The question of San Francisco’s stability, often phrased as whether the city is going to sink, involves a complex interaction between geology, climate change, and human engineering. The city is not uniformly sinking, but it faces multiple, distinct environmental and seismic threats. These forces require continuous, multi-billion dollar mitigation efforts. Understanding the full picture requires separating the slow, vertical movement of the land from the horizontal threat of tectonic plates and the rising pressure of the ocean.
Land Subsidence and Liquefaction Risk
The most direct answer to the concern of the city “sinking” is land subsidence, the gradual compaction and settling of the ground beneath the city’s weight. Much of San Francisco’s eastern waterfront, including the Embarcadero, Mission Bay, and the Marina District, is built on artificial ground. These areas were created over the last 170 years by filling shallow parts of the bay with debris and poorly compacted material. This bay fill sits atop soft bay mud and is slowly compressing under the load of buildings and infrastructure.
A significant portion of the city, including the San Francisco International Airport (SFO) and Treasure Island, is subsiding by as much as 10 millimeters (0.39 inches) per year. This gradual settlement is compounded by the threat of liquefaction during an earthquake. Liquefaction causes water-saturated, loose granular soil to temporarily lose its strength and behave like a liquid.
The Marina District, damaged during the 1989 Loma Prieta earthquake, serves as a historic example of this risk. The artificial ground in these low-lying areas is highly susceptible to this rapid loss of stability, causing buildings to tilt or sink unevenly. The risk is acute where the water table is shallow, compromising the ground’s ability to support itself during seismic shaking.
Sea Level Rise and Coastal Inundation
The perception of San Francisco sinking is also driven by the rising level of the surrounding ocean, a climate hazard distinct from land subsidence. Global heating causes sea levels to rise around San Francisco Bay. Relative sea level rise, the combination of the ocean surface climbing and the land surface falling, creates an accelerated threat to the city’s low-lying coastal areas.
Since 1950, the Bay Area has experienced approximately six inches of sea level rise. Projections indicate the sea level could rise by three to six feet or more by the end of the century, fundamentally reshaping the shoreline. This inundation poses a severe risk to the Sea Level Rise Vulnerability Zone, an area encompassing about four square miles of the city.
This zone is home to more than 37,000 residents, 170,000 jobs, and vital regional infrastructure. Critical assets such as the Port of San Francisco, major roadways, utilities, and wastewater treatment facilities are concentrated along the waterfront. When land subsidence is factored into rising sea levels, the threatened area could be twice as large as estimates based on sea level rise alone.
Seismic Activity and Fault Lines
San Francisco sits astride one of the most active tectonic boundaries in the world, making seismic activity a constant and primary concern. The region is defined by the San Andreas Fault and the Hayward Fault, both right-lateral strike-slip faults. Movement along these fault lines is horizontal, or lateral, as the Pacific Plate grinds northwest past the North American Plate.
The geological risk is not a sudden, vertical drop into a chasm, but intense ground shaking and lateral displacement caused by a major earthquake. The U.S. Geological Survey estimates a high probability of a significant earthquake occurring in the Bay Area in the coming decades. This shaking poses an extreme threat to structures, particularly those built on loose, water-saturated bay fill.
San Francisco has responded by implementing stringent seismic building codes, particularly for new construction. New tall buildings (over 240 feet) must adhere to specialized performance-based design standards. Advanced engineering techniques, such as base isolation, are used to separate a structure’s foundation from the ground to reduce seismic impact. The city also mandated the Mandatory Soft Story Retrofit Program (MSSP) to upgrade thousands of older, vulnerable wood-framed buildings.
Engineering Resilience and Infrastructure Adaptation
San Francisco has embarked on a series of ambitious, long-term engineering and adaptation projects to counter these threats. The most significant is the Waterfront Resilience Program, a multi-billion dollar, 30-year effort to protect the 7.5 miles of shoreline under the Port of San Francisco’s jurisdiction. A primary component of this program is the fortification of the 100-year-old Embarcadero Seawall.
The Seawall is currently unprepared for a major seismic event or modern flood levels. Voters approved a $425 million bond to fund the initial phase of the Seawall Program, addressing immediate life-safety concerns and near-term flood risk. The full cost of comprehensive shoreline adaptation, including deployable flood barriers and raising marine structures, is estimated to be up to $13.5 billion.
The city also coordinates with the U.S. Army Corps of Engineers on studies to better predict and manage coastal flood risk. These efforts ensure that the city’s transportation, utilities, and economic engine can withstand the pressures of a changing climate and inevitable seismic activity.