Mexico City is sinking at an alarming rate of almost two centimeters per month, according to new NASA data released this week. The most visible signs of the problem can already be seen in several historical buildings on the capital’s central square – one of the country’s main landmarks.
On one side of the square stands the imposing Metropolitan Cathedral, visibly tilted. Another church, next to it, leans in another direction. The National Palace, nearby, is also clearly off-axis. The phenomenon of the city’s subsidence is not new, it has been known for more than a century.
Now, the sinking of the city is being recorded in real time thanks to one of the most powerful radar systems ever launched into space. Chado Nisar, the satellite, can detect small changes in the planet’s surface, even amid dense vegetation or skies covered by clouds.
“Nisar has taken Earth observations to another level,” said scientist Marin Govorcin, from NASA’s Jet Propulsion Laboratory. “It can see any change, large or small, that happens on Earth from week to week. No other imaging mission can do that.”
Although it’s not the first time that Mexico City’s subsidence has been recorded from space, the Nisar mission offers more precise data on how far the sinking is advancing and on the differences in subsidence according to terrain type. The system also managed to “see” areas on the city’s outskirts that were difficult to study due to terrain complexity.
According to the new data, some areas of the city, including the airport, are sinking at a rate exceeding two centimeters per month – one of the highest rates ever recorded anywhere in the world.
Among the main examples of this rapid sinking is the “Angel of Independence” statue, on Paseo de la Reforma Avenue, one of the city’s main landmarks. Inaugurated in 1910 to commemorate the 100th anniversary of Mexico’s independence, the 36-meter monument has had 14 steps added to its base as it gradually sank. But the phenomenon can be seen across the metropolis of 22 million people, in the deformed asphalt on streets and highways, in leaning buildings, and in recurring damage to the local subway system.
“The problem affects the entire urban infrastructure of the city: the streets, the pipes for water distribution, and the water reserves themselves,” said engineer Efraín Ovando Shelley of the National Autonomous University of Mexico in an interview with the British newspaper The Guardian.
The subsidence of the city is the result of unregulated groundwater extraction. Mexico City and its surroundings were built on the former bed of a river and, therefore, the soil is not solid. When water is pumped from the aquifer below, the soil forms a kind of mud, leading the city to sink.
The underground aquifer still contributes to about half of the capital’s water supply. As water pumping increased at a rate higher than rainfall could replenish, the aquifer is shrinking.
According to the engineer, this creates a kind of vicious circle: as the city sinks, the old water pipes end up cracking. It is estimated that about 40% of the pumped water is lost due to leaks. Adding to this global warming, which reduced the rainfall regime, the city is moving rapidly toward a disaster
“To stop the subsidence, we would need to suspend water extraction,” Shelley explained. “But if we stop extracting water, what water will we drink? The recurring joke is that if we can’t drink water, well, we’ll drink tequila.”
