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Using satellite technology, scientists have now determined that 25 of 28 of the most-populated cities in the United States are sinking at an alarming rate. Fort Worth, Houston and Dallas are sinking at the highest rates of all the large cities, followed by New York, Chicago, Columbus, Seattle and Denver.
“By comparing multiple images taken over time from the same area, we can detect tiny vertical movements of the ground, down to a few millimetres per year,” said Manoochehr Shirzaei, Associate Professor of Geophysics and Remote Sensing at Virginia Tech. “It’s like taking a high-resolution time-lapse of Earth’s surface and watching how it rises or sinks over time.”
In the study, published in the journal Nature Cities, the researchers state, “While often considered solely a coastal hazard due to relative sea-level rise, subsidence also threatens inland urban areas, causing increased flood risks, structural damage and transportation disruptions. However, spatially dense subsidence rates that capture granular variations at high spatial density are often lacking, hindering assessment of associated infrastructure risks.”
Houston is the fastest sinking city out of the 28 most populated U.S. cities, according to the researchers. “It has 42 percent of its land area subsiding faster than 5 mm per year, and 12 percent subsiding faster than 10 mm per year,” they wrote.
This is a concern because “even modest rates of urban subsidence can profoundly impact the structural integrity of buildings, roads, bridges and dams.” The scientists
“We estimate that at least 20% of the urban area is sinking in all cities, mainly due to groundwater extraction, affecting ~34 million people,” the scientists concluded. “Additionally, more than 29,000 buildings are located in high and very high damage risk areas, indicating a greater likelihood of infrastructure damage.”
The study points out that most of the sinking is caused by groundwater extraction. However, in some cases, like in New York, Philadelphia and Washington D.C., the cause is something called “glacial isostatic adjustment.” In others, such as Seattle, Portland and San Francisco, plate tectonics have played a large role in the subsidence.
“We need to start treating subsidence like the slow-moving disaster it is,” Shirzaei told New Scientist.