Abstract

ALVARADO, Patricia  and  RAMOS, Victor A. The Andean structure of  the Sierras Pampeanas based on earthquake focal mechanisms in their northwestern region. Rev. Asoc. Geol. Argent. [online]. 2010, vol.67, n.4, pp.461-472. ISSN 0004-4822.

Modeling of  broadband seismic waveforms recorded by global and Chilean networks for two moderate crustal earthquakes of  the northwestern Sierras Pampeanas shows their focal mechanisms, depths and seismotectonic features. The magnitude Mw 5.8 earthquake on 28 May 2002, located in the eastern flank of  the sierra de Velasco and the Mw 6.2 earthquake on 7 September 2004, with epicenter in the southwestern part of  the sierra de Ambato, have shallow focal depths of  10 and 8 km, respectively. These results combined with the historical seismicity of  the region allow us to estimate the deep structure of  the Sierras Pampeanas in the study region. The seismic analyses together with interpretations of  the surface structure and previous neotectonic studies ruled out extensional or strike slip deformation as the main responsible mechanism of  the Present structure of  this sector of the sierras de Ambato and Velasco in the northwestern Sierras Pampeanas. The comparison between the Nazca-South America plate convergence orientation as well as GPS velocities in the upper plate with the summation of  the seismic moment tensor for the largest seismic energy released by crustal earthquakes of  this region in the last 30 years, shows a clockwise rotation of  50º of  the average P-axis orientation from the convergence orientation to the northeast suggesting important strain partition. This partition is controlled by the Eopaleozoic basement fabric, which has guided the orientation and vergence of  the Andean faults.

Keywords : Earthquakes; Decollement levels; Andes; Compressive deformation.

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