LIQUEFACTION HAZARD ZONE MAPPING IN THE GREATER SACRAMENTO AREA, SACRAMENTO AND YOLO COUNTIES, NORTHERN CALIFORNIA

The greater Sacramento area, located at the confluence of the Sacramento and American Rivers in northern California, is home to nearly 600,000 residents and is the fastest-growing urban region in the state. With this growth comes increasing exposure to seismic risk associated with the Great Valley fault system and other faults to the west near the boundary with the Coast Ranges.

The region is underlain by Quaternary alluvial sediments deposited by these two major river systems. These include the slightly to moderately consolidated Pleistocene Modesto and Riverbank Formations, along with Holocene deposits such as sandy channel fills, alluvial fans, overbank clays, and soft marsh sediments. Pleistocene units form elevated mesas and terraces, while Holocene alluvium creates a lower, broad alluvial plain with established river channels. Much of Sacramento’s historical core, downtown business/government district, and all of West Sacramento are built on this alluvial plain, which is potentially liquefiable due to shallow groundwater and the nearby seismogenic fault zones. Liquefaction-induced ground failure from a significant earthquake could severely damage infrastructure, disrupt state government operations, and jeopardize the regional levee system critical for flood protection. Identifying and mapping vulnerable sediments is essential to improving the area’s seismic resilience.

The Seismic Hazards Mapping Act (SHMA) directs the California Geological Survey (CGS) to prepare Seismic Hazard Zone maps that identify areas susceptible to liquefaction, landslides, and other seismic hazards, aiding local agencies in land-use and emergency planning. CGS integrates historical groundwater data with surficial geologic mapping and probabilistic ground motion models to delineate potentially liquefiable areas. Currently, CGS is evaluating the Sacramento West and East quadrangles and will soon release preliminary liquefaction hazard maps for public review. The SHMA requires site-specific liquefaction hazard studies within hazard zones and allows local agencies to identify vulnerable infrastructure founded on potentially liquefiable sediments helping this rapidly growing region make well-informed planning decisions.