What does a city weigh?


By Allison Kubo Hutchison

File:San Francisco from the Marin Headlands in March 2019.jpg
Noah Friedlander, San Francisco from the Marin Headlands in March 2019, CC BY-SA 4.0
 As you walk the pavement of your city, the buildings rising around you, the impact of a city on the landscape is clear. It changes the skyline and the view. But how does it change the ground below? Does the weight of a city bend the crust below? Thirty years from now it is estimated that 70% of Earth’s population will concentrate into high-density metropolitan regions most of which are coastal. It is evident that our human activities influence the air we breathe but how do the concrete structures influence the land below?

Recent research published in the AGU Advances estimated the weight of the San Fransisco Bay region home to about 7.75 million people at 1.6×1012 kg. Author Tom Parsons of the US Geologic Survey calculated the weight by satellite-based building footprints across the Bay area then applied average values for the dead load, the weight of the building itself and the live load the weight of the moving people, cars, water, food, etc. He found the most massive building to be the sprawling San Francisco Airport with a mass of 3.35×109 kg making up 0.20% of the total mass of the area.

Using computer models, this concentrated weight was found to cause between 0.2 and 3.2 mm of subsidence. and increase the stresses within the Earth’s crust by 0.015 MPa which is enough to trigger earthquakes if that pressure were applied at once in a small location. However, there is no evidence that cities increased earthquake rates. The biggest hazard from this seemingly small subsidence is flooding. Indeed many largest cities are situated on the coasts where the combination of rising seas and subsidence may see more flooding in the future.

Cities such as San Francisco, Monterey Bay, Los Angeles, and San Diego are all slowly gently sinking relative to their surroundings according to a 2020 paper in Science Advances. Using interferometric synthetic aperture radar (InSAR) analysis, researchers were able to find that the Californian communities containing over 8 million people may be put at increased risk of flooding due to the subsidence of the ground. This sinking could accelerate into the 21st century due to the emptying of aquifers and groundwater and increased migration to the coasts.

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