Hidden below San Francisco’s dense patchwork of homes, stores, and skyscrapers lies millions of years of geological history. Various physical processes over the past two million years have created diverse strata of Bay Mud, consisting of soft, granulated, clay, that lurks underneath much of the region’s shoreline areas. During the last Ice Age, more than 10,000 years ago, sea levels were significantly lower than present-day, with land that far extended into what’s now the Bay.
During this era, sand dunes sculpted by blowing winds on the Peninsula became the visible, defining, feature for much of the topography until humans began erecting permanent structures on top of them beginning in the mid-1800’s. The sand deposits, along with thousands-year-old mud strata that accumulated during deglaciation, now resides underneath San Francisco’s most heavily built-up areas, such as Downtown and Mission Bay.
According to Gray Brechin, a historical geographer and visiting scholar at the University of California, Berkeley’s Department of Geography, scientists have been aware of visible subsidence, or sinking land, in Mission Bay for well more than a century. Early settlers knew about the soft sandy soils, which prompted them to add artificial landfill to create flat terrain more suitable to be built on. Groundwater mixes with underlying soils, creating what Brechin called “pudding” that looks like dry land. The soft soils are going through a gradual process of settling, which can be intensified by other factors.
“I’ve been interested in visible subsidence and what would happen if there was a quake,” said Brechin. “I did walking tours in the neighborhood and building distress was so visible that’s it’s evident that buildings would be badly damaged. The filled Bayfront and Creek would liquify when a quake hits. We’ve known that for some time, even before the quake of 1906. They knew about liquefaction at that time, and the 1906 earthquake really showed it; buildings collapsed into creeks and marshes.”
According to the U.S. Geological Survey, ground shaking is the primary cause of earthquake damage. Certain soil types amplify shaking. The strongest intensification of shaking results from water-saturated mud and artificial fill, a soil category that encompasses Mission Bay apart from Mission Creek. Neighborhoods with sandy soils are considered at risk for liquefaction during an earthquake, when they lose cohesion and behave more like a liquid than a solid.
This soil characteristic is the reason why similar areas were damaged during the 1906 and 1989 earthquakes. In April, using California and U.S. Geological Survey data, the New York Times published a liquefaction risk map for the City illustrating that significant portions of the Marina, North Beach, Financial District, South of Market, Mission District, Mission Bay, Dogpatch and Bayview-Hunters Point have a high likelihood of being liquefied during a tremor.
Brechin said he vividly remembers every one of the 15 seconds of shaking during the Loma Prieta earthquake in 1989, as if a deep roar was emanating from the earth. Although he lauds the seismic mitigation work that’s been done since then to strengthen freeways, he’s deeply concerned about the amount of new construction on fill areas and anticipates widespread devastation in the same City areas that burned during the 1906 earthquake.
Geotechnical engineer, Larry Karp, conducted an engineering review for the Golden State Warriors Arena in Mission Bay in 2015, and wrote that, “The land of the proposed project site (sand and rubble fill over Bay Mud) will be subjected to strong shaking from earthquakes generated along both the active San Andreas (eight miles to the west) and Hayward (10 miles to the east) faults.”
Karp explained that weight from buildings and infrastructure pushes down on settling soil and adds to subsidence. The compression can exacerbate ground water loss already threatened by evaporation due to drought conditions, further intensifying sinking.
In addition to water scarcity, another complicating factor associated with subsidence is sea level rise. Warming global temperatures have resulted in the melting of polar ice caps and glaciers, which, along with the thermal expansion of water, have caused the oceans to rise, threatening the world’s heavily populated coastlines with flooding and salt water inundation.
“Fresh water floats on top of salt water, and because of sea level rise, salt water inundation will push up fresh groundwater and create springs gushing out of the ground,” said Joel Pomerantz, creator of the Seep City project. “Flooding is inevitable in low areas of the City.”
A University of California, Berkeley geophysicist, Roland Bürgmann, recently published a paper in ScienceAdvances that concluded that land subsidence will exacerbate sea level rise risks in many parts of the Bay Area. He and fellow researchers concluded, “Given ongoing land subsidence, we project that an area of 125 to 429 square kilometers will be vulnerable to inundation, as opposed to 51 to 413 square kilometers considering sea level rise alone.”
Historically, reports of sinking in areas such as Mission Bay came from point measurements taken optically over time. With advancements in satellite technology, land subsidence can now be gauged over vast areas from space, providing a more comprehensive view, albeit with less detail for specific sites. One of the outcomes of Bürgmann’s study is a map showing hotspots of sinking across the region, with data around Mission Bay confirming visual reports of sinkage.
The neighborhood isn’t alone. Other hotspots, such as parts of Treasure Island, San Francisco International Airport, and Foster City, are subsiding as fast as 10 millimeters per year. The subsidence rates for most of the coastal San Francisco Bay is less than two millimeters annually.
Mission Bay residents have noticed that parts of their neighborhood are visibly sinking. Philip de Andrade, a Channel Street resident, said that beginning about four years ago people started noticing that buildings along Fourth Street were separating from the sidewalks, leaving gaps that’ve since been patched with white caulk. Although he said neighborhood conversation about subsidence has diminished, he wonders what things will look like in ten years.
“I see a lot of lawsuits in the future, and eventual abandonment,” Pomerantz commented. “There’s a 100 percent chance that there will be issues with every single property on the fill within the next 50 years.”