The San Mateo fault: New paleoseismological evidences of active faulting in the Acambay graben, Mexico
This study focuses on the identification and seismic characterization of the San Mateo fault, herein described for the first time. This fault is located in the central part of the Acambay graben, central Mexico. It is a 13 km long active normal fault with E-W direction and dip to the south. Two trenches were excavated in the locality of La Lechuguilla to determine the recent chronology of paleoearthquakes along this fault and to estimate its seismic parameters. At least three paleo-breaks have been found in one of these trenches and new radiocarbon ages indicate that they occurred from Late Pleistocene to the Holocene times. The oldest event (Event 1) occurred in 31.0-29.3 ky cal BP. Event 2 occurred in 19.1–6.5 ky cal BP and the younger faulting (Event 3) in 6.0 to 4.2 cal ky BP. These results suggest a recurrence interval of surface rup- tures of about 11.57 ± 5.32 ky. A slip rate of 0.085 ± 0.025 mm / year and a magnitude (Mw) of 6.43 to 6.76 is estimated from the maximum coseismic displacement of 0.85 ± 0.163 m and a rupture length of 13 km. If we consider a maximum rupture length of 25 km, including the possible continuity of the fault under the present day sediments filling the graben, a Mw magnitude of 6.7 ± 0.3 could be expected. The paleo- seismic data and fault scaling relationships suggest that the San Mateo fault is a significant seismic source in terms of hazard. This study also highlights the recency of the volcanic activity in the Acambay graben.