The nation’s largest cities, including San Francisco , are sinking, according to a new study that calls the downward spiraling a “widespread” and “slow-moving hazard,” threatening thousands of buildings and millions of people.
The 28 cities studied, each with populations of 600,000 or more, averaged just a few millimeters of vertical movement annually. Still, this small amount of sinking adds up over time. In parts of some cities, the movement is as much as two inches a year.
Known technically as land subsidence, the phenomenon can cause roads, bridges, utilities, dams and building foundations to buckle and increase the likelihood of flooding. Often it is the result of groundwater pumping…