very strongest shaking, may get little or no advance warning at all.15U.S. Geological Survey Open-File Report. Research to improve ShakeAlert earthquake early warning products and their utility
For cities like Palm Springs and San Bernardino, which sit right on or next to the fault, this means the earthquake early warning system is more useful for automated responses (shutting down gas valves, slowing trains, opening fire station doors) than for giving individuals time to take shelter. For Los Angeles and San Francisco, which are farther from the fault, the warning window could be long enough for people to drop, cover, and hold on before the strongest waves arrive. Even a few seconds of warning can reduce injuries from falling objects and allow industrial systems to initiate safe shutdowns.
Communities People Forget About
Media coverage of San Andreas risk tends to focus on Los Angeles and San Francisco, but several mid-size and smaller communities face risks that are arguably more acute per capita. Palmdale and Lancaster, with a combined population of around 300,000, sit directly on or adjacent to the Mojave segment of the fault, one of the segments with the highest accumulated stress. San Bernardino, a city of over 200,000 in an economically disadvantaged region, is located at the convergence of the San Andreas and San Jacinto faults near Cajon Pass, which functions as an “earthquake gate” where ruptures can transfer between the two fault systems.10U.S. Geological Survey. Cajon Pass and the southern San Andreas Fault System: Earthquake cycle stress accumulation and present-day loading
The Coachella Valley, home to about 400,000 permanent residents plus a large seasonal population, has a housing stock that includes many manufactured homes and older construction not designed for strong shaking. Infrastructure corridors running through San Gorgonio Pass carry power lines, highways, rail lines, and aqueducts. A rupture in this area would not only damage local communities but could cut lifelines serving tens of millions of people in coastal southern California.
In northern California, communities along the Santa Cruz Mountains segment of the fault, including towns in San Mateo and Santa Cruz counties, face compounded risks from shaking and landslides. The 1989 Loma Prieta earthquake, though centered on a nearby fault rather than the San Andreas itself, illustrated how damaging a magnitude 6.9 event in this region can be. A full San Andreas rupture on the peninsula segment would involve far more energy.
How the Fault’s Behavior Varies From South to North
The San Andreas is not one uniform structure, and that matters for understanding which cities face which kinds of risk. In the far south, the fault is locked and accumulating strain. The Coachella segment has not ruptured in roughly 300 years, while geologic studies suggest it typically ruptures every 150 to 200 years. This is the segment seismologists often point to as “overdue,” though earthquake forecasting is not precise enough to say when a rupture will come.
Through San Gorgonio Pass, the fault geometry becomes tortured, with slip distributed across multiple strands at varying rates.2Geosphere. New geodetic constraints on southern San Andreas fault-slip rates, San Gorgonio Pass, California Some researchers have questioned whether a rupture that starts in the Coachella Valley can propagate through this complex zone and continue northwest toward Los Angeles, or whether the pass acts as a barrier that stops ruptures. The answer has huge implications for the size of the earthquake that could hit the Los Angeles region. If a rupture gets through San Gorgonio Pass, you are looking at a potential magnitude 8 or greater event that affects the Coachella Valley, the Inland Empire, and Los Angeles simultaneously.
In central California, the fault creeps steadily near Parkfield and Hollister, which releases strain gradually and produces frequent small earthquakes rather than occasional large ones. This creeping behavior is actually good news for the communities in between, as it reduces the chance of a sudden catastrophic rupture. But the creeping section is bounded by locked sections to the north and south, and large ruptures on those locked sections can propagate into the creeping zone.
The northern section, from San Juan Bautista through the San Francisco Peninsula and up to Cape Mendocino, is locked and last ruptured in 1906. San Francisco, Oakland, San Jose, and the entire Bay Area sit on or near this segment. The region’s dense population, aging infrastructure, and soft bay-margin soils make it one of the most seismically vulnerable metropolitan areas in the country.