TITLE:
Stripe of normal mechanisms for crustal earthquakes with M ≤ 3.5 flanking the western side of the thrust front zone in the Andes backarc
AUTHORS:
Silvina Nacif, Enrique Triep, Renzo Furlani, Silvana Spagnotto
KEYWORDS:
Small Magnitudes Seismicity; Normal Focal Mechanism Solutions; Normal Fault Activation; Regional Compressional Regime
JOURNAL NAME:
Natural Science,
Vol.5 No.8A,
August
20,
2013
ABSTRACT: Earthquakes with magnitudes M ≤ 3.5 were registered
in the Andes backarc between ~32.5°S 33.75°S
within a temporary experiment from November 2002 to March 2003. Data were collected
from 15 seismological broad band stations, deployed above flat subduction
section and also above the transitional to normal section of the Nazca plate.
Seismic events were located mostly in part of the Southern Precordillera and
Cerrilladas Pedemontanas of Mendoza Province. Focal mechanism
solutions were obtained, for selected data between15 kmand35 kmdepths, from P wave first motion using FOCMEC software. A band trending NW-NNW
of normal focal mechanism earthquakes is located just by the thrust front
towards West, and covers the Southern tip of the Southern Precordillera and the
Western side of the Cerrilladas Pedemontanas, Mendoza Province, Argentina.
Thrust focal mechanism solutions obtained in the present work for events with
magnitude less than or equal to 3.5 also show that the thrust front South of
~33.1°S is located in the same place as Mw ≥ 3.6 earthquakes. The most significant
findings in this work are these signals of extensional regimen which appear in
a compressional subduction tectonic regimen. Nevertheless the short temporal
experiment precludes firm interpretations about this particular phenomenon.
Probably it is related to a temporary stress relaxation. The normal earthquakes
are likely associated to normal faults, and/or to their 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.