Sacramento is not situated directly atop a major active fault line, but the capital city of California is still part of a seismically active state, meaning the risk of earthquakes is present. The city’s location deep within the Central Valley provides a degree of geographic buffering from the most intense shaking. However, Sacramento’s unique geological setting means it remains susceptible to seismic hazards, especially those originating from powerful, distant earthquakes.
The Sacramento Valley’s Seismic Status
The Sacramento area experiences a significantly lower frequency of earthquakes compared to the coastal regions of California. Local seismic activity is typically characterized by small, infrequent events, with most tremors registering below magnitude 3.5. In the past year, Sacramento has experienced around 100 earthquakes of magnitude 1.5 or greater within a 100 km radius, with most being too small to be felt.
Historically, the largest earthquakes felt in the region have been centered at the margins of the valley, such as the 1892 Vacaville-Winters earthquakes, estimated to be around magnitude 6.5. These events demonstrate that the edges of the valley are not entirely free from moderate seismic potential. However, the central Sacramento area is considered one of the safer metropolitan regions in California regarding immediate, local fault activity.
Distant Threats: Major Fault Influence
Sacramento’s primary earthquake risk stems not from local faults, but from major, high-magnitude fault systems hundreds of miles away. The most significant threat comes from the Northern San Andreas Fault and the complex of faults in the Bay Area, including the Hayward and Calaveras Faults. A major event, such as a magnitude 7.0 or greater earthquake on one of these systems, would send powerful seismic waves eastward toward the Central Valley.
The magnitude 6.9 Loma Prieta earthquake in 1989, centered near Santa Cruz, was felt in Sacramento, and the 2014 South Napa earthquake (magnitude 6.0) also caused noticeable shaking in the county. The seismic waves lose energy as they propagate, but the city’s unique geology then comes into play, creating a different kind of hazard.
The Impact of Local Geology and Soil
The city of Sacramento is built upon a deep sedimentary basin, which is the defining factor in its seismic risk profile. This basin is filled with unconsolidated materials like soft, water-saturated sands, silts, and clays deposited by the Sacramento and American Rivers. When seismic waves from a distant earthquake hit this soft basin material, the shaking is amplified, a phenomenon called the basin effect.
Loose, water-saturated soils intensify the seismic waves, causing the ground motion to last longer and be more damaging than it would be on solid bedrock. This amplification means that even a distant earthquake can produce significant ground shaking in Sacramento, increasing the risk to structures. Downtown Sacramento and the Delta region are particularly susceptible due to the high water table and the nature of the young sedimentary deposits.
This geological setting also introduces the hazard of liquefaction, which occurs when saturated sandy soil loses its strength and behaves like a viscous liquid during intense shaking. Liquefaction can cause severe damage to infrastructure, including buildings and the regional levee system, as the ground underneath loses its ability to support weight. The California Geological Survey has recently been mapping liquefaction hazard zones, particularly for new development.
Preparedness and Building Resilience
Given the risk of amplified shaking and liquefaction, preparedness measures in Sacramento focus on mitigating ground motion effects. Securing heavy household items like water heaters, bookcases, and televisions is a primary step to prevent injuries and property damage from prolonged shaking. Local building codes mandate that water heaters must be anchored or strapped to resist horizontal displacement.
The California Building Code sets minimum standards intended to protect life safety, but it allows for damage to structures in a significant event. For older homes built before modern seismic codes, property owners should consider retrofitting measures to strengthen the structure against lateral forces. Resources like the California Earthquake Authority (CEA) offer guidance and insurance options to help residents prepare for the potential losses.