NEW ORLEANS, LOUISIANA—Darío Solano-Rojas remembers living in Mexico City and riding metro trains that sometimes slowed to a crawl. A geophysicist at the University of Miami in Florida, he now understands one of the causes of those delays: Mexico City, built on an ancient lakebed, is sinking by up to 30 centimeters per year, as groundwater is extracted to support its more than 20 million inhabitants. All of that ground movement has wreaked havoc on the city’s metro system, bending rails and precipitating a train crash, according to a new satellite-based map presented here last week at the fall meeting of the American Geophysical Union. The map, researchers say, can now be used to pinpoint spots where future accidents may occur.
“This study is important because it links surface deformation directly to critical infrastructure,” says Tim Wright, a geophysicist at the University of Leeds in the United Kingdom who was not involved in the research.
Ground subsidence has been implicated in a variety of damage reports and service interruptions across Mexico City’s vast surface and underground metro network, known as the Collective Transport System. For instance, inspectors have noted cracks in the concrete columns that support the above-ground tracks, and vertical shifts in the tracks themselves. In 2015, a crash left 12 people injured after a train traveling down an incline into Oceania Station in the eastern part of the city crashed into a stopped train. “The original design [of the tracks] was for a slope of 3%,” Solano-Rojas says. “But after so many years of subsidence the slope changed [to more than 7%], and the train’s brakes weren’t designed for that slope.”
Now, Solano-Rojas and his collaborators have analyzed satellite data showing how the ground near 93 kilometers of above-ground metro rail track in Mexico City is subsiding. The team used data from an Italian satellite that bounces microwave laser pulses off the ground and records the time it takes the light to return. Armed with these travel times for satellite passes from 2011–12, the scientists calculated the ground heights with millimeter-level precisions—and how they changed over time.
Metro lines in the eastern part of the city are subsiding more rapidly than those elsewhere, Solano-Rojas reported at the meeting. The researchers also analyzed how subsidence rates varied along adjacent 30-meter sections of track. This differential subsidence is the real culprit when it comes to causing track damage and accidents, says Solano-Rojas, because it causes the track to bend and change slope as some sections of track subside more quickly than others.
Solano-Rojas and his colleagues then correlated their maps of differential subsidence with the locations of reported track damage and accidents collected from local newspapers, YouTube, and Twitter. The results were striking: The team found that segments of the southeasterly Line A had both the most problems and the highest levels of differential subsidence. That's troubling because government officials are currently evaluating a plan to extend Line A by 13 kilometers, the team notes. The researchers also found high levels of differential subsidence near Oceania Station, the site of the 2015 crash.
Not all areas of high differential subsidence are associated with reported damage or accidents, the researchers found. But these spots, obvious in the satellite data, should be monitored because the effects of subsidence often take years to manifest, Solano-Rojas says. “We see displacements that could lead to problems in the future.”
The results show that the metro is particularly vulnerable to highly variable rates of subsidence, says Cathleen Jones, a radar scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “This information can be used to identify parts of the system that need repair to prevent accidents.”