All 28 of the biggest cities in the United States seem to be slowly sinking, with potential implications for their 34 million residents, according to a new study.
Scientists at Virginia Tech’s Earth Observation and Innovation Lab used satellite-based radar measurements to measure sinking land – called subsidence – in 28 of the US’s most populous cities.
They found that in all of the cities they measured, at least 20 per cent of the urban area was sinking by at least 2mm per year. In 25 of the 28 cities, nearly two-thirds of the land was sinking.
“The immediate risks of land subsidence include increased exposure to localised flooding, structural damage to buildings, roads, bridges and pipelines, and disruptions to essential services,” said study author Assistant Professor Manoochehr Shirzaei, of Virginia Tech, told BBC Science Focus.
Shirzaei explained that even movement of a few millimetres per year can distort the land enough to weaken the foundations of buildings and create cracks on their surfaces.
“In cities like Houston and New York, localised subsidence has already been linked to infrastructure vulnerability and magnified flood risks during storms,” he said.
This subsidence seems to happen because of underground water. People living in cities need access to freshwater; as the cities grow, so does that need.
Water can be extracted from the ground from aquifers: layers of rock or earth that contain water or allow water to pass through.
But if water is extracted from an aquifer faster than it refills, it can crumble, collapsing the land on top with it.
“City centres built on reclaimed or sedimentary land – for example, parts of San Francisco or New Orleans – are particularly vulnerable,” added Shirzaei.
The scientists found that nine cities were sinking by approximately 2mm per year, including New York, Chicago, Seattle and Denver.
But several cities in Texas were sinking by about 5mm per year – and certain areas of Houston were sinking by as much as 1cm per year.
In Houston, New York, Las Vegas and Washington DC, some localised zones were sinking much faster than nearby areas.
“Over time, subsidence amplifies the effects of climate change, especially in coastal and flood-prone areas,” said Shirzaei. “As land sinks and sea levels rise, floodplains expand, and storm surges reach farther inland.”
He said the study provided “a roadmap for proactive, science-based planning” for experts and policymakers to monitor sinking and mitigate its effects where possible.
The 28 most populated cities in the US are: New York, Los Angeles, Chicago, Houston, Phoenix, Philadelphia, San Antonio, San Diego, Dallas, Jacksonville, Austin, Fort Worth, Columbus, Charlotte, San Jose, Indianapolis, San Francisco, Seattle, Denver, Washington DC, Nashville, Oklahoma City, El Paso, Boston, Portland, Las Vegas, Detroit and Memphis.
The study was published in Nature Cities.
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About our expert
Manoochehr Shirzaei is Associate Professor of Geophysics and Remote Sensing at Virginia Tech's Department of Geosciences. He is a geodesist/geophysicist who specialises in satellite geodesy, inverse theory, signal processing, modelling techniques and the physics of crustal deformation.
