San Diego is earthquake prone, a reality shaped by its location within one of the world’s most active seismic regions. While the city may not experience the frequent, noticeable shaking common to other Southern California areas, it sits firmly within the broad zone of deformation between two tectonic plates. This geological setting means San Diego is subject to shaking from distant, large earthquakes and, more concerningly, from an active fault that runs directly beneath its most densely populated areas. The city’s seismic risk is governed by regional plate mechanics and localized geological hazards.
Tectonic Setting and Regional Fault Systems
San Diego County is positioned on the western edge of the North American continent, at the boundary between the Pacific Plate and the North American Plate. This boundary is a right-lateral transform system where the plates are grinding horizontally past each other, with the Pacific Plate moving northwest. This movement is distributed across a wide network of faults known as the San Andreas Fault system.
This regional system includes major faults east of the city that pose a significant, though distant, threat to San Diego. The San Jacinto Fault, one of California’s most active faults, cuts through the eastern county near Anza and Borrego Springs. Running parallel is the Elsinore Fault Zone, a large right-lateral strike-slip structure extending approximately 110 miles, capable of generating earthquakes up to magnitude 7.5. A major rupture on either the Elsinore or San Jacinto Faults would cause substantial shaking across the entire San Diego region.
The Rose Canyon Fault: San Diego’s Immediate Threat
The most direct seismic threat to San Diego comes from the Rose Canyon Fault Zone, the southern extension of the Newport-Inglewood system. This active fault cuts directly through the urban core, running onshore near La Jolla Shores, passing through Mission Bay, Old Town, downtown San Diego, and underneath San Diego Bay. Its proximity to dense infrastructure means that even a moderate earthquake could cause severe, localized damage.
Geological studies classify the Rose Canyon Fault as capable of generating a major seismic event, with maximum potential estimates ranging from magnitude 6.5 to 7.4. Although the fault has a slow slip rate, moving only about 1 to 2 millimeters per year, the recurrence interval for a major rupture is estimated to be between 500 and 2,000 years.
The impact of a rupture would be amplified by the fault’s shallow depth and the direct transmission of shaking into the city’s foundations. Experts predict that coastal communities and downtown areas could be cut off from essential services for months. Significant damage is expected to transportation corridors like Interstate 5, bridges, and the San Diego International Airport. The fault’s path through the bay also connects it to the offshore Coronado Bank Fault Zone.
Associated Hazards: Liquefaction and Landslides
San Diego faces significant secondary hazards: liquefaction and seismically induced landslides. Liquefaction occurs when loose, water-saturated soil temporarily loses its strength and acts like a liquid when subjected to strong shaking. This hazard is most pronounced in areas built on artificial fill and low-lying river valleys, which have a high water table and unconsolidated sediments.
High-risk zones for liquefaction include the coastal plains, the Midway District, Mission Bay, and the lower valleys of the San Dieguito and Sweetwater Rivers. The loss of soil stability in these areas can cause structures to tilt, sink, or collapse as their foundations fail.
Seismically induced landslides are a major concern due to San Diego’s steep, hilly topography and weak geological formations. Strong ground motion can destabilize ancient landslide deposits and steep coastal bluffs, leading to slope failure. Areas like Mount Soledad, Point Loma, and numerous canyons are susceptible to such failures, which can destroy property and block transportation routes. The risk is heightened where development has occurred on or near these unstable slopes.
Seismic Monitoring and Building Standards
The management of San Diego’s seismic risk is a collaborative effort between scientific bodies and regulatory agencies. Organizations like the United States Geological Survey (USGS) and the California Geological Survey (CGS) continuously monitor regional seismic activity and conduct detailed fault studies to refine hazard assessments. These findings inform local policy and engineering practices, including the city’s official Seismic Safety Study, which maps geologic hazards to guide land use decisions.
Regulatory response is governed by the California Building Code (CBC), which is updated every three years to incorporate the latest research. Modern building standards ensure new construction can withstand the anticipated forces of major earthquakes. For certain high-rise structures, the city mandates the installation of earthquake recording instrumentation, often monitored by the California Strong Motion Instrumentation Program (CSMIP). This equipment measures ground motion and structural response, providing data to improve future building resilience. Structures built before the adoption of more stringent codes in the mid-20th century, particularly before 1951, are the focus of ongoing regulatory efforts to improve their seismic performance.