Ground fractures in Mexico City are indicated in red. Ground fractures in Mexico City are indicated in red. Green areas are mountainous zones; orange are foothills.

Major fractures shown on Mexico City map

Areas with most fractures saw the most buildings collapse during earthquake

Geology experts have created a map that highlights the major ground fractures beneath Mexico City, features that pose a risk for city infrastructure.

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The Center for Geosciences’ geological fracture map reveals that 15 of the city’s 16 boroughs could be affected to some degree by the underlying fractures, particularly Benito Juárez, Cuauhtémoc, Iztapalapa, Tláhuac and Xochimilco.

Comparing the map of fractures with areas damaged during the earthquakes in 1985 and this year indicate that the buildings that collapsed were located in the areas with the most fractures, especially the Benito Juárez and Cuauhtémoc boroughs.

“Both are located in a type of pit bounded by two faults that cross the city from north to south,” explained Dora Carreón Freyre, chief of the Center for Geologic Risk Evaluation (CERG), who led the mapmaking project.

Geologists have known about some of the fractures for some time. In the case of the southern area of Iztapalapa, adjoining Tláhuac, a series of fractures caused vertical land movement of close to one meter during last month’s earthquake.

Carreón said the fractures have their origin in the contrast between the original volcanic subsoil and the materials used to fill Texcoco Lake, where large parts of the city were built.

“We must study each zone . . . in order to assess the effects earthquakes have on the different kinds of soil, because it is important to determine their vulnerability to [future] fractures,” she said.

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The extraction of underground water is one of the factors that trigger and make evident the weaknesses in the soil that has resulted in infrastructure damage during the recent seismic activity.

After water is extracted, explained Carreón, the subsoil compacts on itself, causing existing irregularities and discontinuities to propagate toward the surface.

Carreón added that the appearance of the fractures is not random, and that specialists can map them and even predict where they will spread.

She suggested that reconstruction projects take into consideration that the city sinks an average of 20 to 30 centimeters per year, and that this displacement should be part of their short and mid-term plans.

The Center for Geosciences regards the map as a valuable piece of information as reconstruction gets under way in the aftermath of the September 19 earthquake.

Work on the map began in 2016 as a collaboration by UNAM, the National Autonomous University of México, and Cenapred, the National Disaster Prevention Center.

Source: La Crónica de Hoy (sp)

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