Pliocene to Holocene deformation and earthquake potential of the Mesamávida fault, West Andean Thrust System of central-southern Chile (36ºS)

Ambrosio Vega-Ruiz, Joaquín Cortés-Aranda, Ricardo Aguilera, Luis Astudillo-Sotomayor, Martin Riedel, Mauricio Espinoza, Andres Tassara, Alan Nelson, Daniel Melnick, Matteo Lupi, Danitza Arriagada, Tomás Sepúlveda, Fernanda Álvarez-Amado, Lucas Navarro

Crustal reverse faults are recognized for their potential to generate devastating earthquakes, making them a focus of seismic hazard assessment. Along the Chilean Andes, the Western Andean Thrust System (WATS)—a structure marking the Central Valley-Principal Cordillera border—includes several probable late Quaternary faults. However, evidence for large Holocene earthquakes (M~7) has only been confirmed on two faults at ~32-33°S.
This study focuses on the WATS at ~36°S, where a hectometer-scale escarpment dominates the border between the Central Valley and Principal Cordillera. LiDAR-derived topography, new geologic mapping of Pliocene to Quaternary units, and morphological analysis reveal that the mountain front and surrounding areas have undergone tectonic deformation since at least the Pliocene. Approximately 200 m west of the mountain front, we mapped a 0.7-to-8.7-m-high fault scarp in alluvial deposits dated at ~14 ka with Optically Stimulated Luminescence (OSL). Morphotectonic analysis of the scarp and faults mapped in two trenches demonstrates that the scarp formed during moderate to large Holocene earthquakes along the newly identified Mesamávida fault. A local seismic network also detected crustal seismicity likely attributable to the fault.
At least one branch fault of the WATS at 36°S—the Mesamávida fault—has the potential to generate earthquakes with magnitudes up to ~7.1. The seismic potential of the Mesamávida fault should be incorporated into hazard analyses for central-southern Chile. Study of similar faults within the WATS to the north and south will enhance understanding of this thrust system's seismic hazard and its role in Quaternary tectonics of the Andes.