By Allison Kubo Hutchison

As you walk your city’s sidewalk, the buildings rising up around you, a city’s impact on the landscape is clear. It changes the skyline and the view. But how does it change the soil underneath? Does the weight of a city bend the crust underneath? In thirty years, an estimated 70% of the world’s population will be concentrated in high-density metropolitan areas, most of which are coastal regions. It’s obvious that our human activities affect the air we breathe, but how do the concrete structures affect the land below?

Recent research published in AGU Advances estimated the weight of the San Francisco Bay area, home to approximately 7.75 million people, to be 1.6 x 1012 kg. Author Tom Parsons of the US Geologic Survey calculated the weight from the satellite-based building footprints throughout the Bay Area and then applied averages for the dead weight, the weight of the building itself and the payload, and the weight of moving people, cars and water, Food, etc. He found the most massive building the sprawling San Francisco Airport with a mass of 3.35 x 109 kg, which is 0.20% of the total mass of the area.

Using computer modeling, this concentrated weight was found to cause a drop of between 0.2 and 3.2 mm. and increase the stresses in the earth’s crust by 0.015 MPa, which is enough to cause earthquakes if that pressure is immediately applied to a small area. However, there is no evidence that cities have increased earthquake rates. The greatest danger of this seemingly small subsidence is flooding. In fact, many of the largest cities are on the coasts where the combination of rising seas and subsidence could potentially lead to further flooding in the future.

Cities like San Francisco, Monterey Bay, Los Angeles, and San Diego are slowly declining relative to their surroundings, according to a 2020 paper in Science Advances. Using synthetic aperture interferometric radar analysis (InSAR), the researchers found that California’s communities of more than 8 million people are at increased risk of flooding due to subsidence. This decline could accelerate in the 21st century due to the depletion of aquifers and groundwater and increasing migration to the coasts.



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