Title: Paleoseismology of the 1912 Acambay earthquake and the Acambay‐Tixmadejé fault, Trans‐Mexican Volcanic Belt
Abstract: The Acambay Graben is a major intra‐arc basin of the Trans‐Mexican Volcanic Belt. Trenching studies at four sites along the Acambay‐Tixmadejé fault reveal evidence for the 1912 Acambay earthquake and at least four late Pleistocene and Holocene ground‐rupturing earthquakes with similar displacements to the 1912 event. Evidence for multiple events was recognized by a combination of upward terminating faults, fissure fills, erosionally truncated fissures, colluvial wedge fills, and downward increasing separation of stratigraphic units. Sites along the central part of the ESE‐WNW trending rangefront fault indicates large dip slip separations across a multi event scarp in Holocene colluvium with a penultimate event that occurred since ∼5230 calibrated years before present (cal. yr B.P.). On the Huapango Plain, near the eastern end of the 1912 rupture where slip was oblique, the penultimate earthquake occurred after ∼4700 cal. yr B.P., and there is evidence for at least four more earthquakes in the last ∼34,000 cal. yr B.P. Three‐dimensional excavation of lacustrine and channel deposits at Las Lomas yields a left‐normal slip vector of S40±5°E with 105±10 cm of slip for the last three events there. This slip vector is consistent with NNE‐oriented extension across the graben. The amount and style of slip documented for prehistoric ruptures are also consistent with the 1912 rupture, with a mean of ∼60 cm/event of dip slip on the rangefront and a mean of ∼35 cm/event of oblique slip on the N48°W trending Huapango Plain. Assuming these values represent a characteristic behavior yields an average recurrence interval of ∼3600 years for the last four large earthquakes (since ∼11,570 cal. yr B.P.), and a slip rate of ∼0.17 mm/yr across the rangefront. Empirical earthquake parameters calculated from our paleoseismic data support instrumental measures indicating that the 1912 Acambay earthquake was of M 6.8–7.0. The 1912 event is representative of ground‐rupturing earthquakes on the Acambay‐Tixmadejé fault and demonstrates that large seismic events ( M ∼7) can be generated by faults in the Trans‐Mexican Volcanic Belt and in other intra‐arc settings.