Los Angeles County’s unique geography makes it vulnerable to a wide array of natural hazards. The region is situated near the boundary of the Pacific and North American tectonic plates, creating a seismically active environment. The area also features a mountainous landscape, including the Santa Monica and San Gabriel ranges, which abut the dense urban basin and the Pacific Ocean. This juxtaposition of mountains, sea, and city on a major fault system establishes a high-risk setting for earthquakes, wildfires, and water-related disasters.
Seismic Activity and Earthquake Risk
The threat of a major earthquake is ever-present in Los Angeles, driven by an extensive network of fault systems. While the San Andreas Fault is the most recognized, several other faults lie directly beneath the metropolitan area. The Puente Hills thrust fault system, for example, is considered dangerous because it runs directly under densely populated areas, including downtown Los Angeles. A large earthquake on this system could cause catastrophic damage within the city center.
The intensity of ground shaking is not uniform across the region. The Los Angeles basin is largely composed of soft sediments washed down from the surrounding mountains, which can significantly amplify seismic waves. This basin structure traps energy, leading to larger ground motions and more prolonged shaking than in areas built on bedrock.
Liquefaction is another significant hazard, occurring where the soil is composed of loose, saturated sand or silt deposits. Intense ground shaking causes this soil to temporarily behave like a liquid, leading to a loss of bearing strength and causing buildings to sink or tilt. Large swaths of the LA Basin, particularly in low-lying areas, are mapped as high-risk liquefaction zones. This phenomenon also causes lateral spreading, which ruptures underground utility lines and damages surface infrastructure.
Wildfires and Associated Landslides
Los Angeles faces a yearly threat from wildfires, especially in the urban-wildland interface where development meets forested mountains. These fires are often driven by the Santa Ana winds, a meteorological phenomenon that typically occurs from fall into winter. These powerful, warm, and dry downslope winds blow from the interior Great Basin toward the coast, rapidly drying out vegetation and pushing flames across the landscape.
The Santa Ana winds can reach speeds up to 100 miles per hour in mountainous areas, turning small sparks into massive, fast-moving conflagrations. This combination of dry fuel, rugged topography, and high winds creates an environment where fires are extremely difficult to control until the winds subside. The proximity of these fire-prone areas to residential communities means thousands of homes are situated in high-risk zones.
A devastating secondary hazard follows intense wildfires: the increased risk of landslides and mudslides. The heat from a severe fire can make the soil water-repellent, a condition known as hydrophobicity. Without vegetation to anchor the soil, heavy rainfall runs rapidly over the charred surface, picking up sediment and debris. These debris flows can carry cars and boulders, posing a severe threat to structures and life at the base of burned hillsides. The risk remains high in the burn scars of recent fires for several years.
Coastal Hazards: Tsunamis and Storm Surge
The Los Angeles coastline is susceptible to hazards originating from the Pacific Ocean, primarily tsunamis and storm surge. Tsunamis can be generated by distant seismic events, such as an earthquake off Alaska, or by local sources like offshore faults or underwater landslides. A distant tsunami allows several hours for warning and evacuation, while a local event could strike the coast in mere minutes.
Tsunami inundation zones include low-lying coastal communities like Venice, Santa Monica, Marina del Rey, and the ports of Los Angeles and Long Beach. Even smaller tsunamis have historically caused significant damage to California’s harbors. Coastal residents are advised to evacuate immediately to higher ground if they feel strong, prolonged earthquake shaking, due to the potential for locally generated waves.
Storm surge and sea level rise pose a growing threat to coastal infrastructure. Storm surge is a temporary rise in sea level caused by severe weather, amplified when it coincides with high tide. This causes coastal flooding and threatens low-lying infrastructure, including the Port of Los Angeles and coastal power plants. Expected sea level rise will exacerbate the impacts of these storm surge events, making coastal flooding more frequent and severe.
Extreme Weather Events: Flooding and Heatwaves
Los Angeles is vulnerable to two distinct atmospheric hazards: flash flooding and extreme heat. Heavy rainfall events can quickly overwhelm the urban environment, leading to localized flash flooding. The Los Angeles River, which runs 48 miles largely encased in concrete, is a central component of the flood control system.
While channelization was implemented to prevent catastrophic historical floods, some sections of the system still cannot handle a major storm event. Urban street flooding is common because the widespread use of impermeable surfaces prevents rainwater from soaking into the ground. This rapid runoff overloads storm drains and can lead to dangerous conditions on city streets.
The increasing frequency and intensity of heatwaves represent a serious public health threat. Los Angeles experiences the urban heat island effect, where densely built areas are significantly warmer than surrounding vegetated areas. Building materials absorb and retain heat, causing nighttime temperatures to remain elevated and limiting the body’s ability to recover.
This extreme heat disproportionately affects vulnerable populations and can lead to increased cases of heatstroke, respiratory, and cardiovascular illnesses. The heat island effect places additional strain on the power grid as residents rely heavily on air conditioning, potentially leading to power outages. The risk is particularly high in low-income neighborhoods with less green space.