Paleoseismology of the 1912 Acambay earthquake and the Acambay-Tixmadeje fault, Trans-Mexican Volcanic Belt

The Acambay Graben is a major intra-arc basin of the Trans-Mexican Volcanic Belt. Trenching studies at four sites along the Acambay-Tixmadeje fault re veal evidence for the 1912 Acambay earthquake and at least four late Pleist ocene and Holocene ground-rupturing earthquakes with similar displacements to the 1912 event. Evidence for:multiple:events was recognized by a combina tion of upward terminating faults, fissure fills, erosionally truncated fis sures, colluvial wedge fills, and downward increasing separation of stratig raphic-units. Sites along the central part of the ESE-WNW trending rangefro nt fault indicates large dip slip separations across a multi event scarp in Holocene colluvium with a penultimate event that occurred since similar to 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 penu ltimate earthquake occurred after similar to 4700 cal. yr B.P., and there i s evidence for at least four more earthquakes in the last similar to 34,000 cal. yr B.P.. Three-dimensional excavation of lacustrine and channel depos its at Las Lomas yields a left-normal slip vector of S40 +/- 5 degrees E wi th 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 similar to 60 cm/event of dip slip on the rangefront and a mean of similar to 35 cm/event of oblique slip on the N48 degrees W trending Huapango Plain. Assuming these values represent a chara cteristic behavior yields an average recurrence interval of similar to 3600 years for the last four large earthquakes (since similar to 11,570 cal. yr B.P.), and a slip rate of similar to 0.17 mm/yr across the rangefront. Emp irical 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 o n the Acambay-Tixmadeje fault and demonstrates that large seismic events (M similar to 7) can be generated by faults in the Trans-Mexican Volcanic Bel t and in other intra-arc settings.