Tuesday, May 4, 2021

"The Looming Crisis of Sinking Ground in Mexico City"

Not saying the ground under Mexico City is what caused the overpass collapse. This article just happened to be published April 22 and was in the queue for linking.

As with New Orleans—built below sea level in a very active hurricane zone, imagine having the risk of the 1953 North Sea Flood, 2500 dead, two or three times per year—if you could go back in time and tell folks in Mexico City "Don't build here", before it became one of the world's great conurbations with 22 million residents, you could remove the risk of hundreds of thousands dead the next time a magnitude 8.0 earthquake hits.

But you can't.

From the American Geophysical Union's EOS:

New research reveals the cause of Mexico City’s rapidly sinking and fracturing ground.

The ground in Mexico City is sinking at a rate of almost 50 centimeters (20 inches) per year, and it’s not stopping anytime soon, nor will it rebound, say Chaussard et al. in a new study.

Combining 115 years of ground-based and 24 years of space-based measurements, the team of U.S. and Mexican scientists has concluded that wide swaths of ground beneath the city are steadily compacting after long ago being drained of water. The ground will continue to compact for about 150 years, they forecast, adding up to 30 meters to what is already several meters of subsidence during the 20th century.

Unlike subsidence seen in many other cities of the world, Mexico City’s subsidence does not seem to reflect local groundwater pumping rates, as would be expected. Instead, it reflects the steady compacting of the ancient lake bed on which the city was built.

That lake bed was once Lake Texcoco, home of the Aztec city Tenochtitlán. As water extraction drove groundwater deeper underground, the 100-meter-thick, salty, clay-rich lake bed was left high and dry. Its very fine mineral grains have since been steadily repacking themselves more tightly, causing the ground to shrink and subside....