Interplay of slow-slip faults beneath Mexico City induces intense seismicity over months

In February 2023, a long seismic sequence began in western Mexico City causing widespread panic and some damage to housing infrastructure. On May 11 and December 14, two Mw3.2 mainshocks occurred at less than 700 m depth. Unprecedented satellite interferograms captured tectonic deformations in the two epicentral zones during the days surrounding the earthquakes. Data analysis revealed extended slip with maximum values around 8 cm on two sub-parallel east-west trending normal faults 800 m apart: namely the Barranca del Muerto (BM) fault to the south and the Mixcoac fault to the north. Detailed microseismicity analysis showed that 95 % of the slip on the BM fault was aseismic and initiated at least 6 days before the May 11 earthquake on the main asperity, located 1 km east of the hypocenter and ∼ 1.2 km deep. For the December event on the Mixcoac fault, ∼70 % of the slip was also aseismic but shallower (mostly above 600 m), which can be partially explained by the induced stresses on that fault due to the May slip on the BM fault. A quantitative geomorphological analysis allowed to establish the structural connection between both buried faults and their geomorphic expression to the west, with surface extensions of ∼3.5 and ∼ 4.5 km in the hilly area—where the most intense seismicity concentrates. The spatiotemporal patterns of fast and slow earthquakes suggest that the seismotectonics west of the city comprises two mechanically distinct zones: a stable region prone to aseismic deformation to the east where faults are buried under water-saturated sediments, and an unstable region to the west, prone to seismic radiation where faults are expressed geomorphologically. Thus, the seismic swarms in this area appear to result from the regional extensional regime, the stresses induced by slow slip on the eastern fault segments and interaction between these faults.