Researchers from the Engineering Institute of the National Autonomous University of Mexico (UNAM) found the possible cause of the low intensity earthquakes that originated in December of last year: the Plateros-Mixcoac fault.
This week they published the results of the investigation “The Seismicity Puzzle of the West of Mexico City” for which they installed seismic recording stations with speed and acceleration sensors in the Mixcoac area. The network recorded continuously from the third week of December 2023 until early February.
On December 12, four microseisms with an epicenter in the Álvaro Obregón mayor’s office were recorded in Mexico City. Neighbors reported damage to at least three properties on Los Echave Street, Mixcoac neighborhood, in the Benito Juárez mayor’s office.
After the reviews that the city’s mayor’s office and civil protection carried out in the following days, eight properties were evacuated to analyze the structural risks.
To fully understand this problem, the UNAM group of researchers placed 15 temporary stations at Walmart de Plateros, the National Preparatory School 8 and also in the offices of the Secretariat of Comprehensive Risk Management and Civil Protection of Mexico City.
With the information from these stations, they determined the location of the epicenters of another swarm of earthquakes that occurred in the last days of December 2023. “Once some of the earthquakes were relocated and keeping in mind that the damage occurred closer to the new epicenters, we made a tour of the area near them. To our surprise, we found a ‘crack’ just over a kilometer long that almost uninterruptedly crosses the Mixcoac area,” the investigation says.
From what they located as a crack, the group of experts gathered information from the sensors and complemented it with satellite images to identify displacements on each side of that rupture. Their intention was to find a pattern of ground displacement and they succeeded.
“To the north of the ‘crack’ there are negative subsidences or displacements and to the south there are small positive displacements with a smaller area of influence. The pattern is similar to the permanent displacement generated by a normal type fault, which may be independent and which had not been identified before, but was suggested by many authors.
“This finding, which we will call the Plateros-Mixcoac fault, has been proposed in some way in conversations with academics and is even suggested in the risk map of Mexico City, but it had never been confirmed. We have preliminarily corroborated this through epicenters in the northern area of the Plateros-Mixcoac fault and vertical displacements or subsidence whose pattern is that of a normal fault,” the analysis determines.
The fault runs from the vicinity of the Pan American University, in the Insurgentes Mixcoac neighborhood and crosses to the Torres de Mixcoac. It passes under the Mixcoac fountain, the site where a building, number 959 Avenida Revolución, was at risk of collapse.
Then it continues along Los Echave Street, where three buildings were damaged. In December this newspaper reported that along that street, microseisms caused the opening of a crack that reached Periférico and even caused a water leak.
The owners of the apartments in the building marked with the number 34 had walls falling off. Residents of the property indicated that the microseisms generated several cracks that were in the process of repair.
The fault crosses Periférico and reaches the Torres de Mixcoac, where neighbors also reported cracks.
“The reader’s natural question should be: What else do we need to know and what impact does the presence of the fault have on the buildings, houses and infrastructure in the area?” wrote the group of experts to those who read their report.
To the first question, they responded that they must determine the geometry of the Plateros-Mixcoac fault through continuous GPS monitoring of the speed with which the surface moves. With what results from this monitoring they will be able to answer the second question. “We will continue to monitor and begin to estimate scenarios to determine the impact,” they conclude.