Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

156 PATA Days 2024 InLasCondes district the fault exhibits twobranches, one located at the foot of Calán and Apoquindo hills, and the other located closer to the mountain front (Fig. 2). In the San Carlos de Apoquindo area (Fig. 2b), in the same district, alluvial layers dipping contrary to the slope suggest sediments deformed by a reverse fault associated to the eastern branch of the SRF (site b1; Fig. 2b), with a dip in the order of 20ºE (Fig. 3). Secondary faults affecting sediments have been observed also in other sites in the same district (Fig. 2c, sites c1 and c2), dipping higher to the east. In the southern extreme of Las Condes district, an outcrop of the eastern fault branch (Figs. 2d, site d1), exhibits volcanic rocks of the Cenozoic Abanico Formation overthrusting Quaternary alluvial layers, with a dip of about 40ºE (Fig. 4). A similar dipping has been deduced from the exploration of the western branch of the SRF in the same district area (Fig. 2d, site d2), from the realization of a borehole coupled with ERT profiles (Menares, 2018; ongoing results). From a new trench site in Pirque (Fig. 2f, site f1) it is possible to observe a fault along which Quaternary alluvial layers overthrust sandy sediments, with a dip of about 25ºE (Fig. 5a), including some kinematic indicators consistent with reverse faulting (Fig. 5b). Long and short ERT profiles performed parallel to the trench in this site, show abrupt contact between resistivity units suggesting a dip of about 50-55ºE for the fault at subsurface (Fig. 6). Fig. 6: Long (bottom) and parallel to the paleoseismological trench (top) ERT profiles performed in Pirque (site f1 in Fig. 2). C O N C L U S I O N S Through geomorphological analysis of LiDAR data from the Andean piedmont in the Metropolitan region, coupled with direct observations from trenches and indirect observations from active geophysical methods, it is possible to assess the location of the San Ramón Fault trace as well as its geometry at subsurface. The fault is expressed for at least 47 km long of linear distance between the northern and southern extremes, crossing the Mapocho, Maipo and Clarillo rivers (Figs. 1, 2), exhibiting a complex geometry with a general N-S orientation, with NNE and NNW oriented fault branches in the order of 5-10 km long each. This fault length agrees with previous inferences from seismic data (Ammirati et al., 2019). At the surface, the main fault has been observed with a dip in the order of 20-25ºE when it overthrusts Quaternary sediments,andwithdipsofabout40-45ºEwhenitoverthrusts rocks over sediments. Meanwhile, indirect observations from active geophysical methods show that the fault dip increases at subsurface, with values in the order of 50-55ºE. The continuous improvement of knowledge about the location of the San Ramón fault trace through surface and subsurface exploration is a critical issue regarding seismic hazard assessment of Santiago and the Metropolitan region (Easton et al., 2022). A C K N O W L E D G E M E N T S We thank Fondecyt projects 1230350 and 1241021.