Sacramento is not located directly on a major active fault line. The city is still subject to seismic activity stemming from distant earthquakes across California. Sacramento sits within the expansive Central Valley, a geological setting that influences how seismic events are experienced locally.
Sacramento’s Geological Foundation
Sacramento lies within the Great Valley of California, a vast and flat geomorphic province. This valley is a deep sedimentary basin, filled over millions of years with thousands of feet of unconsolidated sediments. These deposits can reach depths of up to 9.6 kilometers (6 miles) in the Sacramento Valley. The valley floor originated as an offshore area that gradually became a basin due to the subduction of tectonic plates, later filling with eroded materials from surrounding mountain ranges.
While these deep sediments do not typically produce earthquakes, they can significantly influence how seismic waves behave. Unlike solid bedrock, unconsolidated and water-saturated materials tend to amplify ground shaking. This amplification can lead to more pronounced shaking in Sacramento even from earthquakes originating far away. The presence of a high water table further contributes to this potential for amplified shaking.
Major Regional Fault Systems
Although Sacramento does not have major active faults directly beneath it, the city is influenced by several significant fault systems located across California. The San Andreas Fault system, a primary source of seismic activity in the state, is approximately 80 miles southwest of Sacramento. Other Bay Area faults, such as the Hayward Fault and Calaveras Fault, also pose a seismic risk. A substantial earthquake on the Hayward Fault, for instance, could generate noticeable shaking in Sacramento.
The Great Valley Fault System, which marks the tectonic boundary between the Coast Ranges and the Central Valley, also contributes to the regional seismic picture. This system has been the source of notable historical earthquakes, including the 1892 Vacaville-Winters earthquake, which had an estimated magnitude of 6 or greater. Other, less prominent faults nearer to Sacramento include the Cleveland Hill Fault to the north near Oroville, the Sierra Nevada Fault to the east, and the San Joaquin Fault to the south. The Mormon Island Fault Zone, along Sacramento County’s eastern border near Folsom, and the Willows Fault, which has caused detectable shaking.
Understanding Seismic Risk in Sacramento
The primary seismic risk for Sacramento stems from ground shaking caused by distant earthquakes, rather than direct fault rupture beneath the city. One significant concern related to Sacramento’s geology is liquefaction. This phenomenon occurs when loose, water-saturated sandy soils lose their strength during strong ground shaking and behave like a fluid.
Areas within Sacramento, particularly downtown and the Delta region, are susceptible to liquefaction due to their young sedimentary river deposits and high water table. The California Geological Survey has released preliminary seismic hazard maps that identify specific liquefaction zones within Sacramento County. Building codes in California, including the California Building Code, require the evaluation of potential ground failures like liquefaction for new developments in identified hazard zones. Even moderate distant earthquakes, such as a 5.5 magnitude event 160 miles away, can be felt in Sacramento, and the city has experienced effects from significant quakes like the 1989 Loma Prieta earthquake. Older structures, particularly those constructed with unreinforced masonry, may be more vulnerable to damage from strong ground motion.