Paleoseismology of a Major Crustal Seismogenic Source Near Mexico City: The Southern Border of the Acambay Graben

The Trans‐Mexican Volcanic Belt is an active continental volcanic arc related to subduction along the Middle America trench. It is characterized by intra‐arc extension resulting into several major arc‐parallel active fault systems and tectonic basins. The Acambay graben, one of the largest of these basins, is located near Mexico City, in the central part of this province. In 1912, a M 6.9 earthquake ruptured the surface along the northern border of the graben together with at least two other faults. In this paper, we analyze the paleoseismic history of the southern border of the Acambay Graben, with new observations made in one natural outcrop and four paleoseismological trenches excavated across branches of the Venta de Bravo Fault at the site where it overlaps with the Pastores Fault. We present evidence of at least two paleo‐earthquakes that occurred between 12,190 ± 175 and 5,822 ± 87 cal year BP and between 647 ± 77 and 250 cal year BP. On one of these branches, we estimate a minimum slip‐rate value between 0.1 and 0.23 mm/year for the last 12 ka and a mean recurrence interval of 8.5 ± 3 ka. By considering several likely rupture lengths along the Venta de Bravo and Pastores faults, we calculated a maximum possible magnitude of Mw 7.01 ± 0.27. Finally, by correlating events recorded along different faults within the Acambay Graben, we discuss several possible rupture coalescent scenarios and related consequences for Mexico City. Plain Language Summary The Trans‐Mexican Volcanic arc is affected by several fault systems. Activity on these faults results in an extension of the crust and the formation of elongated topographic depressions. The study region, called the Acambay Graben, is one of these depressions and is bounded by normal E‐W active faults. In 1912 an M 6.9 earthquake caused nearly 300 fatalities and generated a surface rupture along three faults which are the boundaries of this depression. The epicenter of this event is in a region where close to 24 million inhabitants are living; therefore, the history of past earthquakes is crucial in evaluating the seismic hazard of central Mexico. We conducted paleoseismological excavations on one of these faults, the Venta de Bravo Fault, to infer the record of past earthquakes. The results show that at least two earthquakes occurred in the past: one between 12,365 and 5,735 cal year BP and the other between 724 and 250 cal year BP. With this new data and the previously published, we compiled the paleose