Stripe of normal mechanisms for crustal earthquakes Mw less than 3.5 flanking the western side of the thrust front zone in the Andes backarc

Abstract

Earthquakes with magnitudes M 3.5 and depths between 15km and 35km were registered in the Andes backarc between 32.5°S and 33.75°S within a temporary experiment from November 2002 to March 2003. A band trending NW-NNW of normal focal mechanism earthquakes is located just by the thrust front towards west, and cover the southern tip of the Southern Precordillera and the western side of the Cerrilladas Pedemontanas, Mendoza Province, Argentina. The short temporal experiment precludes firm interpretations about the existence of this confined tensional stress field within the regional predominant compressional stress field. Probably it is related to a temporary stress relaxation. The normal earthquakes are likely associated to normal faults, and/or to its subsidiaries, that were inverted by contraction and now re-inverted at least transitorily as normal. These faults, which are near and to the East of the suture between Chilenia and Cuyania Paleozoic terranes, were originally involved in the Cuyo Triassic basin formation.