The Weight of Cities

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Those living on or near the coast face natural hazards that will only worsen with climate change: earthquakes and related tsunamis, powerful climate-fueled storms, and relative sea-level rise, to name a few.

A map of the globe in various colors to show how much each country is projected to grow from rural to urban in this century.

Map shows the projected rate of urbanization worldwide, from 1950-2050.

(Credit: Tom Parsons, USGS Pacific Coastal and Marine Science Center. Public domain.)

Adding to these hazards is the weight of cities, as USGS research geophysicist Tom Parsons describes in a recent study titled, “The weight of cities: Urbanization effects on Earth's subsurface.”

Parsons (USGS Pacific Coastal and Marine Science Center) became interested in calculating the weight of cities while visiting in-laws in Belgium. Standing in a city square, he marveled at an ornate cathedral that was constructed entirely with immense blocks of stone. “There are no quarries in that part of Belgium,” said Parsons, “so at some point hundreds of years ago, people essentially took apart a mountain and reassembled it in town.” 

“I wondered, ‘What is this weight doing to the ground below?’ Especially in North America, where urbanization is relatively recent,” said Parsons. “All the stuff in cities is coming from some other place: steel, concrete, cars, all the boxes from Amazon, all of it piled up and concentrated in one specific region.” 

In his study Parsons focused on the San Francisco Bay Area, where much of the city is constructed near the waterline of San Francisco Bay. Parsons was interested in whether the weight of San Francisco was contributing to coastal subsidence, the sinking of land relative to sea level that is caused by a variety of factors, including groundwater withdrawal and tectonic activity. 

Active faults underlie much of the Bay Area. “As an earthquake person, I was curious about whether the urban concentration might influence stress distribution on active faults in the area,” said Parsons, “but the main reason I chose this study area was data availability.” 

Parsons chanced upon an openly available and highly detailed GIS database created by the Microsoft Corporation that provided the area and height of major buildings in the Bay Area.

“Drawing on uniform building codes—and assuming each building was built to code—I calculated the total floor area and mass of the buildings, combining the ‘dead’ weight of the structures themselves with their ‘live’ weights, the total mass of the persons and property each building was designed to hold,” said Parsons. 

Golden Gate Bridge lit at night

Golden Gate Bridge, San Francisco, California. (Credit: Scott Haefner, USGS. Public domain.)

Perhaps unsurprisingly, the buildings and infrastructure of cities weigh a lot. Parsons found that the weight of San Francisco and its 7.75 million residents is comparable to the weight of water impounded by a large dam, which exerts considerable pressure on the ground below, contributing to coastal subsidence and potentially influencing tectonic activity. 

While his study focuses on San Francisco, Parsons notes that this issue is global: People around the world are increasingly choosing to live in cities, and 37 percent of the global population lives within 100 kilometers of coastlines.  

“The weight of these cities and their occupants contributes to coastal subsidence, which in some areas is accelerating due to other factors,” Parsons said. “Add to that rising sea levels due to global warming, and the weight of cities becomes a significant concern to those living in coastal areas.”

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Date published: April 29, 2021
Status: Active

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