Major Cities Near the San Andreas Fault: Risks and Preparedness

The San Andreas Fault is one of the most studied and feared geological features on Earth. Stretching roughly 800 miles through California, it marks the boundary between the Pacific and North American tectonic plates. The fault's potential to generate powerful earthquakes poses a direct threat to millions of people living in its shadow. Understanding which major cities are most at risk, the specific hazards they face, and the comprehensive preparedness measures in place is critical for residents, policymakers, and emergency managers.

Major Cities Along the Fault Zone

Several of California's largest and most economically vital cities lie within striking distance of the San Andreas Fault. Their proximity to active segments means that even a moderate event could cause significant disruption, while a major rupture could be catastrophic.

Los Angeles

Los Angeles, the second-most populous city in the United States, is situated near several active fault strands, including the San Andreas itself, which runs through the San Gabriel Mountains north of the city. The southernmost locked segment of the San Andreas, known as the "Mojave segment," is considered capable of producing a magnitude 7.8 earthquake. Such an event would subject the Los Angeles Basin to severe shaking, liquefaction, and potential tsunami hazards along the coast. The city's dense infrastructure, including freeways, water systems, and high-rise buildings, makes it particularly vulnerable.

San Francisco

San Francisco sits near the northern reach of the San Andreas Fault. The 1906 earthquake (estimated magnitude 7.8) and the 1989 Loma Prieta earthquake (magnitude 6.9) remain stark reminders of the region's seismic risk. The San Andreas Fault runs just west of the city through the Pacific Ocean and the San Francisco Peninsula. Another major earthquake on this section could cause widespread ground failure in areas built on landfill, extensive damage to older buildings, and prolonged disruption to transportation and utilities. The city's dense urban fabric and high property values compound the challenge.

San Jose

San Jose, the heart of Silicon Valley, is located near multiple fault systems, including the San Andreas to the west and the Calaveras and Hayward faults to the east. While the San Andreas is further away than in other cities, its potential to generate a large earthquake could still produce strong shaking in San Jose. The region's high-tech economy and critical data centers make business continuity planning a top priority. The likelihood of ruptures on the adjacent Hayward Fault (with an estimated 14–30% chance of a magnitude 6.7 or greater event in the next 30 years) also directly threatens San Jose.

Other Urban Areas

Other significant population centers near the fault include Palm Springs, San Bernardino, and the cities of the Salinas Valley. Palm Springs lies directly on the San Andreas Fault's Coachella Valley segment, which is the southernmost locked section and is considered overdue for a large earthquake. San Bernardino, located east of Los Angeles, sits near the Cajon Pass where several fault segments converge. These areas face unique challenges related to older housing stock, desert infrastructure, and limited emergency access routes.

Risks Associated with the Fault

The primary risk from the San Andreas Fault is, of course, ground shaking from earthquakes. However, the cascade of secondary hazards can be equally destructive. The risks vary by location and depend on geology, urban density, and building standards.

Ground Shaking and Surface Rupture

During a large earthquake, the ground can shake violently, causing buildings and bridges to collapse. The intensity of shaking depends on distance from the rupture, local soil conditions, and building construction. Areas with soft sedimentary soil, such as the Los Angeles Basin and San Francisco Bay, experience amplified shaking. Surface rupture along the fault can cut directly through buildings, roads, and pipelines. For example, the 1992 Landers earthquake caused a visible surface rupture that crossed highways and structures.

Liquefaction

Liquefaction occurs when water-saturated soil temporarily loses strength and behaves like a liquid during an earthquake. This leads to ground settlement, lateral spreading, and damage to underground utilities. Many urban areas near the San Andreas Fault—especially those built on fill or former wetlands, such as parts of San Francisco, Oakland, and Los Angeles near the Los Angeles River—are at high risk of liquefaction.

Landslides

Steep slopes in the Coast Ranges and the San Gabriel Mountains are destabilized by strong shaking. Landslides triggered by earthquakes can block roads, destroy homes, and disrupt water supplies. The 1994 Northridge earthquake, while not on the San Andreas, triggered over 11,000 landslides in a relatively small area. A major San Andreas earthquake could trigger far more.

Fires and Tsunamis

Gas line ruptures combined with downed power lines often ignite fires. After the 1906 earthquake, fire caused the majority of destruction in San Francisco. Modern infrastructure is designed to reduce fire risk, but the potential for a conflagration remains high in densely built areas. Additionally, while the San Andreas Fault is primarily a strike-slip fault, models suggest that a rupture along certain offshore segments could displace the seafloor enough to generate a minor tsunami that could affect coastal cities like Santa Monica, Long Beach, and San Francisco.

Infrastructure Vulnerability

Water supply systems are a particular concern. Los Angeles imports much of its water via aqueducts that cross the San Andreas Fault. A major earthquake could sever these lines, leaving the city without water for weeks or months. Similarly, natural gas pipelines, electrical transmission lines, and fiber-optic cables often cross the fault and are at risk of rupture. The economic impact of such damage would be enormous, affecting national and global supply chains.

Historical Earthquakes: Lessons from the Past

The San Andreas Fault has produced some of the most significant earthquakes in American history. Studying these events helps scientists refine risk models and emergency planners improve response protocols.

The 1906 San Francisco Earthquake

On April 18, 1906, a magnitude 7.8 earthquake ruptured along 296 miles of the northern San Andreas Fault. The quake and subsequent fires killed an estimated 3,000 people and destroyed over 80% of San Francisco. This event led to the development of modern earthquake science in the United States and the eventual creation of the San Andreas Fault Observatory at Depth (SAFOD). The devastation prompted stricter building codes in San Francisco, but it also highlighted the deadly combination of shaking and fire.

The 1989 Loma Prieta Earthquake

This magnitude 6.9 quake struck during the World Series, killing 63 people and causing $6 billion in damage. It severely damaged the Bay Bridge and the Cypress Street Viaduct in Oakland, exposing the weaknesses in the region's transportation infrastructure. The Loma Prieta earthquake accelerated retrofitting of bridges and freeways and spurred the creation of early warning systems. It also reinforced the importance of community preparedness and public education.

The 1857 Fort Tejon Earthquake

One of the largest historical earthquakes on the southern San Andreas Fault. Estimated at magnitude 7.9, it ruptured from the Carrizo Plain to the San Gabriel Mountains. Although the area was sparsely populated, the event serves as a baseline for understanding the potential impact of a modern-day repeat on the southern section. Studies of the 1857 rupture inform current hazard assessments for Los Angeles.

Preparedness Measures: Building Resilience

Effective preparedness requires a multi-layered approach involving government, businesses, and individuals. Cities near the San Andreas Fault have invested heavily in mitigation and response capabilities, but gaps remain.

Building Codes and Retrofits

California has some of the strictest building codes in the world for seismic resilience. The Uniform Building Code and subsequent California Building Standards Code mandate that new buildings be designed to resist specific ground motion levels. However, many older structures predate these codes. Unreinforced masonry buildings, soft-story apartment buildings, and non-ductile concrete structures are especially vulnerable. Cities like Los Angeles, San Francisco, and San Jose have enacted mandatory retrofit ordinances for these at-risk building types. For instance, Los Angeles' Mandatory Retrofit Program targets over 13,000 soft-story buildings and nearly 1,000 non-ductile concrete buildings. Similar programs exist in San Francisco through the Community Action Plan for Seismic Safety (CAPSS).

Public Education and Drills

The "Drop, Cover, and Hold On" campaign is widely promoted by the Earthquake Country Alliance and other organizations. California holds the annual Great ShakeOut drill, with participation from schools, businesses, and government agencies. These drills reinforce proper protective actions and help people mentally rehearse what to do during the shaking. Local emergency departments also conduct community workshops, distribute preparedness guides, and host tabletop exercises.

Emergency Response Plans

Every city near the fault has a comprehensive emergency operations plan (EOP) that outlines the roles of police, fire, medical, and public works personnel. These plans include procedures for search and rescue, mass care, evacuation, and damage assessment. Mutual aid agreements between counties and states enable rapid deployment of resources. The California Governor's Office of Emergency Services (Cal OES) coordinates statewide response, and the Federal Emergency Management Agency (FEMA) stands ready to support recovery. Urban search and rescue teams, such as California Task Force 5 in Orange County, are specially trained for collapsed-structure incidents.

Earthquake Early Warning (ShakeAlert)

The ShakeAlert system, operated by the U.S. Geological Survey (USGS), in partnership with universities and state agencies, provides seconds to tens of seconds of warning before strong shaking arrives. This system detects the initial seismic waves (P-waves) using a network of sensors and sends alerts via cell phones, public address systems, and automated systems. Although the warning time is short, it can allow trains to slow down, surgeons to stop delicate procedures, and people to take cover. Los Angeles has integrated ShakeAlert into its automated systems for opening firehouse doors and controlling elevators. Public adoption of the MyShake app has increased, but coverage and alert latency continue to improve.

Infrastructure Hardening

Critical infrastructure—such as hospitals, fire stations, and emergency operations centers—is required to meet higher seismic design standards. The California Seismic Safety Commission oversees the Alfred E. Alquist Hospital Facilities Seismic Safety Act, which mandates that acute care hospitals be capable of continuing operations after an earthquake. Water agencies are retrofitting aqueducts and building redundancy into supply systems. For example, the Los Angeles Department of Water and Power (LADWP) has invested billions in reinforcing the Los Angeles Aqueduct and expanding groundwater storage to provide backup supply. Power utilities are replacing wooden poles with stronger materials and adding automated switches to isolate damaged lines. The California Public Utilities Commission regulates these efforts for investor-owned utilities.

Community Resilience and Individual Preparedness

No amount of infrastructure hardening can replace the need for individual and family preparedness. Residents near the San Andreas Fault should take proactive steps. The following list outlines key actions:

  • Secure heavy furniture and appliances – Anchor bookshelves, water heaters, televisions, and large mirrors to walls. Use flexible connectors for gas appliances.
  • Identify safe spots in each room – Under sturdy tables, desks, or interior doorways away from windows and heavy objects.
  • Have an emergency communication plan – Designate an out-of-state contact, exchange information with family and neighbors, and know where to meet if separated.
  • Maintain an emergency supply kit – Stock at least two weeks of food, water (one gallon per person per day), medications, first aid supplies, flashlights, batteries, a battery-powered radio, cash, and important documents in a waterproof container.
  • Learn first aid and CPR – In the immediate aftermath of a major earthquake, emergency services may be overwhelmed. Basic medical skills save lives.
  • Know how to shut off gas and water – A wrench near the gas meter and knowledge of the main water valve can prevent fires and water damage. Only shut off gas if you smell a leak.
  • Participate in community emergency response training (CERT) – Many cities offer CERT programs that teach basic disaster response skills such as fire safety, light search and rescue, and team organization.

Future Fault Activity and Risk Projections

Scientists from the USGS and the Southern California Earthquake Center (SCEC) continuously model the probability of future earthquakes on the San Andreas Fault. The Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) estimates that there is a 72% probability of a magnitude 6.7 or greater earthquake in California within the next 30 years. For the San Andreas Fault specifically, the southern section has a roughly 20% chance of a magnitude 7.5 or larger event in that timeframe. The Hayward Fault, which is not part of the San Andreas system but is adjacent, has an even higher probability due to its recurrence interval.

A major focus of research is the "Big One"—a magnitude 8.0 or greater earthquake on the San Andreas. While such an event is statistically unlikely in the short term, it cannot be ruled out. Paleoseismic studies indicate that large ruptures on the San Andreas occur, on average, every 150 to 200 years on the southern section, and the last one there was in 1857—over 160 years ago. The northern section last ruptured in 1906, but other segments like the Carrizo Plain have not ruptured since 1857. The accumulated strain increases the risk.

The impact of a future San Andreas earthquake will depend on factors such as magnitude, location, time of day, and the success of mitigation efforts. The USGS simulation of a magnitude 7.8 scenario on the southern San Andreas, known as the ShakeOut Scenario, estimates about 1,800 deaths, 50,000 injuries, and $200 billion in damage. The most severe losses would occur in the Los Angeles region, with building collapses, fires, and damage to transportation corridors. Power, water, and telecommunications could be disrupted for weeks or months in some areas.

Conclusion

Living near the San Andreas Fault is a reality for millions of Californians. While the risk of a major earthquake is ever-present, significant progress has been made in understanding the fault, strengthening buildings, and preparing the public. The combination of stringent building codes, early warning systems, rigorous emergency planning, and individual preparedness can dramatically reduce the human and economic toll. Residents and authorities alike must remain vigilant, continue to invest in mitigation, and practice the simple but life-saving steps that make communities more resilient.