Slow-motion catastrophe: The 25 largest US cities are collapsing.

In early April, a geological study revealed that the underside of the tectonic plate on which North America sits is "leaking" into the Earth's mantle, causing the entire continent to slowly sink. This geological phenomenon, however, will take millions of years to affect the surface.

But now, new research conducted in the 28 largest cities in the United States confirms that virtually all of them are subsiding, albeit for very different reasons. This is, in fact, a much faster subsidence than the slow "trickle" of rock detected in the previous study. A subsidence that, according to the researchers in a recently published article in Nature Cities, could already be affecting thousands of buildings and millions of people across the country.

The problem had already been detected about a year ago in various cities, although only in coastal areas, where increasingly low-lying terrain favors the gradual increase in flooding. But new satellite technologies, which send radar signals to the Earth's surface and measure the time it takes for them to bounce back and return to the instruments, have allowed scientists to discover that the phenomenon also affects inland areas. In fact, 25 of the 28 North American cities with more than 600,000 inhabitants experience some degree of subsidence, the technical term for this sinking.

"By comparing multiple images of the same area taken over time," explains Manoochehr Shirzaei, co-author of the article, "we can detect tiny vertical movements of the ground, even just a few millimeters per year. It's like taking a high-resolution time-lapse of the Earth's surface and watching it rise or fall over time."

To compound the problem, the data reveal that the subsidence of some cities is not uniform, meaning they are sinking at different rates in different places, and even rising in others, creating stresses that could seriously affect buildings and infrastructure.

The cities of Fort Worth, Houston, and Dallas, all in Texas, have the highest subsidence rates, exceeding an average of 4 millimeters per year. New York, Chicago, Columbus, Seattle, and Denver also show a significant decline, exceeding 2 millimeters per year. Particularly worrying is the data from Houston, where researchers have found that a staggering 42% of its land area is sinking at a rate exceeding 5 millimeters per year, while 12% is sinking at more than 10 millimeters per year. A difference that, in just a few decades, could have fatal consequences for buildings and infrastructure.

But what's causing this situation? According to the researchers, it's a "mosaic of factors." In fact, and despite the fact that massive groundwater extraction is, in general, the main cause of this subsidence, the study reveals that in some cities, such as New York, Philadelphia, and Washington DC, the phenomenon is primarily due to what is known as "glacial isostatic adjustment." A concept that, although complex, can be understood if we imagine the Earth as a memory foam mattress. During the last Ice Age, huge masses of ice covered these areas, exerting immense pressure on the Earth's crust and deforming it, just as a very heavy person would do to a mattress. "The weight of the ice," Shirzaei explains, "pushed down on the Earth's crust, just as if we were sitting on a memory foam mattress." When the ice melted thousands of years ago, the pressure disappeared, and the ground (like the mattress) began to slowly rise, regaining its original shape.

However, this "bounce back" isn't uniform. "In some areas," the scientist continues, "such as the East Coast and the Midwest of the United States, the land is still sinking rather than rising, because they are near an area that had been pushed up by the weight of nearby ice and is now collapsing." It's as if, as we get off the mattress, the adjacent areas that had risen slightly now begin to descend.

But that's by no means the only cause identified. In cities like Seattle, Portland, and San Francisco, plate tectonics, the constant movement of the large pieces that make up the Earth's crust, is the main culprit behind the subsidence. These cities, in fact, are located in seismically active zones, where plates interact with each other and deform, which can cause vertical ground movements.

Finally, other factors are also contributing to subsidence in some areas. A 2023 study, for example, suggested that the combined weight of more than one million buildings in New York City could be contributing to its subsidence. And another recent study, in Miami, found that the construction of new buildings in close proximity to existing ones could be disturbing the subsoil and causing structures to sink. It's as if the constant construction and increasing urban mass are adding additional stress to the ground, accelerating its decline in certain spots. "We need to start treating subsidence as the slow-motion disaster that it is," Shirzaei warns.

Scientists have also discovered that some cities, like the aforementioned Houston, are sinking at different rates in different locations, or even sinking in some places and rising in others. This uneven movement, known as differential motion, is especially concerning. In Shirzaei's words, "This uneven movement creates angular distortion and stress, which can potentially lead to cracks in walls and foundations, misaligned windows and doors, or, worse, structural failure."

The study estimates that approximately 1% of the total surface area of ​​the 28 cities studied is located in areas where differential movement could affect buildings, roads, rail lines, and other structures. Although this percentage may seem small, these areas tend to coincide with the densest urban cores and, together, are currently home to approximately 29,000 buildings. The cities most at risk in this regard are San Antonio, where the researchers estimate that 1 in 45 buildings is at elevated risk; Austin (1 in 71); Fort Worth (1 in 143); and Memphis (1 in 167).

The new study also reveals that eight cities (New York, Chicago, Los Angeles, Phoenix, Houston, Philadelphia, San Antonio, and Dallas) have more than 60% of their population living on subsiding land. Significantly, these same eight cities have experienced more than 90 major floods since 2000, a phenomenon likely fueled by the decrease in land elevation due to subsidence. If land subsides, coastal and low-lying areas become more vulnerable to flooding, as water has less capacity to drain and the elevation of the land relative to sea level decreases.

The study concludes with a call to action for cities to use this new information to find solutions. In many places, flooding could be mitigated, according to the researchers, by elevating land, improving drainage systems, and using green infrastructure, such as constructed wetlands, to absorb stormwater. Cities susceptible to the danger of tilting could, meanwhile, focus on rehabilitating existing structures, integrating ground motions into building codes, and limiting new construction in the most threatened areas.

"Rather than simply saying it's a problem, we can respond, address, mitigate, and adapt," says Leonard Ohenhen, lead author of the study. "We need to move on to solutions." A detailed understanding of how and where land subsidence is occurring in major US cities is the first step toward addressing this "silent catastrophe" and ensuring the safety and sustainability of urban infrastructure for future generations. Ignoring this phenomenon would be like building sandcastles amid a slowly rising tide, a strategy destined for long-term failure. Science, once again, has warned. It remains to be seen whether anyone will listen this time.