San Diego is located in Southern California, a region defined by active geology, meaning the possibility of an earthquake is real and ever-present. While the city does not experience the same frequency of large earthquakes as areas closer to the San Andreas Fault, local systems pose a significant threat. The seismic risk in San Diego is moderate compared to other major Californian metropolitan areas. However, the close proximity of active fault lines to densely populated areas necessitates preparation and understanding the city’s specific local geology.
The Local Fault Systems Driving San Diego’s Seismic Risk
Seismic activity in the San Diego metropolitan area is driven primarily by fault systems that run through and immediately offshore of the city. The most significant local structure is the Rose Canyon Fault Zone, a complex network of segments running directly through the developed coastal corridor. This right-lateral strike-slip fault extends from San Diego Bay, through Old Town, and along the Interstate 5 corridor near La Jolla before trending offshore.
The Rose Canyon Fault is the southern continuation of the larger Newport-Inglewood Fault, forming a system that poses a hazard to coastal Southern California. Movement along this fault system is predominantly horizontal. This local fault is considered active because it has shown evidence of movement within the last 11,000 years, making it the most likely source of a damaging earthquake in the immediate city area.
Further contributing to the regional risk are the offshore Coronado Bank Fault Zone and the San Diego Trough Fault Zone. These faults are part of the Inner Continental Borderlands, accommodating movement between the Pacific and North American tectonic plates. The Coronado Bank fault is linked to the Rose Canyon Fault and the Agua Blanca Fault in Baja California. The San Diego Trough Fault Zone is a group of connected right-lateral strike-slip faults that run parallel to the coast, with portions coming within 19 miles of populated cities.
While the San Andreas Fault is approximately 56 miles to the northeast, the Rose Canyon and Coronado Bank faults are the most concerning due to their direct proximity to the urban center. A major rupture on the Rose Canyon Fault could cause significant damage from ground shaking and surface rupture directly beneath downtown and surrounding neighborhoods. The geological structures in the San Diego Bay area also create the potential for liquefaction and landslides in susceptible soils during a major event.
San Diego’s History of Seismic Activity
San Diego’s historical record shows the region has been affected by both local and distant large-magnitude events. Paleoseismic trenching studies on the Rose Canyon Fault found evidence of multiple large surface-rupturing earthquakes over the past 3,300 years. One notable historical event occurred in 1862, estimated to be a magnitude 6 to 6.5 earthquake likely associated with the Rose Canyon Fault.
More recently, the city felt significant shaking from the 2010 Sierra El Mayor earthquake in Baja California, which caused minor damage. Distant earthquakes like the 1994 Northridge and 1992 Landers events were also felt in San Diego, though they did not cause local disruption. Shaking from faults outside the county, such as the Elsinore and San Jacinto fault zones, can still impact the area.
Seismic risk assessments indicate that the Rose Canyon Fault is capable of producing a magnitude 6.9 earthquake, which could cause substantial economic losses and damage. The probability of a major event (Magnitude 6.5 or greater) on one of the region’s faults is a serious consideration for city planners. Estimates for the average return period for a large rupture on the Rose Canyon Fault are 1,500 to 3,000 years, but the fault is believed to rupture in clusters.
Structural Safety and Infrastructure Resilience
San Diego enforces the stringent standards of the California Building Code (CBC) to mitigate earthquake damage in new construction. Since the discovery of the Rose Canyon Fault’s activity, minimum building codes were raised to the highest seismic zone level, matching those of Los Angeles and San Francisco. Modern buildings are designed not only to prevent collapse but also to ensure they remain functional and safe following a major seismic event.
The city also addresses the risk posed by older structures built before enhanced codes were in place. The California Existing Building Code provides guidelines for the seismic retrofitting of older buildings, and local jurisdictions can implement mandatory programs. San Diego has specific regulations concerning vulnerable structures like unreinforced masonry (URM) buildings, which are susceptible to collapse during strong ground shaking.
Critical infrastructure, including hospitals, bridges, and utilities, is a major focus for resilience planning. Hospitals in California are subject to enhanced requirements, such as a rigorous review process and the need to store several days’ worth of potable water on-site. The city and region have plans dedicated to ensuring the continued operation of transportation hubs and defense installations after a disaster. Research facilities, such as the UC San Diego earthquake shake table, contribute to this resilience by testing new materials and designs.
Essential Steps for Personal Earthquake Preparedness
Personal preparation is a necessary component of earthquake safety, starting with knowing the immediate action to take when shaking begins. The universally recommended response is “Drop, Cover, and Hold On.” This involves dropping to the floor, taking cover under sturdy furniture, and holding on until the shaking stops. This action protects against injury from falling debris, which is the most common cause of harm during an earthquake.
Securing the contents of your home prevents property damage and hazards. Heavy items, such as bookcases, mirrors, and televisions, should be anchored to the wall studs to prevent them from toppling. Water heaters, which can be a source of fire or water damage if they fall, should be strapped securely to the wall frame.
Building a comprehensive emergency supply kit ensures you can be self-sufficient for a minimum of three days following a major event. The kit should include:
- Non-perishable food
- At least one gallon of water per person per day
- A first-aid kit
- A whistle to signal for help
It is also important to establish a local and out-of-state communication plan so family members can reconnect if local services are disrupted.