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

42 PATA Days 2024 The impact of these events has been profound, for example, the Mw 7.4 1944 San Juan earthquake resulted in the destruction of over 90% of San Juan’s buildings and tragically claimed the lives of 10,000 people (Instituto Nacional de Prevención Sísmica (INPRES), 1997, Alvarado, et al., 2006). The region experienced another earthquake on January 18th, 2021, just a few days after the 77th anniversary of the destructive 1944 earthquake (Costa, 2021). Along with the high seismicity, the region is also characterized by blind faulting and intense Quaternary deformation (Costa et al., 2006; Costa et al., 2021; Rockwell et al., 2014). This deformation is manifested by several east-dipping, west vergent thrust faults, namely, the Zonda Fault, Marquesado Fault, La Laja Fault, and La Rinconada Fault that merges with the basement fault at approximately 10-15 km (Ammirati et al., 2022). Among these faults, the La Laja Fault experienced 30 cm of vertical displacement during the 1944 event, which nucleated at a focal depth of ~11 m and propagated westward, updip, and ruptured the east-dipping La Laja Fault (Alvarado and Beck, 2006; Rockwell et al., 2021, 2014). Because the rest of the faults seem to have remained locked and unruptured, it is crucial to establish the chronology of prehistoric earthquakes along these faults and comprehend their rupture dynamics. In this study we focused on the east-dipping Marquesado Fault to examine its rupture history. The paleoseismic investigation along the fault allowed us to uncover at least four paleoearthquakes. Based on the geomorphic analysis, geochronology and comparison of previously published earthquake chronology data we have produced chronological framework of faulting spanning to over 14 ka. Based on the comparison of paleoseimsc data from a nearby trench excavated by Rockwell et a., (2021), we suggest the entire fault length likely ruptured during last four events. M E T H O D S The Marquesado Trench Site is located at 31°30'29.84"S, 68°40'6.02"W along the 400 m long, and 25° - 205° striking west- facing scarp (Fig. 2). To investigate the timing of prehistoric earthquakes, we hand-dug a 6 m long and 2-2.5 m deep trench perpendicular to the identified scarp trace. Nejiri gama handheld scrapers and paint brushes were used to meticulously clean and flatten the trench wall. Extra caution was taken around potential fault splays in order to avoid the loss of imbricated pebbles or other importantfeatures. The trench was gridded at 1 m ´ 1 m scale using fluorescent string and logged manually on Mylar paper and grid paper at a scale of 1:10. The wall was photographed to generate a photomosaic documenting the units, and we utilized Agisoft Metashape to construct photorealistic 3D models of the trenches and the surface topography through Structure fromMotion (sfm) technique. Further, each unit in the trench was distinguished based on clast size, texture, color, composition and depositional environment. Due to the lack of detrital charcoal from the trench, the Optically Stimulated Luminescence (OSL) samples in metal pipes were collected and processed at the University of Texas for age assessment.