Seismicity within the actively deforming Eastern Sierras Pampeanas, Argentina



The Eastern Sierras Pampeanas of central Argentina have been interpreted by previous studies to represent the easternmost expression of continental deformation resulting from the flat-slab subduction of the Nazca Plate beneath the South American Plate. Yet, the seismic activity and deformation of this area, as well as its relationship to subduction more than 700 km to the west, are poorly understood. Using seismic data recorded by the Eastern Sierras Pampeanas broadband seismic array between 2008 August and 2009 August, we identified and located 445 local crustal earthquakes, most with magnitudes of 2.5 or less. We employed a double-difference location method to clarify the hypocentral distribution of local seismicity and to define zones of both clustered and diffuse seismicity. The relocated events define active structures in the region, including a horizontal zone of seismicity between 15 and 25 km depth, a prominent west-dipping structure beneath the central ranges and a vertical structure along the eastern margin of the study area. Overall, seismicity in this region during the study period is constrained to the upper ∼25 km of the crust and is mostly concentrated along minor structures and not along the major range-bounding faults. Focal mechanism solutions from the nine clearest events show mainly normal deformation with an oblique component and E–W oriented P-axes that likely result from shortening along structures with oblique orientations to the direction of plate convergence. This is the first seismic study to document that the Eastern Sierras Pampeanas are actively deforming in response to compression from the convergent margin.