The answer to whether Los Angeles is on a fault line is definitively yes. Southern California is situated within one of the world’s most seismically active regions due to the movement of massive tectonic plates. The Los Angeles metropolitan area is not only near a major plate boundary, but it is also crisscrossed by an extensive network of active faults. This complex geological setting means the region faces a constant risk from earthquakes.
The Direct Answer: LA’s Tectonic Setting
Los Angeles experiences intense seismic activity because it is located near the boundary between the Pacific Plate and the North American Plate. This boundary is a transform fault system, where the two plates slide horizontally past one another. The Pacific Plate, which Los Angeles sits upon, is moving northwestward relative to the North American Plate.
This horizontal scraping motion generates enormous stress that constantly builds up in the Earth’s crust. The plates move at a rate of approximately 50 millimeters, or about two inches, per year. This movement does not happen smoothly; instead, the plates often lock up at various points along the boundary. When the accumulated stress finally overcomes the friction holding the rocks together, the sudden release of energy is experienced as an earthquake.
The regional tectonic forces have created a complex network of secondary faults that accommodate the overall plate motion. While the primary plate boundary is the San Andreas Fault, the stress is distributed across many smaller, local faults that lie directly underneath the urban landscape. This distributed strain is why the Los Angeles area experiences frequent earthquakes originating on local faults, not just on the distant main plate boundary.
Major Fault Systems Affecting the LA Region
The seismic risk stems from the distant plate boundary and several localized fault systems.
San Andreas Fault
The most well-known is the San Andreas Fault, which defines the main boundary between the Pacific and North American plates. Although it lies roughly 60 kilometers northeast of downtown Los Angeles, it is capable of producing a massive, regional earthquake, potentially reaching a magnitude of 7.8 or greater. This strike-slip fault is considered the master fault of the region. The last major rupture on the southern section was in 1857, and it is currently storing energy for a future large event.
Newport-Inglewood Fault Zone
Closer to the population center is the Newport-Inglewood Fault Zone, an active fault that runs directly beneath the densely populated Los Angeles Basin. This fault extends from offshore Newport Beach northwestward through Long Beach, Inglewood, and past Culver City. The system was responsible for the devastating 1933 Long Beach earthquake, which highlighted the danger of faults running directly through urban areas. An earthquake on this fault would produce intense shaking across a massive area due to its proximity.
Puente Hills Thrust Fault
The Puente Hills Thrust Fault presents a distinct type of risk. This system is a “blind thrust fault,” meaning it does not break the surface and remains hidden beneath the basin’s sedimentary layers. It lies directly underneath a vast area, including downtown Los Angeles, the San Gabriel Valley, and northern Orange County. This fault is capable of producing a major earthquake, and the 1987 Whittier Narrows earthquake was associated with this system.
Localized Faults
The Hollywood Fault and the Elysian Park Fault are examples of smaller, localized faults that contribute to the overall seismic hazard. The Hollywood Fault runs through Beverly Hills and Hollywood, accommodating north-south contractional forces in the region. Even moderate earthquakes on faults directly beneath a city can cause significant damage due to their proximity to infrastructure and dense population centers. The complexity of the local fault network means that seismic hazard is highly distributed across the metropolitan area.
Seismic Hazards Unique to the Los Angeles Basin
The geological makeup of the Los Angeles Basin significantly increases the destructive potential of any earthquake.
Basin Amplification
The basin is a deep, bowl-shaped depression filled with soft, unconsolidated sediments up to nine kilometers deep in certain areas. This structure causes basin amplification, often described as the “jelly bowl” effect. When seismic waves enter this soft, deep sediment, they slow down but their amplitude increases substantially, resulting in more intense ground motion.
The soft basin materials also trap the seismic energy, causing the shaking to resonate and last for a much longer duration than in areas built on bedrock. In the deepest parts of the basin, shaking can be prolonged by more than 60 seconds, and the amplification factor can increase ground motion intensity by up to six times. This prolonged shaking poses an extreme risk to tall buildings, which are susceptible to long-duration vibrations.
Liquefaction
Another significant hazard is liquefaction, which occurs when saturated, loose soils temporarily lose their strength and behave like a liquid during strong shaking. This happens because vigorous ground motion increases the water pressure between soil particles, causing the ground to fail. Areas most prone to liquefaction are those with young sediment deposits and a shallow water table, such as coastal areas and former riverbeds.
High-risk zones include areas near the Los Angeles and San Gabriel Rivers. Communities in the Southgate and Whittier quadrangles, such as Compton, Huntington Park, and Downey, are identified as having a high risk of liquefaction. This ground failure can cause structures to settle, tilt, or collapse, even if they have survived the primary ground shaking. Building codes now mandate special foundation designs in these areas to mitigate this hazard.