Ammirati, Jean-Baptiste ; Mackaman-Lofland, Chelsea ; Zeckra, Martin

Talk presented at AGU Fall Meeting 2021, New Orleans, USA on 2021-12-14

Abstract: Understanding the mechanisms of crustal deformation along convergent margins is critical to identifying potential seismogenic structures and evaluating earthquake hazards for nearby urban centers. In the southern central Andes (28–33ºS), along-strike variations in subduction dip angle have been linked to high upper-crustal shortening and characterized by an elevated retroarc seismicity. Over the years, several large magnitude earthquakes were responsible for many casualties and caused infrastructural damage to cities in western Argentina. Although several studies largely contributed to better understand the nature and evolution of the Argentine retroarc, some key aspects remain poorly understood: 1) The discrepancy between subduction-parallel ENE-WSW regional stress evidenced by 30 years of seismic and geodetics monitoring and the EW trench-normal compressional deformation markers characterizing the retroarc. 2) The presence of an abnormally thick crust (~60 km) with deep crustal seismicity within its basement (20-40 km). The January 19th 2021 (Mw6.4) San Juan earthquake can be analyzed in the framework of historical and modern seismicity to investigate and clarify the mechanisms controlling the tectonic evolution of the Argentine retroarc. The deep crustal character (23-24 km depth) of this earthquake and its strike-slip, right-lateral focal mechanism strengthens the idea of deformation partitioning along basement faults and upper-crust structures in response to the oblique convergence of the Nazca and South American plates. Seismogenic basement faults such as the mid-crustal structure activated during the 2021 San Juan earthquake have significant implications for seismic risk in western Argentina, and may transfer shortening to active upper-crustal faults associated with historically devastating shallow earthquakes.

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