San Francisco is highly seismically active, meaning earthquakes are a certainty for the region. The city is situated directly on one of the world’s most active tectonic boundaries, where the Pacific Plate and the North American Plate meet. They move horizontally past one another in a colossal, grinding motion. This constant geological stress must be released periodically, resulting in the earthquakes that define the region’s risk profile.
The Major Fault Systems Impacting the Bay Area
The seismic activity in the San Francisco Bay Area is distributed across a network of subparallel fault lines that accommodate the relative motion between the two major plates. The two most significant fault systems posing a threat are the San Andreas Fault and the Hayward Fault. Both are right-lateral strike-slip faults. The San Andreas Fault runs along the San Francisco Peninsula, mostly offshore, before coming ashore farther north.
The Hayward Fault is located on the eastern side of the Bay and runs directly beneath densely populated East Bay communities. This fault is particularly dangerous because it extends for over 70 miles through cities like Oakland and Berkeley, maximizing the risk to urban infrastructure. Both fault systems are part of a larger network that includes the Calaveras and Rodgers Creek faults, all contributing to the strain accumulating beneath the region.
Defining Historical Seismic Events
San Francisco’s seismic risk is shaped by two catastrophic historical events. The first was the 1906 San Francisco earthquake, which struck with an estimated magnitude of 7.9. The rupture occurred along a significant segment of the northern San Andreas Fault, causing intense shaking.
The immediate destruction was compounded by widespread fires that burned for days, ultimately destroying over 80% of the city and leading to a death toll exceeding 3,000 people. The second defining event was the 1989 Loma Prieta earthquake, which registered a magnitude of 6.9 on a segment of the San Andreas Fault system near the Santa Cruz Mountains. Though its epicenter was distant, the quake caused significant damage in San Francisco, including the collapse of a section of the Bay Bridge and the failure of the Cypress Street Viaduct in Oakland. This event revealed the vulnerability of areas built on loose, water-saturated ground, such as the Marina District.
Current Probability and Hazard Assessment
Current scientific models, such as the Uniform California Earthquake Rupture Forecast (UCERF3), provide a statistical basis for assessing the future seismic threat. The latest forecast indicates a 72% probability that the San Francisco Bay Area will experience an earthquake of magnitude 6.7 or greater within the next 30 years. The likelihood of a magnitude 7.0 or larger event is calculated at 51% within that same timeframe. These probabilities reflect the continuous buildup of strain across the entire fault network, particularly on the Hayward-Rodgers Creek and Calaveras faults, which have been relatively quiet since the massive stress release of 1906.
Beyond the intensity of ground shaking, San Francisco faces specific geological hazards that amplify the risk. Liquefaction is a major concern, particularly in areas along the waterfront, including the Marina District and the Embarcadero, which were built on artificial landfill. During intense shaking, this loose, water-saturated soil can temporarily lose its strength and behave like a liquid, causing structures to sink or collapse. This ground failure means that even a moderate earthquake could cause disproportionate damage in these vulnerable landfill zones.
Immediate Safety and Structural Considerations
For residents, the most immediate and effective personal safety action during an earthquake is the protocol known as “Drop, Cover, and Hold On.” When shaking begins, individuals should immediately drop to their hands and knees, take cover under a sturdy piece of furniture, and hold on until the shaking stops. This action significantly reduces the risk of injury from falling objects.
To address structural vulnerabilities, San Francisco has implemented a mandatory soft-story retrofitting program. This ordinance targets older wood-frame apartment buildings that feature a “soft story,” typically an open ground floor used for parking that lacks the shear walls needed to withstand lateral seismic forces. By requiring owners to strengthen these weak first stories, the city aims to prevent the catastrophic pancake-style collapse seen in similar buildings during past earthquakes.