Tech

Mexico City could sink up to 65 Feet

When Darío Solano-Rojas he moved from his native city of Cuernavaca to Mexico City to study at the National Autonomous University of Mexico, the scheme of the metropolis confused him. Not the grid itself, keep that in mind, but the way the built environment seemed to be in turmoil, like a surreal painting. “What surprised me was that everything was a little crooked and sloping,” says Solano – Rojas. “At the time, I didn’t know what it was all about. I just thought, “Oh, well, the city is so different from my hometown.”

Different, it turned out, in a bad way. Returning to the study of geology at university, Solano – Rojas met the geophysicist Enrique Cabral-Cano, who was actually looking for the surprising reason for this infrastructural chaos: The city was sinking – big time. It is the result of a geological phenomenon called subsidence, which usually occurs when too much water is taken from the ground, and the soil above it begins to compact. According to the new model from the two researchers and their colleagues, parts of the city sink up to 20 centimeters a year. In the next century and a half, they calculate, the zones could drop as low as 65 feet. Spots just outside of Mexico City themselves could sink 100 feet. That torsion and tilt that Solano – Rojas noted was just the beginning of a slowing crisis for 9.2 million people in the city that sank the fastest on Earth.

The foundation of the problem is the poor foundation of Mexico City. The Aztec people built their capital Tenochtitlan on an island in Lake Texcoco, which lies in a basin surrounded by mountains. When the Spaniards arrived, they destroyed Tenochtitlan, and massacred its people, they began to drain the lake and build on it. Gradually, the metropolis that has become today’s Mexico City spreads out, until the lake is gone.

And that brought to light the physical changes that have initiated the sinking of the city. When the sediment of the lake beneath Mexico City was still wet, its component clay particles were arranged in a disorganized manner. Think about launching dishes in a sink, want-to-want– their random orientations allow a lot of flow to circulate between them. But drain the water – as the planners of Mexico City did when they drained the lake in the first place, and as the city has since done by touching the ground as an aquifer – and those particles are reorganizing. to stack neatly, like the plates placed in a cupboard. With less space between the particles, the sediment compacts. Or think of it as applying a clay face mask. When the mask dries, you may feel it tighten against your skin. “It loses water and loses volume,” says Solano – Rojas.

Mexico City officials actually recognized the subsidence problem in the late 1800s, when they saw buildings collapse and began measuring. This has given Solano – Rojas and Cabral-Cano valuable historical data, which they have combined with satellite measurements taken over the last 25 years. By pulling radar waves ashore, these orbiters measure in fine detail – a resolution of 100 feet – how surface elevations have been altered throughout the city.

Using these data, the researchers calculated that it would take another 150 years for Mexico City’s sediments to be completely compact, although their new modeling shows that subsidence rates will vary from block to block. (That’s why Solano-Rojas noticed the sloping architecture when he first arrived.) The thicker the clay in a given area, the deeper it sinks. Other areas, particularly on the outskirts of the city, might not sink much because they are sitting on rock instead of sediment.

It sounds like a relief, but in reality aggravates the situation because it creates a dangerous differential. If the whole city sank evenly, it would be a problem, to be sure. But because some parts are falling dramatically and others are not, the infrastructure that extends to both areas is sinking in some areas but staying at the same elevation in others. And that threatens to break roads, subway networks and sewer systems. “Living by itself can’t be a terrible problem,” Cabral-Cano says. “But it’s the difference in this speed of subsistence that really puts all civilian structures under different stresses ”.

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This is not just the problem of Mexico City. Wherever man draws too much water from aquifers, the earth calms down in response. Jakarta, Indonesia sinks up to ten inches a year, and California’s San Joaquin Valley has sunk 28 feet. “It’s been going on for centuries.” Human thinking was that [water] it’s an unlimited supply, ”says Arizona State University geophysicist Manoochehr Shirzaei, who study subsidence but he was not involved in this new research. “Wherever you want, you can put a hole in the ground and vacuum it.” Historically, groundwater pumping has solved the immediate problems of communities – keeping people and cultures alive – but it has created a long-term disaster. A study earlier this year found that by the year 2040, 1.6 billion people could be affected by subsidence.


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