Thanks to unprecedented satellite interferograms, scientists managed to capture tectonic deformations in epicentral areas during the days before and after the earthquakes. The data analysis revealed that two normal failures, separated by just 800 meters and with a east-west orientation, were responsible: the Barranca del Muerto (BM) fault to the south and the Mixcoac fault to the north.
Both recorded landslides up to 8 centimeters, but with a surprising detail: 95% of the movement in the BM fault was asism, that is, it did not generate perceptible seismic waves, and began at least six days before the earthquake of May 11. “We discovered that the efforts generated by the sliding in the BM fault in May 2023 could explain why the December slip was more superficial and concentrated east of the epicenter of the earthquake of December 14 in the Mixcoac failure,” explains Solano.
This phenomenon of slow sliding, which went unnoticed by the population, not only prepared the stage for later tremors, but also influenced the activity of the Mixcoac failure. Approximately 70% of the sliding in the latter was also assism, although more superficial, and it is believed that it was driven by the efforts generated by the previous movement in the BM fault.
The researchers found that the two failures exhibit different mechanical behaviors. The oriental segments of failures, buried under saturated water sediments, are prone to assistory deformation. On the contrary, the western segments, which have geomorphic expressions, are prone to seismic radiation. The study suggests that seismic swarms are the result of the regional extensive regime, slow sliding tensions in oriental fault segments and interaction between these failures.
But what makes these failures behave so differently? The answer seems to be in your geological environment. To the east, where the failures are buried under saturated sediments of water, the Asist deformation predominates.
On the other hand, to the west, in a mountainous area where failures have a clear geomorphological expression, seismic radiation is more intense. This explains why seismic swarms are concentrated in this area, where the failures are superficially extended about 3.5 and 4.5 kilometers.
The investigation concludes that the seismic crisis of 2023-2024 in Mexico City was influenced by slow landslides in two faults with this-west orientation. Experts identified two regions in the west zone of the city: one more stable to the east, where deformation occurs without generating perceptible earthquakes, and a more unstable to the west, where seismic waves are released. The swarms of earthquakes seem to be the result of the combination of three factors: the extension regime in the region, the tensions caused by slow landslides and the interaction between these main failures.
The study highlights the importance of understanding slow landslides to assess seismic hazards in cities. By using advanced data analysis techniques, scientists can better understand processes behind seismic activity and improve the ability to forecast and reduce earthquake risks.
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