Buildings cause Bay Area to sink under their weight
As seas rise, 3.5 trillion pounds of sprawl push land down, scientist shows
It’s not just San Francisco’s Millennium Tower that’s sinking.
The entire Bay Area is plunging downward under the weight of its own sprawl. And that’s a concern as sea levels rise and cities try to figure out how they’ll stay above water in the coming decades.
Tom Parsons, a geophysicist with the U.S. Geological Survey, documented the problem recently. He calculated the weight of every building in the Bay Area and found the total to be so great, about 3.5 trillion pounds or the equivalent of more than 7 million Boeing 747s, that it’s pushing the Earth’s surface down. His research shows the region has sunk as much as 3.1 inches, on average, as a
result of a century’s worth of development.
“We’ve got all that fill on the bay that’s susceptible to being compressed, and we’re building on it, and we’re seeing some of the impacts of leaning buildings and subsidence,” Parsons told The Chronicle.
His findings were published last month in the journal AGU Advances.
While 645foot Millennium Tower in downtown San Francisco stands alone in how much it’s dropped — about 16 inches over a decade, with one side sagging more than the other — big buildings are known to exert downward pressure. Engineers even plan for it.
However, the aggregate impacts of building weights haven’t been well studied.
By focusing on the Bay Area, Parsons learned that the cumulative stress of buildings is generally less than that of other forces that cause elevation changes, such as such as erosion, drainage and tectonic action. But it’s still significant, he noted, presenting one more drag on coastal areas that are already threatened by rising seas.
“The worry is storm surges and flooding and things like that,” he said.
The findings have implications for coastal regions around the globe. Other areas seeing significant development are also likely to have sinking buildings while similarly having to contend with sea level rise, the paper says.
Making matters worse, the sinking is likely to increase as people migrate to cities and further concentrate the weight of development. About 50% of the world’s population now lives in urban areas, and that is expected to jump to 70% by 2050, the United Nations estimates.
Parsons did his research by tapping databases of Bay Area building sizes and figuring out the weight of the structures and their contents. As might be expected, weights were greatest in the downtown areas of San Francisco, Oakland and San Jose. Of nearly 1 million buildings evaluated, the heaviest was San Francisco International Airport, followed by the Tesla factory in Fremont, UCSF Parnassus medical campus in San Francisco and Levi’s Stadium in Santa Clara.
The weight calculation did not consider roads, bridges and other infrastructure, so the total is assumed to be a significant underestimate.
Based on the calculated weights, Parsons computed the downward pressure exerted by buildings and then modeled how the Earth’s surface was responding.
San Francisco’s 779foot skyscraper at 555 California St., formerly known as the Bank of America Center, was found to be exerting the most pressure relative to the area it covers, followed by Millennium Tower and the 725foot 345 California Center, also in downtown San Francisco.
Across the Bay Area, the amount of subsidence that has resulted from the weight of buildings was typically between about onequarter inch and just over 3 inches, according to the modeling. In some spots, however, subsidence was a lot more. Downtown San Francisco has recorded drops of as much as threequarters of an inch per year.
Gary Griggs, a professor of Earth and planetary sciences at UC Santa Cruz and a leading expert on California’s coastal geology who was not affiliated with the research, said the new calculations help round out the picture of the Earth’s shifting surface.
He added that sources of subsidence are often countered by the Bay Area’s plentiful fault action, which can cause sinking but also uplift, an inadvertent benefit when it comes to sea level rise.
“We’re blessed a little bit here in that respect,” Griggs said. “It doesn’t mean we’re home free. We still have SFO within 2 feet of high tide.”
Sea levels in the Bay Area are widely expected to rise as much as a foot by 2050. They could increase 3 feet by the end of the century.
Historically, the most worrisome human source of subsidence, and a major concern amid rising seas, is groundwater pumping. While pumping hasn’t been a widespread problem in the Bay Area recently, parts of the Santa Clara Valley dropped 13 feet last century as water was pulled from the ground to meet local needs. Unlike the elevation changes caused by buildings, sinking caused by pumping can be restored when rain recharges the aquifer.
David Lewis, executive director of Save the Bay, who has been working with communities to plan for sea level rise, said the more information he has about obstacles like subsidence, whether it’s from buildings or something else, the better.
“Sea level rise is an enormous threat to the Bay Area,” he said. “And the effects are already being seen in king tides and extreme storms.”