3 rd INQUA-IGCP-567 International Workshop on Active Tectonics, PaleoseismologyandArchaeoseismology, Morelia, Mexico (2012) I INQUA-TERPRO PALEOSEISMOLOGY EARTHQUAKE ARCHAEOLOGY 133 HOLOCENE EARTHQUAKES RECORDED AT THE TIP OF THE PASTORES FAULT SYSTEM (CENTRAL MEXICO) Ortuæo, Mara (1, 2), Zœæiga, Francisco Ramn (1), Aguirre, Gerardo (1), Carren, Dora (1), Cerca, Mariano (1), Roverato, Matteo(1) (1) Centro de Geociencias, Universidad Nacional Autnoma de MØxico, Blvd. Juriquilla, 3001, 76230, Juriquilla, QuerØtaro, MEXICO (2) Dept. deGeodinmica i Geofsica, Universitat de Barcelona, C/ Mart i FranquLs s/n, 08028, Barcelona, SPAIN. ortuno.maria@gmail.com Abstract (Holocene earthquakesrecorded at the tip of thePastores fault system (Central Mexico)):The Pastores fault is one of the most active structures within the Intra-arc zone of the TransmexicanVolcanic Belt. It defines the southern margin of the Acambaygraben, extending E-W along near 30 km. this study, we mapped the fault western termination, which corresponds to a fault splay, and performed a paleoseismological researchon one of the fault branches. We relied on GPR data to properly locate the fault on depth. The trenches exposed an igimbritic deposit interbedded withfluvio- lacustrine and coluvial deposits. The deformation of this sequence suggests that this part of the Pastores fault moved at a maximum dip slip rate of 0.10 -0.15 mm/yr (three to four times greater than the rate previously estimated) and produced at least three (and possibly five) earthquakes during the last 2.4 ! 3.67 kyr. Some of the deformation observed in the trenches could be triggered by seismic ruptures in nearby faults, as it is the Acambay- TixmadejØ fault rupture on 19 th November 1912. Key words: Acambay graben, slow faults, paleoearthquake chronology, Ground Penetration Radar INTRODUCTION The Transmexican Volcanic Belt (TMVB) is a 900 km long and up to 200 km wide extensional zone that crosses continental Mexico from E to W, from the Mexican Gulf to the Pacific Coast (Fig. 1). This active deformational zone has been associated to the subduction of the Rivera and Cocos plates underneath the Northamerican plate. The deformation rate of the individual structures within the TMVB do not overpass 0.3 mm/a (Suter et al., 2001), which leads to consider the structures as slow faults. Among the main fault systems that comprise the TMVB are the Chapala, Morelia, Acambay and Queretaro grabens. The historical seismic catalogue shows that the faults in this region are capable to produce earthquakes of up to M = 7. Some good examples of these intraplate earthquakes are the Chapala (1568, M ~ 7), Jalisco, 1875 (M ~ 7.1), Acambay, 1912 ( mB = 6.9) and Jalapa, 1920 (mb = 6.9; Suarez, 1992) events. This study focuses on the Pastores fault(Fig. 2), one of the most active faults in the intra-arc fault system(Suter et al. 1995, 2001). This fault is part of the Morelia- Acambay fault system (Martnez-Reyes and Nieto- Samaniego, 1990; Suter et al., 1992; 1995; 2001; Ramrez-Herrera et al. 1994; Garduæo et al., 2009) and is the southern bounding fault of the Acambaygraben (Langridge et al. 2000). It dips 50-70” to the N and is characterized a by a very rectilinear trace, mainly developed on lava flows of different composition. The fault has a ~ 30 km trace length and an up to 250 m high scarp in its central part. Based on the age of lava flow deposits affected by the eastern prolongation of the fault, Sutter et al. (2001) approximated the vertical slip rate of the Pastores fault in 0.04 mm/yr. Historical activity of the Pastores fault was reported by Urbina and Camacho (1913), who observed the formation of discrete scarps parallel to the fault main trace during the 19th November 1912 Acambay earthquake. More recently, Persaud et al. (2006) performed a paleoseismic study in its eastern termination, concluding that the fault moved at 0.03 mm/yr during the Late- Pleistocene-Holocene and that it produced at least three earthquakes in the last 32 kyr. The study suggested that the fault segment did not move during the Acambay earthquake. RESULTS In this study, we search to evaluate the seismogenic activity of the Pastores fault in its westernmost termination. To do so, we performed an integrated study combining basic geomorphological and structural cartography to refine the location of the fault trace and to identify a site suitable for paleoseismological trenching. Then, we obtained subsurface GPR profiles across the Fig. 1: Location of the Acambay graben within the Trans- Mexican volcanic belt (TMVB).