Holocene paleo-earthquakes recorded at the transfer zone of two major faults: The Pastores and Venta de Bravo faults (Trans-Mexican Volcanic Belt)

We present evidence of five late Holocene earthquake ruptures observed at two paleo-seismological trenches in the Laguna Bani sag pond (Trans-Mexican Volcanic Belt, central Mexico). The trenches exposed two fault branches of the western termination of the Pastores fault, one of the major fault systems within the central Trans-Mexican Volcanic Belt. The site was studied by combining geomorphological and structural approaches, volcanic mapping, ground-penetrating radar, and paleoseismological analysis. The study revealed that coseismic surface rupture was noncharacteristic, and that the exposed fault branches had not always moved simultaneously. The fault tip has ruptured at least 5 times within the past 4 k.y., and the rupture events followed and preceded the deposition of an ignimbrite. The close temporal relationship of the seismic rupture with the volcanic activity of the area could be the result of volcanism triggered by faulting and its associated seismicity. The relatively high recurrence of seismic events (1.1-2.6 k.y.) and the noncharacteristic fault behavior observed at this tip of the Pastores fault suggest that the fault might have been active as a primary fault rupturing along segments of variable length or depth, and/or that the fault ruptured eventually as a secondary fault. The secondary ruptures would likely be related to earthquakes produced at major neighboring faults such as the Acambay fault, which moved during the 1912 Acambay earthquake, or the Venta de Bravo fault. A relatively large slip rate estimated for this fault branch (0.23-0.37 mm/yr) leads us to contemplate the possible connection at depth between the Pastores and the Venta de Bravo faults, increasing the maximum expected magnitude for central Mexico.