San Diego, California, is situated within the Southern California seismic zone, a region characterized by active fault lines due to the boundary between the Pacific and North American tectonic plates. This location means the area is highly susceptible to seismic activity. The city’s geography places it in a region where strain constantly builds up along numerous faults, setting the stage for potential tremors that affect both residents and visitors.
Historical Context and Frequency
San Diego County experiences a substantial number of small earthquakes each year, though the vast majority are too minor to be felt. The area has a high level of seismic activity, with approximately 17 earthquakes of magnitude 4.0 or greater occurring within 186 miles of San Diego annually. This means a relatively strong tremor occurs roughly once a month in the wider region.
While the city itself is often quieter than the Los Angeles or Bay Area regions, it has a history of experiencing significant shaking. A notable historical event was the 1968 Borrego Mountain earthquake, which reached a magnitude of 6.5 and was widely felt in San Diego. Furthermore, a magnitude 6.2 quake may have struck the area in 1862, demonstrating the local faults are capable of generating damaging events.
The Primary Seismic Threats
The most immediate and dangerous threat to the metropolitan area comes from the Rose Canyon Fault Zone (RCFZ), which runs directly through the heart of the city. This 35-mile-long right-lateral strike-slip fault comes ashore near downtown San Diego, passing beneath Old Town, Mission Bay, Pacific Beach, and La Jolla before extending offshore. The fault’s urban path means that a rupture would occur directly beneath densely populated areas, leading to intense shaking.
The Rose Canyon Fault Zone is capable of generating earthquakes up to a magnitude of 6.9, which would be highly destructive. San Diego is also exposed to seismic energy from major regional faults located further east. The San Jacinto and Elsinore faults, which slice through the eastern parts of San Diego County, are capable of generating large earthquakes up to magnitude 7.3. A major event on either could still cause significant shaking, especially in communities in North and East County.
Assessing the Local Hazard
The impact of an earthquake in San Diego depends not only on the fault’s magnitude but also on local geological conditions, which are mapped to assess the seismic hazard. Areas built on soft, water-saturated sediments, particularly along the coast and in Mission Bay, face a specific danger known as liquefaction. Liquefaction occurs when intense shaking causes the soil to temporarily lose its strength and behave like a liquid, leading to severe damage to buildings and infrastructure.
Geological surveys estimate an 18% probability of a magnitude 6.7 or greater earthquake occurring on or near the Rose Canyon Fault Zone within a 30-year period. For the entire Southern California region, there is over a 99% probability of at least one magnitude 6.7 or greater earthquake striking somewhere in the state within the same 30-year timeframe. Scientists use these probabilistic models to forecast the potential for severe ground shaking intensity.
Essential Earthquake Preparedness
In light of the identified risks, preparedness is the most effective way for the public to mitigate potential harm. Every household should prepare an emergency kit and secure the home environment. During an earthquake, the accepted procedure is to “Drop, Cover, and Hold On,” which means immediately dropping to the floor, taking cover under a sturdy piece of furniture, and holding on until the shaking stops.
Securing Your Home and Planning
- Prepare an emergency kit with a three-day supply of water, non-perishable food, flashlights, and a battery-powered radio for communication.
- Secure tall furniture, such as bookshelves and china cabinets, by bolting them to wall studs.
- Secure water heaters by strapping them to the wall to prevent gas leaks and water loss.
- Establish a family communication plan, including an out-of-area contact, to coordinate locations if separated after a disaster.