Long‐Term Lithospheric Strength and Upper‐Plate Seismicity in the Southern Central Andes, 29°–39°S.

We examined the relationship between the mechanical strength of the lithosphere and the distribution of seismicity within the overriding continental plate of the southern Central Andes (SCA, 29°–39°S), where the oceanic Nazca Plate changes its subduction angle between 33°S and 35°S, from subhorizontal in the north (<5°) to steep in the south (∼30°). We computed the long‐term lithospheric strength based on an existing 3D model describing variations in thickness, density, and temperature of the main geological units forming the lithosphere of the SCA and adjacent forearc and foreland regions. The comparison between our results and seismicity within the overriding plate (upper‐plate seismicity) shows that most of the events occur within the modeled brittle domain of the lithosphere. The depth where the deformation mode switches from brittle frictional to thermally activated ductile creep provides a conservative lower bound to the seismogenic zone in the overriding plate of the study area. We also found that the majority of upper‐plate earthquakes occurs within the realm of first‐order contrasts in integrated strength (12.7–13.3 log Pam in the Andean orogen vs. 13.5–13.9 log Pam in the forearc and the foreland). Specific conditions characterize the mechanically strong northern foreland of the Andes, where seismicity is likely explained by the effects of slab steepening. Plain Language Summary: The southern Central Andes (29°–39°S) are one of the most seismically active regions along the South‐American subduction zone. However, the causative dynamics behind localization of earthquakes in the South American plate are still not well understood. In this study, we computed the strength of the lithosphere (i.e., the resistance to deformation) with a specialized computer code and showcase that there exist a causative relationship with seismicity in the area. We found that earthquakes in the upper plate occur: (a) above the transition between domains with brittle to ductile deformation; and (b) where large and sharp horizontal strength contrasts exist. Specific conditions characterize the mechanically strong northern foreland of the Andes, where seismicity is likely explained by the additional effects of the oceanic subducted plate, where it transitions from a subhorizontal to a steep subduction angle. These results show the importance of quantifying the strength of the lithosphere and its spatial variations to better understand the distribution of seismicity records in the area. Key Points: The yield strength of the lithosphere was computed and compared with the upper‐plate seismicity distribution of the southern Central AndesThe brittle‐ductile transition effectively bounds the depth extent of the seismogenic zoneSeismicity localizes at the boundaries between weak and strong domains; slab steepening likely explains seismicity in the strong foreland [ABSTRACT FROM AUTHOR]